We demonstrated that the epsilon-near-zero (ENZ) aluminum-doped zinc oxide (AZO) thin film exhibited ultrafast nonlinear optical response and efficient third-harmonic generation (THG) experimentally. The AZO film showed sub-picosecond response and broadband wavelength-dependent nonlinear absorption and refraction properties. In addition, the AZO thin film can produce efficient THG with an efficiency of 0.63 × 10-6 at the ENZ wavelength. The experimental results revealed the exceptional nonlinear optical behavior in the AZO thin film, and may provide insights for designing all-optical ultrafast optoelectronic devices.

Nonlinear photonic crystals (NPCs) with modulated second-order nonlinear coefficients (χ(2)) enable quasi-phase-matching (QPM) for efficient frequency conversion. Traditional electric-field poling is limited to two-dimensional domain engineering and cannot achieve three-dimensional (3D) χ(2) distributions, while femtosecond laser writing (FLW) offers greater control but introduces crystal damage. In this work, we use the pyroelectric-based fabrication process by performing the cooling step in a vacuum after FLW, suppressing thermal fluctuations, and maximizing the pyroelectric field. Vacuum cooling significantly improves domain inversion probability and uniformity compared to air cooling, making the periodicity close to electrical poling. Real-time polarized microscopy reveals improved domain growth, while nonlinear diffraction analysis confirms negligible refractive index changes. We demonstrate domain-inverted NPCs with a periodicity of 4 µm, achieving QPM at near-infrared wavelengths. This method provides a scalable and efficient pathway for advanced nonlinear photonic devices.

We make a comparison study of linear and nonlinear diffraction by a periodically poled lithium niobate (PPLN) dual linear–nonlinear thin-plate grating with weak surface corrugation against four types of white light sources. They are the ordinary halogen lamp thermal radiation source (WL1), the silica photonic crystal fiber supercontinuum nanosecond laser source (WL2), and two femtosecond white lasers made by a Ti:sapphire pump laser beam passing through a fused silica plate with significant spectrum broadening (WL3) and through a cascaded silica plate and a chirped PPLN crystal with ultrabroadband second-harmonic generation (WL4). The experiments show that the coherence, peak power, spectral bandwidth, profile, and flatness of the pump white light all contribute to shaping the characteristics of linear and nonlinear optical diffraction patterns. In particular, for WL1, WL2, and WL4, the peak power is not sufficiently large; thus, only linear diffraction occurs. For incoherent WL1, the interference is absent. Remarkably, when a femtosecond white laser with sufficiently large peak power (WL3) shines upon the PPLN grating, bright and sharp linear Bragg scattering spots, dispersive colored linear diffraction patterns, and nonlinear Cherenkov radiation nonlinear diffraction patterns are observed simultaneously by naked eyes. The experiments would enrich the basic physical and optical understanding of linear and nonlinear optical diffraction characteristics of ultrabroadband white laser sources.

In classical linear optics, when light shines upon a grating in the normal configuration where the incident plane of light is perpendicular to the optical axis of the grating, ordinary Bragg diffraction will occur. However, when light is incident in the general conical configuration where the incident plane of light is oblique to the optical axis of the grating, the Bragg diffraction becomes much more complicated. What happens to the nonlinear diffraction of a laser beam by a nonlinear grating? In this Letter, we wish to answer this interesting and fundamental question in the realm of nonlinear optics. We shine a Ti:sapphire femtosecond pulse laser beam (with a central wavelength at 800 nm) upon a periodically poled lithium niobate (PPLN) thin plate nonlinear grating and systematically investigate the Raman–Nath nonlinear diffraction (NRND) pattern of a second-harmonic generation (SHG) laser beam in various “conical” (or off-plane) incidence configurations characterized by both the polar angle α and azimuthal angle φ. By analyzing the diffraction characteristic and uncovering the underlying mechanisms of conical NRND nonlinear diffraction, we have provided a comprehensive understanding of its spatial behavior. The study of conical nonlinear diffraction enriches the understanding of the complicated interaction between the pump laser beam and the structured nonlinear medium and broadens the arena of nonlinear optics.

We fabricated a three-dimensional nonlinear photonic crystal in a Sr0.61Ba0.39Nb2O6 (SBN) crystal using femtosecond laser direct writing of ferroelectric domain structures. The crystal features three layers of fork-shaped gratings, each oriented differently. These gratings convert an incident vortex beam into a second-harmonic Gaussian beam in specific directions. By altering the vortex beam’s topological charge, we can control the emission direction of the second-harmonic Gaussian beam, enabling flexible all-optical switching and manipulation. This work provides a foundation for controlling photon angular momentum in nonlinear optical frequency-conversion processes.

Transition metal dichalcogenide two-dimensional (2D) materials, exhibiting extraordinary properties absent in their bulk forms, have garnered significant attention in the field of nonlinear optical devices. However, the atomic-level thickness limits the light absorption, which makes the intensity of the nonlinear signal extremely weak. Through transferring monolayer WS2 onto a silica microsphere, we report a giant second-harmonic generation (SHG) enhancement for approximately 1.46 × 107 times. The second-harmonic (SH) signal reaches 2.56 MHz pumped by a continuous wave laser of 2.5 mW. It is attributed to an enhancement of the pump laser due to the whispering gallery mode of the microsphere cavity. This work demonstrates the potential of microcavity-integrated monolayer 2D materials for nonlinear optics in integrated photonics.

The whispering gallery resonator (WGR) represents a promising avenue for the miniaturization of optical devices, while cascaded optical parameter oscillator (OPO) processes have not been realized in the WGR, to the best of our knowledge. We present a microdisk with quality factors up to 3.2 × 107, then embed two quasi-phase-matching structures inside it to demonstrate cascaded OPO. The cascaded OPO exhibits the same idler light output with the threshold of 32.7 mW at 36°C (1063.8 nm → 1566.6 nm + 3314.6 nm/1566.6 nm → 2970.4 nm + 3314.6 nm), while the operating threshold of OPO without cascade process is 4.32 mW. Moreover, diverse cascaded processes are observed, with the longest output wavelength reaching 4802.9 nm. Our results suggest the potential for a low-threshold cascade OPO based on WGR.

Nonlinear Cherenkov radiation is a phenomenon of light first observed in 1970 that can be manipulated by phase matching conditions. However, under a rotatory symmetry, the nonlinear Cherenkov radiation was still untouched, where the rotation parameters in optics would introduce an additional phase to the beam, change the phase velocity of the electromagnetic wave, and lead to novel optical phenomena. Here, we introduce rotation as a new freedom and study the nonlinear Cherenkov radiation in optically rotatory crystals in theory. With a quartz crystal as the representative, we derive theoretical variations, which show that the phase velocity of the crystal-coupled wave is found to be accelerated or decelerated by the rotational angular velocity, corresponding to the change of the Cherenkov radiation angle. In addition, the variation on the effective nonlinear coefficient of quartz crystals with rotational polarization direction is analyzed theoretically and used to simulate the Cherenkov ring distribution in rotatory nonlinear optics. This work introduces the rotation parameter into the non-collinear phase matching process and may inspire the development of modern photonics and physics in rotatory frames.

Terahertz (THz) radiation generation by two-color femtosecond laser filamentation is a promising path for high-intensity THz source development. The intrinsic characteristics of the filament, especially its length, play a crucial role in determining the THz radiation strength. However, a detailed analysis of the underlying physical mechanism and the quantitative correlation between the laser filament length and the THz radiation intensity under a high-peak-power driving laser is still lacking. In this paper, the effect of filament length on the THz radiation is investigated by modulating the basic characteristics of the two-color laser field and changing the focal length. Experimental results show that the long filament length is advantageous for improving THz radiation intensity. The theoretical simulation indicates that enhancement of THz radiation arises from coherent accumulation of THz wave produced at each cross-section along the filament. These insights suggest that extending the filament length is an effective scheme to enhance the intensity of THz radiation generated by the two-color femtosecond laser filament.

Recently, in the field of nonlinear optics of the terahertz frequency range, numerous unique features have been discovered that distinguish it advantageously from nonlinear optics of the optical frequency range. This study demonstrates that the interference of radiations generated at triple frequencies and those due to self-phase modulation in a cubic nonlinear medium can be either constructive or destructive, depending on the parameters of the pulse at the input of the medium. As a result, for a single-cycle pulse, mutual attenuation of these effects is observed by a factor of 20, while for a single and a half-cycle pulse, mutual enhancement occurs by a factor of 1.7. The obtained features are in good agreement with existing experimental data. Thus, by varying the parameters of few-cycle terahertz waves, it is possible to control the nonlinear processes observed in optical media. This will allow for the future development of light-to-light control devices based on these principles.

Zinc oxide (ZnO) plays a crucial role in the application of all-optical devices due to its broadband nonlinear optical response. In this work, the broadband nonlinear optical response of Ga-doped ZnO (GZO) was investigated using femtosecond transient absorption spectroscopy (450–800 nm), revealing an ultrahigh modulation depth (∼86%) and ultrafast response (∼280 fs) at a pump wavelength of 750 nm. Non-degenerate two-photon absorption (TPA) spectra corrected by dispersion revealed significantly stronger TPA in the visible range. An energy level model based on Ga-related defects explained the enhancement mechanism of TPA and the photoluminescence spectrum. This study highlights the significant potential of GZO in future broadband all-optical device applications.

For the first time, to our knowledge, the cascading effects of self-phase modulation and second-harmonic generation (SPM-SHG) in a nonlinear optical medium were used to conveniently convert a near-infrared ultrafast laser with a fixed center wavelength into a visible to deep-ultraviolet (DUV) laser with a continuously tunable wavelength. When a β-BaB2O4 (BBO) crystal was used as the nonlinear optical medium, and a Ti:sapphire laser (800 nm, 38 fs) was used as the fundamental light source, the output wavelength had a tunable range of 225–460 nm, and the highest optical conversion efficiency reached 18.1% at 361 nm. For a 1030 nm fundamental light source, the shortest output wavelength was also 225 nm by one-step frequency conversion of the BBO crystal. By further frequency conversions, the tunable wavelength can extend to the vacuum ultraviolet (VUV) waveband, as short as 193 nm. These results demonstrated that SPM-SHG could be used as an extremely simple and effective frequency conversion method to obtain a wideband tunable ultraviolet laser.

We experimentally demonstrate a continuous-wave (CW) injection-seeded cascaded optical parametric amplifier (OPA) for generating femtosecond pulses in the NIR-I spectral region. Utilizing a cascaded two-stage configuration, our system achieves an output of 347 mW of NIR radiation centered at 792 nm, combined with a pulse duration of 171 fs at a repetition rate of 50 MHz. The CW seeding intrinsically ensures superior pulse-to-pulse and long-term power stability. Our measurements indicate a relative intensity noise (RIN) of 2.2% root mean square (RMS) (integrated from 3.3 Hz to 2.5 MHz) and an RMS power stability as low as 0.63% over a duration of 90 min. Moreover, the beam quality of the output beam is near-diffraction-limited, with M2 factors of MX2 = 1.11 and MY2 = 1.29. We believe that this type of laser source is capable of delivering stable femtosecond pulses within the NIR-I spectral range and can serve as an ideal solution for various applications including biophotonics, microscopy, and laser processing.

The development of nonlinear optical materials with strong multiphoton absorption (MPA) is crucial for the design of ultrafast nonlinear optical devices, such as optical limiters and all-optical switchers. In this study, we present the wavelength-dependent coefficients of two-photon absorption (2PA) and three-photon absorption (3PA) in a GaS film across a broad range of wavelengths from 540 to 1600 nm. The observed dispersions in the 2PA and 3PA coefficients align well with the widely used two-band approximation model applied to direct bandgap semiconductors. Notably, the GaS film exhibits exceptional MPA properties with a maximum 2PA coefficient of 19.89 cm/GW at 620 nm and a maximum 3PA coefficient of 4.88 cm3/GW2 at 1500 nm. The GaS film surpasses those found in traditional wide-bandgap semiconductors like β-Ga2O3, GaN, ZnO, and ZnS while remaining comparable to monolayer MoS2, CsPbBr3, and (C4H9NH3)2PbBr4 perovskites. By employing a simplified two-energy-level model analysis, our results indicate that these large MPA coefficients are primarily determined by the remarkable absorption cross sections, which are approximately 4.82 × 10-52·cm4·s·photon-1 at 620 nm for 2PA and 8.17 × 10-80·cm6·s2·photon-2 at 1500 nm for 3PA. Our findings demonstrate significant potential for utilizing GaS films in nonlinear optical applications.

We use a broad Gaussian beam with perturbations to motivate rogue waves (RWs) in a two-dimensional optical-induced lattice. In a linear situation, we fail to observe RWs. Nevertheless, under a nonlinear condition, the probability of RWs in the lattice is less than that in a homogeneous medium. Additionally, we obtain a shorter long-tail distribution of probability density function in an optical lattice.

Stimulated Brillouin scattering in planar integrated circuits promises to realize compact and highly coherent lasers. Here we report efficient Brillouin lasing at telecommunication wavelength from a planar Ge25Sb10S65 chalcogenide (ChG) resonator with a high quality factor above 106. A low lasing threshold of 24.8 mW is achieved with a slope efficiency of 8.3%. An 8-kHz linewidth is measured for 1.56-mW on-chip output power. This work offers a good opportunity to enrich the versatility and functionality of the ChG photonics on account of their intrinsic advantages of low loss, high third-order nonlinearity, and potential capacity for wafer-scale fabrication.

A 3D nonlinear photonic crystal containing four parallel segments of periodic χ(2) grating structure is fabricated employing the femtosecond laser poling of ferroelectric Ca0.28Ba0.72Nb2O6 crystal. The second harmonic generation from this four-segment structure is studied with a fundamental Gaussian wave. By tuning the wavelength of the fundamental wave, the second harmonic varies from the Laguerre–Gaussian beam (topological charge lc = 1) to the higher-order Hermite–Gaussian beam and Laguerre–Gaussian again (lc = -1). This effect is caused by the wavelength-dependent phase delays introduced by the four-grating structure. Our study contributes to a deeper understanding of nonlinear wave interactions in 3D nonlinear photonic crystals. It also offers new possibilities for special beam generation at new frequencies and their control.

We report the generation of quasi-cw vacuum ultraviolet (VUV) light at 160 nm with a repetition rate of 82 MHz by two second-harmonic generations and one sum frequency mixing. The VUV laser light is produced as a fifth-harmonic generation of a mode-locked ps Ti:sapphire laser system by successive stages with nonlinear crystals of LBO and KBBF. A stable generation of laser light at 200 nm for more than 6 h is the most important step for obtaining the generation of light at a wavelength of 160 nm.

Three-dimensional (3D) nonlinear photonic crystals have received intensive interest as an ideal platform to study nonlinear wave interactions and explore their applications. Periodic fork-shaped gratings are extremely important in this context because they are capable of generating second-harmonic vortex beams from a fundamental Gaussian wave, which has versatile applications in optical trapping and materials engineering. However, previous studies mainly focused on the normal incidence of the fundamental Gaussian beam, resulting in symmetric emissions of the second-harmonic vortices. Here we present an experimental study on second-harmonic vortex generation in periodic fork-shaped gratings at oblique incidence, in comparison with the case of normal incidence. More quasi-phase-matching resonant wavelengths have been observed at oblique incidence, and the second-harmonic emissions become asymmetric against the incident beam. These results agree well with theoretic explanations. The oblique incidence of the fundamental wave is also used for the generation of second-harmonic Bessel beams with uniform azimuthal intensity distributions. Our study is important for a deeper understanding of nonlinear interactions in a 3D periodic medium. It also paves the way toward achieving high-quality structured beams at new frequencies, which is important for manipulation of the orbital angular momentum of light.

Fast and stable phase control is essential for many applications in optics. Here, we propose an all-fiber all-optical phase modulation scheme based on a Fabry–Perot interferometer (FPI) and an Er/Yb co-doped fiber (EYDF). By using the EYDF as an F-P cavity via rational design, a phase shift with a modulation sensitivity of 0.0312π/mW is introduced to the modulator. The phase shifts in the EYDF consist of a thermal phase shift and a nonlinear phase shift with a ratio of 19:1, and the corresponding temporal responses of the modulation are 204 ms and 2.5 ms, respectively. In addition, the compact FPI is encapsulated to provide excellent stability for the modulator.

Whispering-gallery-mode (WGM) microresonators can greatly enhance light–matter interaction, making them indispensable units for frequency conversion in nonlinear optics. Efficient nonlinear wave mixing in microresonators requires stringent simultaneous optical resonance and phase-matching conditions. Thus, it is challenging to achieve efficient frequency conversion over a broad bandwidth. Here, we demonstrate broadband second-harmonic generation (SHG) in the x-cut thin-film lithium niobate (TFLN) microdisk with a quality factor above 107 by applying the cyclic quasi-phase-matching (CQPM) mechanism, which is intrinsically applicable for broadband operation. Broadband SHG of continuous-wave laser with a maximum normalized conversion efficiency of ∼15%/mW is achieved with a bandwidth spanning over 100 nm in the telecommunication band. Furthermore, broadband SHG of femtosecond lasers, supercontinuum lasers, and amplified spontaneous emission in the telecommunication band is also experimentally observed. The work is beneficial for integrated nonlinear photonics devices like frequency converters and optical frequency comb generator based on second-order nonlinearity on the TFLN platform.

The design of nonlinear photonic Vogel’s spiral based on quasi-crystal theory was demonstrated. Two main parameters of Vogel’s spiral were arranged to obtain multi-reciprocal circles. Typical structure was fabricated by the near-infrared femtosecond laser poling technique, forming a nonlinear photonic structure, and multiple ring-like nonlinear Raman–Nath second-harmonic generation processes were realized and analyzed in detail. The structure for the cascaded third-harmonic generation process was predicted. The results could help deepen the understanding of Vogel’s spiral and quasi-crystal and pave the way for the combination of quasi-crystal theory with more aperiodic structures.

We investigated the Talbot effect in an anti-parity-time (PT) symmetric synthetic photonic lattice composed of two coupled fiber loops. We calculated the band structures and found that with an increase in the gain-loss parameter, the band transitions from a real spectrum to a complex spectrum. We study the influence of phase in the Hermitian operator on the Talbot effect, and the Talbot effect disappears when the period of the input field is N > 8. Further study shows that the variation of Talbot distance can also be modulated by non-Hermitian coefficients of gain and loss. This work may find significant applications in pulse repetition-rate multiplication, temporal invisibility, and tunable intensity amplifiers.

Entangled photon pairs are crucial resources for quantum information processing protocols. Via the process of spontaneous parametric downconversion (SPDC), we can generate these photon pairs using bulk nonlinear crystals. Traditionally, the crystal is designed to satisfy a specific type of phase-matching condition. Here, we report controllable transitions among different types of phase matching in a single periodically poled potassium titanyl phosphate crystal. By carefully selecting pump conditions, we can satisfy different phase-matching conditions. This allows us to observe first-order Type-II, fifth-order Type-I, third-order Type-0, and fifth-order Type-II SPDCs. The temperature-dependent spectra of our source were also analyzed in detail. Finally, we discussed the possibility of observing more than nine SPDCs in this crystal. Our work not only deepens the understanding of the physics behind phase-matching conditions, but also offers the potential for a highly versatile entangled biphoton source for quantum information research.

Dissipative Kerr cavity solitons (CSs) are localized temporal structures generated in coherently driven Kerr resonators which have attracted widespread attention for their rich nonlinear dynamics and key role in the generation of optical frequency combs. Akin to the complexity of the dissipative solitons in mode-locked lasers, the nonlinear dynamics of the CSs present distinctive evolutionary behaviors that may create new potential for understanding interdisciplinary nonlinear problems. Here, we leverage real-time spectroscopy to study the transient behaviors of the CSs in a Kerr fiber cavity with coherent driving. The real-time spectroscopy is implemented with the emerging dispersive Fourier transform (DFT) technology with a large dispersion of -10.2 ns/nm, which provides a sampling spectral resolution of ∼1 pm. Under perturbations, the complete birth-to-annihilation process of the CS is visualized in real time as the Kerr fiber cavity specifically locked around the boundary of modulation instability (MI) and bistable regimes. Fruitful transient dynamics are observed, including MI, soliton breathing, stationary CS, and its annihilation. The mechanism of the observed transient dynamics is theoretically studied through numerical simulation, and we find that the cavity detuning variation resulting from the external perturbation plays a dominant role in the evolution of the CS. More importantly, there exists a visible energy drop accompanied by the CS breathing, wherein the collision between the solitons triggers the subsequent drift and annihilation of the CS. The spectral interferograms of multiple CSs that are analyzed by their field autocorrelation also verify the annihilation of the CS.

Recently, artificial intelligence has been proven as an effective modeling tool in ultrafast optics; its application in the design of ultrafast laser systems is a promising issue. In this Letter, a method based on a feed-forward neural network (FNN) model with a simple structure is adopted to inversely predict the full-field supercontinuum generation and recover the initial pulse. The performance of the FNN and its dependence on the predicted pulse features are further explored by a reconstruction test. The generalization ability of the proposed method is further demonstrated in the case with an initial chirp.

The dependence of nonlinear optical absorption and carrier dynamics on the thickness of chromium thiophosphate (CrPS4) is investigated. Utilizing the I-scan system, we have observed the typical two-photon absorption (TPA) at 780 nm for three different thicknesses. The TPA coefficient, third-order nonlinear optical susceptibility Imχ(3), and the figure of merit have been obtained by fitting the I-scan data. Using nondegenerate pump-probe measurements, the photoinduced absorption has been observed, and the carrier relaxation processes are phonon-assisted. This study provides deep insights into the nonlinear optical properties of CrPS4, which is of great significance for potential applications in ultrafast optical devices.

We propose a spatially chirped quasi-phase-matching (QPM) scheme that enables ultrabroadband second-harmonic-generation (SHG) by using a fan-out QPM grating to frequency-convert a spatially chirped fundamental wave. A “zero-dispersion” 4f system maps the spectral contents of ultrabroadband fundamental onto different spatial coordinates in the Fourier plane, where the fundamental is quasi-monochromatic locally in picosecond duration, fundamentally canceling high-order phase mismatch. A fan-out QPM grating characterized by a linear variation of the poling period along the transverse direction exactly supports the QPM of the spatially chirped beam. We theoretically demonstrate the SHG of an 810-nm, 12.1-fs pulse into a 405-nm, 10.2-fs pulse with a conversion efficiency of 77%.

In this study, a batch of indium tin oxide (ITO)/Sn composites with different ratios was obtained based on the principle of thermal evaporation by an electron beam. The crystalline structure, surface shape, and optical characterization of the films were researched using an X-ray diffractometer, an atomic force microscope, a UV-Vis-NIR dual-beam spectrophotometer, and an open-hole Z-scan system. By varying the relative thickness ratio of the ITO/Sn bilayer film, tunable nonlinear optical properties were achieved. The nonlinear saturation absorption coefficient β maximum of the ITO/Sn composites is -10.5×10-7 cm/W, approximately 21 and 1.72 times more enhanced compared to monolayer ITO and Sn, respectively. Moreover, the improvement of the sample nonlinear performance was verified using finite-difference in temporal domain simulations.

Compact terahertz (THz) devices, especially for nonlinear THz components, have received more and more attention due to their potential applications in THz nonlinearity-based sensing, communications, and computing devices. However, effective means to enhance, control, and confine the nonlinear harmonics of THz waves remain a great challenge for micro-scale THz nonlinear devices. In this work, we have established a technique for nonlinear harmonic generation of THz waves based on phonon polariton-enhanced giant THz nonlinearity in a 2D-topologically protected valley photonic microcavity. Effective THz harmonic generation has been observed in both noncentrosymmetric and centrosymmetric nonlinear materials. These results can provide a valuable reference for the generation and control of THz high-harmonics, thus developing new nonlinear devices in the THz regime.

The self-focusing phenomenon of partially coherent beams (PCBs) was simulated using the complex screen method combined with the split-step Fourier method to solve the nonlinear Schrödinger equation. Considering the propagation of Gaussian Schell-model beams in a nonlinear medium as an example, the suppression effects of intensity, propagation distance, and spatial coherence on small-scale self-focusing were demonstrated. Simulations of overall and small-scale self-focusing using this method were compared with the existing literature to demonstrate the validity of the method. This method can numerically analyze the degree of self-focusing in PCBs and advance the study of their nonlinearity.

The compact and reliable ultraviolet (UV) source has attracted remarkable attention for its potential use in optical measurement systems, high-density optical storage, and biomedical applications. We demonstrate ultraviolet generation by frequency doubling in a lithium-tantalate-on-insulator (LTOI) microdisk via modal phase matching. The 50-µm-diameter microdisk was milled by a focused ion beam (FIB) and followed by chemo-mechanical polishing (CMP) to smooth the disk surface and edge, and the Q-factor reaches 2.74×105 in the visible band. On-chip UV coherent light with a wavelength of 384.3 nm was achieved, which shows great promise for using LTOIs in integrated ultraviolet source platforms.

A circular-sided square microcavity laser etched a central hole has achieved chaos operation with a bandwidth of 20.8 GHz without external optical feedback or injection, in which the intensity probability distribution of a chaotic signal with a two-peak pattern was observed. Based on the self-chaotic microlaser, physical random numbers at 400 Gb/s were generated by extracting the four least significant bits without other complex post-processing methods. The solitary chaos laser and minimal post-processing have predicted a simpler and low-cost on-chip random number generator in the future.

Relativistic electrons moving over a periodic metal grating can lead to an intriguing emission of light, known as Smith–Purcell radiation (SPR), the precursor of the free-electron laser. The speed of light plays a critical role in the far-field emission spectrum. Inspired by this photonic SPR, here we experimentally demonstrate a photoacoustic phased array using laser-induced shock waves. We observe acoustic radiation spectrum in the far field, perfectly predicted by a universal theory for the SPR. This scheme provides a tool to control the acoustic radiation in the near field, paving the way toward coherent acoustic wave generation and microstructure metrology.

An exceptionally high stimulated Raman scattering (SRS) conversion efficiency to the first Stokes component associated with the secondary (low-frequency and low intensity) vibrational mode ν2 (∼330 cm-1) was observed in a BaWO4 crystal in a highly transient regime of interaction. The effect takes place in the range of pump pulse energy from ∼0.1 to ∼0.5 µJ with maximum energy conversion efficiency up to 35% at 0.2 µJ. The nature of the observed effects is explained by interference of SRS and self-phase modulation, where the latter is related to a noninstantaneous orientational Kerr nonlinearity in the BaWO4 crystal.

In this article, we use a convolutional autoencoder neural network to reduce data dimensioning and rebuild soliton dynamics in a passively mode-locked fiber laser. Based on the particle characteristic in double solitons and triple solitons interactions, we found that there is a strict correspondence between the number of minimum compression parameters and the number of independent parameters of soliton interaction. This shows that our network effectively coarsens the high-dimensional data in nonlinear systems. Our work not only introduces new prospects for the laser self-optimization algorithm, but also brings new insights into the modeling of nonlinear systems and description of soliton interactions.

We investigate the nonlinear optical limiting effect of novel graphene dispersions and graphene dispersions in carbon tetrachloride at a wavelength of 3.8 µm. The transmittances of graphene dispersions in carbon tetrachloride under two different concentrations (0.004 and 0.008 mg/mL) and two different solution thicknesses (10 and 20 mm) are measured. The influences of concentration and thickness on the optical limiting effect of graphene dispersion are analyzed. Theoretical analysis of the experimental data shows that the main optical limiting mechanism of the new graphene dispersions is Mie scattering. The limiting capacity coefficient of the new graphene dispersion in carbon tetrachloride reaches 0.405 and the minimum transmittance reaches 0.292 when the concentration is 0.004 mg/mL and the thickness is 20 mm. The graphene dispersions in carbon tetrachloride can be used to achieve good nonlinear optical limiting effects at the wavelength of 3.8 µm.

Frequency conversion based on three-wave mixing is a critical nonlinear optic application, extending the frequency range of existing lasers and realizing frequency-transduced detectors in a wavelength range that lacks an effective detector. Phase matching is vital for effective frequency conversion. The advantages of quasi-phase matching (QPM) over birefringent phase matching are a lack of walk-off effect, a maximum nonlinear coefficient, and phase matching in the entire transparency window. Herein, using different types and orders of QPM, four kinds of effective frequency doubling processes are realized in a periodically poled potassium titanyl phosphate (KTP) crystal with a single period, and three kinds of frequency doubling processes are experimentally verified. We also show a feasible way to construct an RGB color generator based on two different QPM processes. This study significantly expands the feasible frequency conversion of existing lasers to different wavelengths, providing an effective method for multi-color laser generation based on periodically poled KTP crystals.

Optical frequency conversion based on the second-order nonlinearity (χ(2)) only occurs in anisotropic media (or at interfaces) and thus is intrinsically polarization-dependent. But for practical applications, polarization-insensitive or independent operation is highly sought after. Here, by leveraging polarization coupling and second-order nonlinearity, we experimentally demonstrate a paradigm of TE/TM polarization-independent frequency upconversion, i.e., sum frequency generation, in the periodically poled lithium niobate-on-insulator ridge waveguide. The cascading of quasi-phase-matched polarization coupling and nonlinear frequency conversion is exploited. With a proper transverse electric field, TE and TM mode fundamental waves can be frequency-upconverted with an equal efficiency in the frequency converter. The proposed method may find ready application in all-optical wavelength conversion and upconversion detection technologies.

Microcavities constructed from materials with a second-order nonlinear coefficient have enabled efficient second-harmonic (SH) generation at a low power level. However, it is still technically challenging to realize double resonance with large nonlinear modal overlap in a microcavity. Here, we propose a design for a robust, tunable, and easy coupling double-resonance SH generation based on the combination of a newly developed fiber-based Fabry–Perot microcavity and a sandwich structure, whose numerical SH conversion efficiency is up to 3000% W-1. This proposal provides a feasible way to construct ultra-efficient nonlinear devices for generation of classical and quantum light sources.

The nonlinear physics dynamics of temporal dissipative solitons in a microcavity hinder them from attaining high power from pump lasers with a typical nonlinear energy conversion efficiency of less than 1%. Here, we experimentally demonstrate a straightforward method for improving the output power of soliton combs using a silica microrod cavity with high coupling strength, large mode volume, and high-Q factor, resulting in a low-repetition-rate dissipative soliton (∼21 GHz) with an energy conversion efficiency exceeding 20%. Furthermore, by generating an ∼105 GHz5×FSR (free spectral range) soliton crystal comb in the microcavity, the energy conversion efficiency can be further increased up to 56%.

The technological innovation of thin-film lithium niobate (TFLN) is supplanting the traditional lithium niobate industry and generating a vast array of ultra-compact and low-loss optical waveguide devices, providing an unprecedented prospect for chip-scale integrated optics. Because of its unique strong quadratic nonlinearity, TFLN is widely used to create new coherent light, which significantly promotes all-optical signal processes, especially in terms of speed. Herein, we review recent advances in TFLN, review the thorough optimization strategies of all-optical devices with unique characteristics based on TFLN, and discuss the challenges and perspectives of the developed nonlinear devices.

The spatial rogue waves (RWs) generated by a wide Gaussian beam in a saturated nonlinear system are experimentally observed. Our observations show that RWs are most likely to occur when Gaussian light evolves to the critical state of filament splitting, and then the probability of RWs decreases with voltage fluctuations. The occurrence probability of RWs after splitting is related to the nonlinear breathing phenomenon of optical filament, and the statistics of RWs satisfy the long-tailed L-shaped distribution. The experiment proves that the presence of high-frequency components and the aggregation of low-frequency components can serve as a prerequisite for the occurrence of extreme events (EEs).

We report an experimental generation of few-cycle pulses at 53 MHz repetition rate. Femtosecond pulses with pulse duration of 181 fs are firstly generated from an optical parametric oscillator (OPO). Then, the pulses are compressed to sub-three-cycle with a hybrid compressor composed of a commercial single-mode fiber and a pair of prisms, taking advantage of the tunability of the OPO and the numerical simulating of the nonlinear compression system. Our compressed optical pulses possess an ultrabroadband spectrum covering over 470 nm bandwidth (at -10 dB), and the output intensity fluctuation of our system is less than 0.8%. These results show that our system can effectively generate few-cycle pulses at a repetition rate of tens of megahertz with excellent long-term stability, which could benefit future possible applications.

Light bullets (LBs) are localized nonlinear waves propagating in high spatial dimensions. Finding stable LBs and realizing their control are desirable due to the interesting physics and potential applications. Here, we show that nonlocal LBs generated in a cold Rydberg atomic gas via the balance among the dispersion, diffraction, and giant nonlocal Kerr nonlinearity contributed by long-range Rydberg-Rydberg interaction can be actively manipulated by using a weak gradient magnetic field. Nonlocal LBs are generated by a balance among dispersion, diffraction, and large nonlocal Kerr nonlinearities contributed by long-range Rydberg-Rydberg interactions. Here, we find that active manipulation can be achieved by weak gradient magnetic fields in cold Rydberg atomic gases. Especially, the LBs may undergo significant Stern–Gerlach deflections, and their motion trajectories can be controlled by adjusting the magnetic-field gradient. The results reported here are helpful not only for understanding unique properties of LBs in nonlocal optical media but also for finding ways for precision measurements of magnetic fields.

The linewidth of the BaGa4Se7 (BGSe) optical parametric oscillator (OPO) was narrowed for the first time, to the best of our knowledge, by inserting a Fabry–Perot (FP) etalon into an L-shaped cavity. When a 15 mm long BGSe (56.3°, 0°) was pumped by a 1064 nm laser, the peak wavelength was ∼3529 nm, and the linewidth was 4.53 nm (3.64 cm-1) under type I phase matching. After inserting a 350 µm thick FP etalon, the linewidth was decreased to 1.27–2.05 nm. When the tilt angle of the etalon was 2.34°, the linewidth was 2.05 nm (1.65 cm-1), and the peak wavelength was still ∼3529 nm. When the tilt angle of the etalon was 3.90°, the peak wavelength was 3534.9 nm, and the linewidth was 1.27 nm (1.02 cm-1), which was the narrowest linewidth of a BGSe OPO, to the best of our knowledge. The beam quality was also improved after inserting the FP etalon.

Nano-focusing structures based on hybrid plasmonic waveguides are likely to play a key role in strong nonlinear optical devices. Although the insertion loss is considerable, a significant nonlinear phase shift may be achieved by decreasing the nano-focusing device footprint and careful parameter optimization. Here, we study the Kerr effect in hybrid plasmonic waveguides by analyzing the mode effective area, energy velocity, and insertion loss. Particularly, by utilizing plasmonics to manipulate the effective index and mode similarity, the TM mode is reflected and absorbed, while the TE mode passes through with relatively low propagation loss. By providing a deep understanding of hybrid plasmonic waveguides for nonlinear applications, we indicate pathways for their future optimization.

A novel infrared broadband nonlinear optical limiting (NOL) technology based on stimulated Brillouin scattering (SBS) in As2Se3 fiber is proposed. The As2Se3 fiber allows a weak infrared laser to pass through, but blocks an intense laser with the same wavelength due to the SBS effect. This NOL technology has been experimentally proved to have excellent NOL performance for incident pulsed lasers with a typical infrared wavelength of 3.6 µm. The linear transmissions of 1 m and 0.5 m As2Se3 fibers are higher than 90%, and the lowest nonlinear transmissions are reduced to 0.89% and 1.23%, respectively.

When two synchronized laser beams illuminate the inner surface of bulk lithium niobate crystals with magnesium doping (5%/mol MgO:LiNbO3) under the condition of total reflection, semi-degenerate four-wave mixing (FWM) is generated. On this basis, a more sophisticated frequency conversion process on the interface of nonlinear crystal has been researched. The generation mechanism of FWM is associated with the fundamental waves reflected on the inner surface of the nonlinear crystal. Analysis of the phase-matching mechanism confirms that the FWM is radiated by the third-order nonlinear polarized waves, which are stimulated by the third-order nonlinear susceptibility coefficient of the nonlinear crystal. Theoretically calculated and experimentally measured corresponding data have been presented in this article. These results are expected to provide new inspiration for further experimental and theoretical research on frequency conversion in nonlinear crystals.

The optical limiting performances of few-layer transitional metal dichalcogenides (TMDs) nanosheets in the VB group (VS2, VSe2, NbS2, NbSe2, TaS2, and TaSe2) were systematically investigated for the first time, to the best of our knowledge. It was found that these TMDs nanosheets showed a normalized transmittance in the range of 20%–40% at the input energy of 1.28 GW/cm2. Ultralow initial threshold FS (0.05–0.10 J/cm2) and optical limiting threshold FOL (0.82–2.23 J/cm2) were achieved in the TMDs nanosheets, which surpassed most of the optical limiting materials. This work showed the potential of TMDs beyond MoS2 in optical limiting field.

Lithium-niobate microcavities have not only the ability to resonantly enhance light–matter interaction but also excellent nonlinear optical properties, thereby providing an important platform for nonlinear optical investigations. In this paper, we report the observation of multi-peak spectra in the near infrared range in lithium-niobate microcavities on a chip under the pump of a 1550 nm continuous laser. Such a multi-peak spectrum was attributed to the sum-frequency of the pump laser and its background. The conversion efficiencies of the sum-frequency processes are of the order of 61.5%W-1. The influences of the phenomenon on nonlinear processes were further discussed.

We demonstrate manipulating the interactions of a second-order soliton with a weak probe pulse under the condition of group velocity match and group velocity mismatch (GVMM). During these interactions, the second-order soliton acting as an effective periodic refractive-index barrier leads to the polychromatic scattering of the probe pulse, which is represented as unequally spaced narrow-band sources with adjustable spectral width. In the case of GVMM, almost all the spectral components of the narrow-band sources meet the nonlinear frequency conversion relationship by using the wavenumber-matching relationship due to the robustness of the second-order soliton under moderate high-order-dispersion perturbations, so this case is more conducive to the study of the soliton wells. In addition, different transmission states of a soliton well are demonstrated under different probe pulse properties in the fiber-optical analog of the event horizon. When the power of the probe pulse is strong enough, a dispersive wave can be generated from the collision of two fundamental solitons split from the two second-order solitons. These interesting phenomena investigated in this work as a combination of white- and black-hole horizons can be considered as promising candidates for frequency conversion and broadband supercontinuum generation.

We demonstrate integrated lithium niobate (LN) microring resonators with Q factors close to the intrinsic material absorption limit of LN. The microrings are fabricated on pristine LN thin-film wafers thinned from LN bulk via chemo-mechanical etching without ion slicing and ion etching. A record-high Q factor up to 108 at the wavelength of 1550 nm is achieved because of the ultra-smooth interface of the microrings and the absence of ion-induced lattice damage, indicating an ultra-low waveguide propagation loss of ∼0.0034 dB/cm. The ultra-high Q microrings will pave the way for integrated quantum light source, frequency comb generation, and nonlinear optical processes.

A new unsaturated wind-chime model is proposed for calculating the formation time of the diffraction rings induced by spatial self-phase modulation (SSPM) in molybdenum disulfide suspension. To optimize the traditional wind-chime model, the concentration variable of 2D materials was introduced. The results of the unsaturated wind-chime model match quite well with the SSPM experimental results of molybdenum disulfide. Based on this model, the shortest formation time of diffraction rings and their corresponding concentration and light intensity can be predicted using limited data. Theoretically, by increasing the viscosity coefficient of the solution, the response time of the diffraction ring, to reach the maximum value, can be significantly reduced. It has advanced significance in shortening the response time of photonic diodes.

Recent years have witnessed the exploration of fiber laser technology focused on numerous pivotal optoelectronic applications from laser processing and remote sensing to optical communication. Here, using cobalt oxyfluoride (CoOF) as the nonlinear material, a 156 fs mode-locked fiber laser with strong stability is obtained. The rapid thermal annealing technique is used to fabricate the CoOF, which is subsequently transferred to the tapered region of the microfiber to form the effective pulse modulation device. CoOF interacts with the pulsed laser through the evanescent field to realize the intracavity pulse shaping, and then the stable mode-locked pulse is obtained. The mode-locked operation is maintained with the pulse duration of 156 fs and repetition rate of 49 MHz. In addition, the signal-to-noise ratio is about 90 dB. Those experimental results confirm the attractive nonlinear optical properties of CoOF and lay a foundation for the ultrafast application of low-dimensional transition metal oxides.

All-optical analog-to-digital conversion is a paramount issue in modern science. How to implement real-time and ultrafast quantization to optical pulses with different intensities in an all-optical domain is a central problem. Here, we report a real-time demonstration of an all-optical quantization scheme based on slicing the supercontinuum in a nonlinear fiber. In comparison with previous schemes through off-line analysis of the power of different optical spectral components in the supercontinuum, this, to the best of our knowledge, is the first demonstration of such functionality online in the time domain. Moreover, the extinction ratio among the quantized outputs can exceed 10 dB, which further confirms the feasibility of the proposed quantization scheme. The current 3 bit resolution in the proof-of-principle experiment is limited by the current experimental condition, but it can be expected to be greatly enhanced through improving both the spectral width of the generated supercontinuum and the number of filtering channels used.

We demonstrate a novel method to control the free spectral range (FSR) of silica micro-rod resonators precisely. This method is accomplished by iteratively applying laser annealing on the already-fabricated micro-rod resonators. Fine and repeatable increasing of resonator FSR is demonstrated, and the best resolution is smaller than 5 MHz, while the resonator quality-factor is only slightly affected by the iterative annealing procedure. Using the fabricated micro-rod resonators, single dissipative Kerr soliton microcombs are generated, and soliton repetition frequencies are tuned precisely by the iterative annealing process. The demonstrated method can be used for dual-comb spectroscopy and coherent optical communications.

We use the nonlinear coupled-mode theory to theoretically investigate second-harmonic generation (SHG) in subwavelength x-cut and z-cut lithium niobate (LN) thin-film waveguides and derive the analytical formula to calculate SHG efficiency in x-cut and z-cut LN thin-film waveguides explicitly. Under the scheme of optimal modal phase matching (MPM), two types of LN thin films can achieve highly efficient frequency doubling of a 1064 nm laser with a comparable conversion efficiency due to very consistent modal field distribution of the fundamental wave and second-harmonic wave with efficient overlap between them. Such a robust MPM for high-efficiency SHG in both the subwavelength x-cut and z-cut LN thin-film waveguides is further confirmed in a broad wavelength range, which might facilitate design and application of micro–nano nonlinear optical devices based on the subwavelength LN thin film.

Based on the transverse second-harmonic generation (TSHG) effect, we demonstrate a method for in-situ modal inspection of nonlinear micro/nanowaveguides. Pumping lights are equally split and coupled into two ends of a single CdS nanobelt (NB). As pumping light counter-propagates along the NB, transverse second-harmonic (TSH) interference patterns are observed. The influence of multimode interaction on the TSHG effect is discussed in detail. Using fast Fourier transform, TSH interference patterns are analyzed, indicating the existence of at least four modes inside the NB. Experimental beat lengths are found to be in agreement with calculated results.

We propose a spatial diffraction diagnostic method via inserting a millimeter-gap double slit into the collimated terahertz beam to monitor the minute variation of the terahertz beam in strong-field terahertz sources, which is difficult to be resolved in conventional terahertz imaging systems. To verify the method, we intentionally fabricate tiny variations of the terahertz beam through tuning the iris for the infrared pumping beam before the tilted-pulse-front pumping setups. The phenomena can be well explained by the theory based on the tilted-pulse-front technique and terahertz diffraction.

At the surfaces of crystals, linear susceptibility tensors would differ from their counterparts in the interior of the bulk crystal. However, this phenomenon has not been shown in a visible way yet. In previous researches, numerous types of nonlinear Cherenkov radiation based on different materials have been studied, while linear Cherenkov radiation is barely reported. We experimentally prove the generation of linear Cherenkov radiation on the potassium dihydrogen phosphate (KDP) crystal surface and theoretically analyze its phase-matching scheme. In our study, o-polarized light and e-polarized light can mutually convert through the linear Cherenkov process. According to this result, we figure out new nonzero elements at off-diagonal positions in the linear susceptibility tensor matrix at crystal surfaces, compared with the normal form of a bulk KDP.

The temperature tuning of BaGa4Se7 (BGSe) was demonstrated for the first time, to the best of our knowledge. When the temperature of BGSe (56.3°,0°) was raised from 30°C to 140°C, the idler light under type I raised from 3637 nm to 3989 nm, the tunable range reached 352 nm, and Δλ2/ΔT reached 3.20 nm/°C. We calculated the phase matching curve of BGSe when ? and T took different values. The relationship between θ and Δλ2/ΔT was obtained by fixing ? at 0°. The maximum Δλ2/ΔT and its corresponding (θ, ?) phase matching type were reported under different fixed λ2 (3 μm, 3.2 μm,…, 5 μm).

The vector dynamics of solitons are crucial but easily neglected for realizing vortex solitons. In this Letter, we investigate the effect of vector dynamics on cylindrical vector beams (CVBs) implementation and propose a novel technical method to realize femtosecond CVBs based on vector-locked solitons, which are presented as group-velocity-locked vector solitons (GVLVSs) in the experiment. The outstanding vector properties of GVLVSs not only greatly improve the efficiency of solitons converted into CVBs and output power of CVBs (2.4 times and 4.1 times that of scalar solitons and vector change periodical solitons, with the purity of 97.2%), but also relax the obstacle of ultrafast CVBs from the fundamental frequency to the harmonic regime (up to 198 MHz) for the first time, to the best of our knowledge. This is the highest repetition rate reported for ultrafast CVBs based on passive mode-locking. The investigation of the influence of solitons vector dynamics evolution on the realization of CVBs provides guidance for the excellent performance of ultrafast CVBs.

Tilted-pulse-front-pumping (TPFP) lithium-niobate terahertz (THz) pulse sources are widely used in pump-probe and control experiments since they can generate broadband THz pulses with tens of microjoules of energy. However, the conventional TPFP setup suffers from limitations, hindering the generation of THz pulses with peak electric field strength over 1 MV/cm. Recently, a few setups were suggested to mitigate or even eliminate these limitations. In this paper, we shortly review the setups that are suitable for the generation of single-cycle THz pulses with up to a few tens of megavolts/centimeter focused electric field strength. The THz pulses available with the new layouts pave the way for previously unattainable applications that require extremely high electric field strength and pulse energy in the multi-millijoule range.

We demonstrate comprehensive investigation of the injection locking dynamics of a backscattered Brillouin laser in silica whispering-gallery-mode microcavity. Via injection locking, the Brillouin laser acquires highly correlated phase with the seed laser, enabling ultra-narrow bandwidth, high gain, and coherent optical amplification. Also, for the first time, to the best of our knowledge, the injection locked Brillouin laser is utilized to implement all-optical carrier recovery from coherent optical data signals. We show that by using the injection locked Brillouin laser as a local oscillator for self-homodyne detection, high-quality data receiving can be realized, even without traditional electrical compensations for carrier frequency and phase drifts.

One-step precipitation of Ag nanoparticles in Ag+-doped silicate glasses was achieved through a focused picosecond laser with a high repetition rate. Absorption spectra and transmission electron microscopy (TEM) confirmed that metallic Ag nanoparticles were precipitated within glass samples in the laser-written domain. The surface plasmon absorbance fits well with the experimental absorption spectrum. The nonlinear absorption coefficient β is determined to be 2.47 × 10-14 m/W by fitting the open aperture Z-scan curve, which originated from the intraband transition in the s-p Ag band. The formation mechanism of Ag-glass nanocomposites is discussed as well.

We investigate femtosecond laser trapping dynamics of two-photon absorbing hollow-core nanoparticles with different volume fractions and two-photon absorption (TPA) coefficients. Numerical simulations show that the hollow-core particles with low and high-volume fractions can easily be trapped and bounced by the tightly focused Gaussian laser pulses, respectively. Further studies show that the hollow-core particles with and without TPA can be identified, because the TPA effect enhances the radiation force, and subsequently the longitudinal force destabilizes the trap by pushing the particle away from the focal point. The results may find direct applications in particle sorting and characterizing the TPA coefficient of single nanoparticles.

Goodness of fit is demonstrated for theoretical calculation of z-scan data based on beams propagating in the nonlinear medium and the Fresnel–Kirchhoff diffraction integral in experiments with high nonlinear refraction and absorption. The constancy of nonlinear optical parameters is achieved regardless of sample thickness and laser intensity, which clarifies the physical significance of optical parameters. We have obtained γ = 2.0 × 10-19 m2/W and β = 5.0 × 10-13 m/W for carbon disulfide excited by a pulsed laser at 800 nm with pulse duration of 35 fs, which are independent of sample thickness and laser intensity. Affirming constancy of the extracted parameters to the incident light intensity may become a practice to verify the goodness of the z-scan experiment.

In this work, we propose a new scheme to generate frequency-doubled vortex beams from a radially poled LiNbO3 micro-ring resonator based on nonlinear Cherenkov radiation. The near-infrared fundamental wave is resonant in the micro-ring, while the second harmonic is emitted from the resonator along the Cherenkov phase-matching direction. The topological charge of the emitted second-harmonic vortex beam is determined by both the azimuthal order of the whispering galley modes and the number of nonlinear grating elements. The field distribution and the conversion efficiency of the emitted vortex beam are investigated.

The fluorescence evolution along Tm3+-doped ZrF4–BaF2–LaF3–AlF3–NaF (ZBLAN) optical fibers, as well as amplified spontaneous emission in the UV-IR region with emphasis on 350 nm, 365 nm, and 450 nm, is studied, estimating optimal fiber lengths for amplification within the region. The fibers were diode-pumped with single and double lines (687 and/or 645 nm). Double-line pumping presents a quite superior efficiency for producing UV-blue signals with the benefit of requiring very short fibers, around 20 cm, compared to single-line pumping requiring more than 50 cm. A virtual cycle in which the pumps enhance each other’s absorption is the key to these systems.

A high repetition rate, picosecond terahertz (THz) parametric amplifier with a LiNbO3 (LN) crystal has been demonstrated in this work. At a 10 kHz repetition rate, a peak power of 200 W and an average power of 12 μW have been obtained over a wide range of around 2 THz; at a 100 kHz repetition rate, a maximum peak power of 18 W and an average power of 10.8 μW have been obtained. The parametric gain of the LN crystal was also investigated, and a modified Schwarz–Maier model was introduced to interpret the experimental results.

Alloying in two-dimension has been a hot spot in the development of new, versatile systems of optics and electronics. Alloys have been demonstrated to be a fascinating strategy to modulate the chemical and electronic properties of two-dimensional nanosheets. We firstly reported ultra-broadband enhanced nonlinear saturable absorption of Mo0.53W0.47Te2 alloy at 0.6, 1.0, and 2.0 μm. The nonlinear saturable absorption of Mo0.53W0.47Te2 saturable absorber (SA) was measured by the open aperture Z-scan technique. Compared to MoTe2 and WTe2 SAs, the Mo0.53W0.47Te2 SA showed five times deeper modulation depth, 8.6% lower saturable intensity, and one order larger figure of merit. Thus, our research provides a method of alloys to find novel materials with more outstanding properties for optics and optoelectronic applications.

In this Letter, we report the existence and relaxation properties of a critical phenomenon on called a 3D super crystal that emerges at T = TC ? 3.5°C, that is, in the proximity of the Curie temperature of a Cu:KTN sample. The dynamics processes of a 3D super crystal manifest in its formation containing polarized nanometric regions and/or polarized clusters. However, with strong coupling and interaction of microcomponents, the characteristic relaxation time measured by dynamic light scattering demonstrates a fully new relaxation mechanism with a much longer relaxation time. As the relaxation mechanism of a relaxator is so-far undetermined, this research provides a novel perspective. These results can help structure a fundamental theory of ferroelectric relaxation.

We report an observation of the second-order correlation between twin beams generated by amplified spontaneous parametric down-conversion operating above threshold with kilowatt-level peak power, from a periodically poled LiTaO3 crystal via a single-pass scheme. Photocurrent correlation was measured because of the bright photon streams, with raw visibility of 37.9% or 97.3% after electronic filtering. As expected in our theory, this correlation is robust and insensitive to parametric gain and detection loss, enabling important applications in optical communications, precision measurement, and nonlocal imaging.

In this work, a neural network (NN) method is developed for pulse duration inferring for an erbium-doped fiber laser at 1550 nm. Experimentally, the interferometric autocorrelation trace is observed clearly with the use of the two-photon absorption (TPA) effect in a GaAs photodiode. The intensity autocorrelation function is curve-fitted by the NN with an appropriate performance, and the measuring accuracy is consistent with a commercial autocorrelator. Compared with the Levenberg–Marquardt curve-fitting method, the NN can retrieve the intensity autocorrelation function more stably and has a certain noise reduction ability, simplifying the signal processing for a TPA photodiode-based autocorrelator.

We report efficient generation of 671 nm red light based on quasi-phase-matched second harmonic generation of 1342 nm in LiNbO3 waveguides. The design method and fabrication process of the high-quality annealed proton-exchanged periodically poled channel waveguides were presented. A continuous-wave 1.71 mW red light was obtained with a single-pass conversion efficiency of 47%·W-1·cm-2, which is 88% that of the theoretical value. While for 1 mW quasi-continuous-laser input, the corresponding peak power being 2 W, the conversion efficiency reached up to 60%. Our results indicate that the annealed proton-exchanged periodically poled LiNbO3 waveguide is promising for high-efficiency and low power consumption nonlinear generation of visible light.

We demonstrated the efficient plasmon-induced nonlinear absorption of liquid metal GaInSn nanospheres prepared by a facile liquid-phase method. With GaInSn as saturable absorbers, a passively Q-switching operation was obtained at both 1.3 and 2 μm. The pulse width of 32 ns was achieved at 1.3 μm with repetition rate of 44 kHz, single pulse energy of 51.9 μJ, and output power of 425 mW. Meanwhile, 510 ns and 92 kHz pulses with energy of 36.1 μJ and output power of 2.48 W were obtained at 2 μm. This work provides the potential of liquid metal for improving metal functions and flexible optical devices.

Bi2S3-xSex/poly(methyl methacrylate) (PMMA) nanocomposite films were prepared using microwave assisted synthesis with different compositions of x. Crystal structure, surface morphology, and optical properties were investigated to characterize the prepared nanocomposite films. The crystallinity and optical band gap of the prepared Bi2S3-xSex/PMMA were affected by x. The prepared samples showed a blue shift in the absorption edge. The laser power dependent nonlinear refraction and absorption of Bi2S3-xSex/PMMA nanocomposite films were investigated by using the Z-scan technique. The optical nonlinearity of the nanocomposite films exhibited switchover from negative nonlinear refraction to positive nonlinear refraction to negative nonlinear refraction effects, and from saturable absorption to reverse saturable absorption to saturable absorption with an increase and decrease in the composition. An interesting all-optical figure of merit was reported to assess the nanocomposite films for a practical device. It was calculated that the device all-figures of merit were based on the nonlinear response, which is important for the all-optical switching device. The results demonstrate that the optimized all-optical figures of merit can be achieved by adjusting the composition and input laser power, which can be used for the design of different all-photonic devices, and the results of nonlinear switching behavior can open new possibilities for using the nanocomposite films in laser Q-switching and mode-locking.

The dipole resonances of gold nanocages were investigated theoretically using finite difference time domain method. The results show that field enhancement is obtained at the walls of the gold nanocages. It is believed that the effect can cause a strong optical nonlinear property. To test the hypothesis, nonlinear absorption was investigated using a broadband 5 ns Z scan. It was found that at low intensities the sample shows saturable absorption (SA), while at higher intensities a switch from SA to reverse SA occurs. Moreover, the nonlinear absorption of the sample is sensitively wavelength-dependent, and, in the resonant region, saturation intensity is the largest.

We demonstrate the full vectorial feature of second-harmonic generation (SHG), i.e., from infrared full Poincaré beams to visible full Poincaré beams, based on two cascading type I phase-matching beta barium borate crystals of orthogonal optical axes. We visualize the structured features of the vectorial SHG wave by using Stokes polarimetry and show the interesting doubling effect of the polarization topological index, i.e., a low-order full Poincaré beam is converted to a high-order one. However, the polarization singularities of both C points and L lines are found to keep invariant during the SHG process. Our scheme could offer a deeper understanding on the interaction of vectorial light fields with media and can be generalized to other nonlinear optical effects.

In this Letter, a new method is presented to calculate the interactive length between the fundamental wave and second harmonic generation (SHG) for the configuration of total internal reflection on the inner surface of a nonlinear crystal. Three independent experiments are designed to measure the bandwidths of this second harmonic wave. The theoretical expression of the intensity of SHG is obtained through a nonlinear coupled wave equation. The interactive length of this phase-matched SHG can be calculated mathematically by utilizing the measured bandwidths and the intensity equation. There is no existing method to obtain the interactive length either from theoretical calculations or by experimental measurement. This method can be applied to estimate the extremely short interactive volume in nonlinear processes.

Mechanical exfoliation (ME) and chemical vapor deposition (CVD) MoS2 monolayers have been extensively studied, but the large differences of nonlinear optical performance between them have never been clarified. Here, we prepared MoS2 monolayers using ME and CVD methods and investigated the two-photon absorption (TPA) response and its saturation. We found that the TPA coefficient of the ME monolayer was about (1.88 ± 0.21) × 103 cm/GW, nearly two times that of the CVD one at (1.04 ± 0.15) × 103 cm/GW. Furthermore, we simulated and compared the TPA-induced optical pulse modulation in multilayer cascaded structures, which is instructive and meaningful for the design of optical devices such as a beam shaper and optical limiter.

Understanding the nonlinear optical effect of novel materials plays a crucial role in the fields of photonics and optoelectronics. Herein, we theoretically and experimentally investigate the simultaneous presence of third-order locally refractive nonlinearity and thermally induced nonlocal nonlinearity saturation. We present analytical expressions for the closed-aperture Z-scan trace and the number of spatial self-phase modulation (SSPM) rings, which allows one to unambiguously and conveniently separate the contributions of local and nonlocal nonlinear refraction in the case that both effects occur simultaneously. As a test, we study both the local and thermally induced nonlocal nonlinear refraction in fullerene/toluene solution by performing continuous-wave Z-scan and SSPM measurements at two different wavelengths. This work enriches the understanding of the physical mechanism of the optical nonlinear refraction effect in solution dispersions of nanomaterials, which can be exploited for nonlinear photonic devices.

We report the observation of ultralow-power absorption saturation in a tapered optical fiber (TOF) mounted in a hot cesium (Cs) vapor in a vacuum chamber. The small optical mode area of TOF produces a great influence on optical properties, allowing optical interactions with nanowatt-level power. The comparison of transmission characteristics for the TOF system and free-space vapor is investigated at different input power and atomic density. The unique performance of the Cs-TOF system makes it a promising candidate in resonant nonlinear optical applications with ultralow power.

Mechanisms of upconversion luminescence (UCL) of SrF2:Er phosphors corresponding to the G411/2→I415/2, H29/2→I415/2, F45/2→I415/2, F47/2→I415/2, H211/2→I4<

A highly efficient laser system output at the H-β Fraunhofer line of 486.1 nm has been demonstrated. A high pulse energy single-frequency hybrid 1064 nm master oscillator power amplifier was frequency-tripled to achieve 355 nm laser pulses, which acted as the pump source of the beta barium borate nanosecond pulse optical parametric oscillator. With pump energy of 190 mJ, the laser system generated a maximum output of 62 mJ blue laser pulses at 486.1 nm, corresponding to conversion efficiency of 32.6%. The laser spectrum width was measured to be around 0.1 nm, being in conformity with the spectrum width of the solar Fraunhofer line.

We experimentally demonstrate a heralded single photon source at 1290 nm by exploiting the spontaneous four wave mixing in a taper-drawn micro/nano-fiber (MNF). Because the frequency detuning between the pump and heralded single photons is ～58 THz, the contamination by Raman scattering is significantly reduced at room temperature. Since the MNF is naturally connected to standard single mode fibers via fiber tapers, the source would be compatible with the existing fiber networks. When the emission rate of heralded signal photons is about 4.6 kHz, the measured second-order intensity correlation function g(2)(0) is 0.017±0.002, which is suppressed by a factor of more than 55, relative to the classical limit.

The confocal microscopy technique was applied for nonlinear optical characterization of single β-barium-borate (β-BBO) nanocrystals. The experimental setup allows measurements of the laser polarization-selective second-harmonic (SH) generation, and the results can be used to determine the nanocrystals’ c-axis orientation, as well as to obtain information about their second-order susceptibility χ(2). The dependence of the SH signal on the laser polarization allowed the discrimination of individual particles from aggregates. The data were fitted using a model that takes into account the BBO properties and the experimental setup characteristics considering (i) the electrostatic approximation, (ii) the effects of the microscope objective used to focus the light on the sample in an epi-geometry configuration, and (iii) the symmetry of χ(2) for the β-BBO nanocrystals. A signal at the third-harmonic frequency was also detected, but it was too weak to be studied in detail.

We systematically study the optimization of highly efficient terahertz (THz) generation in lithium niobate (LN) crystal pumped by 800 nm laser pulses with 30 fs pulse duration. At room temperature, we obtain a record optical-to-THz energy conversion efficiency of 0.43% by chirping the pump laser pulses. Our method provides a new technique for producing millijoule THz radiation in LN via optical rectification driven by joule-level Ti:sapphire laser systems, which deliver sub-50-fs pulse durations.

Quasi-parametric chirped-pulse amplification (QPCPA) can improve the signal amplification efficiency and stability by inhibiting the back-conversion, in which the idler absorption plays a critical role. This Letter theoretically studies the impacts of idler absorption on the QPCPA performance in both the small-signal and saturation regimes. We demonstrate that there exists an optimal idler absorption that enables the achievement of maximum pump depletion within a minimum crystal length. To overcome the reduction in small-signal gain induced by idler absorption, the configuration of gradient idler absorption is proposed and demonstrated as a superior alternative to constant idler absorption. The results provide guidelines to the design of state-of-the-art QPCPA lasers.

We propose a high-gain and frequency-selective amplifier for a weak optical signal based on stimulated Brillouin scattering in a single mode fiber. To be able to satisfy the needs of high gain and high signal-to-noise ratio laser pulse amplification, different fiber lengths and core diameters are used to fulfill this experiment. In the experiment, a 430 nW (peak power) pulsed signal is amplified by 70 dB with a signal-to-noise ratio of 14 dB. The small size, high gain, low cost, and low noise of the fiber Brillouin amplifier make it a promising weak signal amplification method for practical applications such as lidar.

An injection-seeded single-resonant optical parametric oscillator (SROPO) with single frequency nanosecond pulsed 2.05 μm wavelength output is presented. Based on two potassium titanyl phosphate crystals and pumped by a 1064 nm single frequency laser pulse, injection seeding is performed successfully by using the ramp-hold-fire technique in a ring cavity with a bow-tie configuration. The SROPO provides 2.65 mJ single frequency signal pulse output with a 17.6 ns pulse duration at a 20 Hz repetition rate. A near-diffraction-limited beam is achieved with a beam quality factor M2 of about 1.2. The spectrum linewidth of the signal pulse is around 26.4 MHz, which is almost the Fourier-transform-limited value.

Developing natural “free space” frequency upconversion is essential for photonic integrated circuits. In a single-crystal lithium niobate thin film planar waveguide of less than 1 μm thickness, we achieve type I and type II mode phase-matching conditions simultaneously for this thin film planar waveguide. Finally, by employing the mode phase matching of e+e→e with d33 at 1018 nm, we successfully achieve a green second-harmonic wave output with the conversion efficiency of 0.12%/(W·cm2), which verifies one of our simulation results. The rich mode phase matching for three-wave mixing in a thin film planar waveguide may provide a potential application in on-chip frequency upconversions for integrated photonic and quantum devices.

The influence of laser temporal contrast on high-order harmonic generation from intense laser interactions with solid-density plasma surfaces is experimentally studied. A switchable plasma mirror system is set up to improve the contrast by two orders of magnitude at 10 ps prior to the main peak. By using the plasma mirror and tuning the prepulse, the dependence of high-order harmonic generation on laser contrast is investigated. Harmonics up to the 21st order via the mechanism of coherent wake emission are observed only when the targets are irradiated by high contrast laser pulses by applying the plasma mirror.

In optical studies on layered structures, quantitative analysis of radiating interfaces is often challenging due to multiple interferences. We present here a general and analytical method for computing the radiation from two-dimensional polarization sheets in multilayer structures of arbitrary compositions. It is based on the standard characteristic matrix formalism of thin films, and incorporates boundary conditions of interfacial polarization sheets. We use the method to evaluate the second harmonic generation from a nonlinear thin film, and the sum-frequency generation from a water/oxide interface, showing that the signal of interest can be strongly enhanced with optimal structural parameters.

We theoretically investigate the attosecond pulse generation in an orthogonal multicycle midinfrared two-color laser field. It is demonstrated that multiple continuum-like humps, which consist of about twenty orders of harmonics and an intensity of about one order higher than the adjacent normal harmonics, are generated when longer wavelength driving fields are used. By filtering these humps, intense isolated attosecond pulses (IAPs) are directly generated without any phase compensation. Our proposal provides a simple technique to generate intense IAPs with various central photon energies covering the multi-keV spectral regime by using multicycle midinfrared driving pulses with high pump energy in the experiment.

We observe the third-harmonic generation and second-harmonic generation together with element fluorescence from the interaction of a femtosecond laser filament with a rough surface sample (sandy soil) in non-phase-matched directions. The harmonics prove to originate from the phase-matched surface harmonics and air filament, then scatter in non-phase-matched directions due to the rough surface. These harmonics occurr when the sample is in the region before and after the laser filament, where the laser intensity is not high enough to excite the element fluorescence. The observed harmonics are related to the element spectroscopy, which will benefit the understanding of the interaction of the laser filament with a solid and be helpful for the application on filament induced breakdown spectroscopy.

We experimentally investigate the resonant and nonresonant second-harmonic generation in a single cadmium sulfide (CdS) nanowire. The second-order susceptibility tensor is determined by analyzing the forward second-harmonic signals of the CdS nanowire. Our results show that (1) d33/d31= 2.5 at a nonresonant input wavelength of 1050 nm; (2) d33/d31= 1.9 at a resonant wavelength of 740 nm. The difference can be attributed to the polarization-dependent resonance.

Five conical harmonic beams are generated from the interaction of femtosecond mid-infrared (mid-IR) pulses at a nominal input wavelength of 1997 nm with a 2D LiNbO3 nonlinear photonic crystal with Sierpinski fractal superlattices. The main diffraction orders and the corresponding reciprocal vectors involved in the interaction are ascertained. Second and third harmonics emerging at external angles of 23.82° and 36.75° result from nonlinear erenkov and Bragg diffractions, respectively. Three pathways of fourth-harmonic generation are observed at external angles of 14.21°, 36.5°, and 53.48°, with the first one resulting from nonlinear erenkov diffraction, and the other two harmonics are generated via different cascaded processes.

Back conversion is an intrinsic phenomenon in nonlinear frequency down-conversion processes. However, the physical reason for its occurrence is not well understood. Here, we theoretically reveal that back conversion is the result of a π-phase jump associated with the depletion of one interacting wave. By suppressing the idler phase jump through a deliberate crystal absorption, the back conversion can be inhabited, thus enhancing the conversion efficiency from the pump to the signal. The results presented in this Letter will further the understanding of nonlinear parametric processes and pave the way toward the design of highly efficient down-conversion systems.

We observe conical sum-frequency generation in a bulk anomalous-like dispersion medium, which is attributed to complete phase-matching of one fundamental wave and the scattering wave of the other fundamental wave. In addition, efficient sum-frequency output is achieved making use of total internal reflection with conversion efficiency of 7.9% by only one reflection. The experiment proposes a new phase-matching mode under an anomalous-like dispersion condition, which suggests potential applications in efficient frequency conversion.

We demonstrate a tunable optical parametric oscillator in a periodically poled congruently grown lithium tantalite whispering gallery mode resonator. The resonator is mechanically polished to millimeter size, and the quality factor is approximately 107 at 1064 nm. Our experiments show that this kind of resonator is capable of reaching a very low threshold and having a wide tuning range. Combined with its narrow resonant linewidth, it is potentially used as a compact, widely tunable, and narrow-linewidth infrared to mid-infrared laser source.

We obtain the output of a 284 ps pulse duration without tail modulation based on stimulated Brillouin scattering (SBS) pulse compression pumped by an 8 ns-pulse-duration, 1064 nm-wavelength Q-switched Nd:YAG laser. To suppress the tail modulation in SBS pulse compression, proper attenuators, which can control the pump energy within a rational range, are added in a generator-amplifier setup. The experimental result shows that the effective energy conversion efficiency triples when the pump energy reaches 700 mJ to 51%, compared with the conventional generator-amplifier setup.

The stochastic resonance based on optical bistability in the semiconductor optical amplifier is numerically investigated to extract a weak pulse signal buried in noise. The output property of optical bistability under different system parameters is analyzed, which determines the performance of the stochastic resonance. Through optimizing these parameters, the noise-hidden signal is extracted via stochastic resonance, in which the maximum cross-correlation gain higher than nine is obtained. This provides a novel technology for detecting a weak optical signal in various signal processing fields.

We report the measurements of Brillouin gain coefficients in FC-770, FC-40, FC-43, and FC-70 using a Brillouin oscillator and amplifier system. In contrast to the traditional way, the novel method provides direct measurements of these coefficients with the medium electro-strictive coefficient or with the phonon lifetime absent. Additionally, the Brillouin gain coefficient of FC-70 in this experiment is different from the theoretical work.

We experimentally demonstrate the application of MoSe2 thin film as a nonlinear medium and stabilizer to generate a multi-wavelength erbium-doped fiber laser. The cooperation of a photonic crystal fiber and a polarization-dependent isolator induces unstable multi-wavelength oscillations based on the nonlinear polarization rotation effect. A MoSe2 thin film is further incorporated into the cavity to achieve a stable multi-wavelength. The laser generates 7 lasings with a constant spacing of 0.47 nm at a pump power of 250 mW. The multi-wavelength erbium-doped fiber laser is stable with power fluctuations of less than 5 dB over 30 min.

Efficient second harmonic generation (SHG) in a nonlinear transparent conducting oxide (TCO) stripe waveguide that incorporates an organic polymer is theoretically investigated. The phase match condition between the fundamental photonic mode at the second harmonic and the fundamental long-range plasmonic mode at the fundamental frequency can be satisfied by dynamically or statically tuning the free carrier concentration of the TCO. The theoretically generated signal reaches its maximum up to 56.4 mW at a propagation distance of 34.8 μm for a pumping power of 1 W. The corresponding normalized conversion efficiency of the phase-matched SHG is up to 4.65×103 W 1 cm 2.

The electron and heavy hole energy levels of two vertically coupled InAs hemispherical quantum dots/wetting layers embedded in a GaAs barrier are calculated numerically. As the radius increases, the electronic energies increase for the small base radii and decrease for the larger ones. The energies decrease as the dot height increases. The intersubband and interband transitions of the system are also studied. For both, a spectral peak position shift to lower energies is seen due to the vertical coupling of dots. The interband transition energy decreases as the dot size increases, decreases for the dot shapes with larger heights, and reaches a minimum for coupled semisphere dots.

Temporal cavity solitons (CSs) have excellent properties that can sustain their shape in a temporal profile and with a broadband, smooth-frequency spectrum. We propose a method for controllable frequency line spacing soliton formation in a microresonator using two continuous-wave (CW) pumps with multi-free-spectral-range (FSR) spacing. The method we propose has better control over the amount and location of the solitons traveling in the cavity compared to the tuning pump method. We also find that by introducing a second pump with frequency N FSR from the first pump, solitons with N FSR comb spacing can be generated.

We report the transformation of a linear electro-optically tunable non-phase-matched second-order nonlinear process into a cascaded second-order nonlinear process in a bulk KTP crystal to generate the effect of electro-optically tunable Kerr-type nonlinearity. By applying an electric field on the x–y plane, parallel to the z-axis of the crystal, phase mismatch is created, which introduces a nonlinear phase shift between the launched and reconverted fundamental waves from the generated second harmonic wave. Due to the nonuniform radial intensity distribution of a Gaussian beam, a curvature will be introduced into the fundamental wavefront, which focuses or defocuses the incident beam while propagating through the crystal.

In this Letter, we report, for the first time to our knowledge, on a continuous-wave, singly resonant optical parametric oscillator using an MgO: PPLN crystal pumped by an all-fiberized master-oscillator power amplifier structured amplified random fiber laser. An idler output power of 2.46 W at 3752 nm is achieved with excellent beam quality, and the corresponding pump-to-idler conversion efficiency is 9.6% at room temperature. The idler output power exhibits a peak-to-peak power stability better than 12.7%, and the corresponding standard deviation is better than 3.6% RMS in about 20 min at the maximum output power. Meanwhile, other characteristics of the generated signal and idler laser are studied in detail and not only offered an effective guide in the research of optical parametric processes in the case of a continuous spectrum, but also broadened the range of random fiber laser applications.

A low-threshold Raman effect in a kilowatt ytterbium-doped narrowband fiber amplifier system is reported. The Raman Stokes light at 1120 nm is achieved with the total output power of only ～400 W, indicating that the Raman threshold of this kilowatt codirectional pumped continuous wave fiber amplifier is much lower than the predicted value estimated by the classic formula. To figure out the mechanism of this phenomenon, simulations based on the general stimulated Raman scattering (SRS) model are analyzed indicating that the key factor is the coupling between four-wave mixing (FWM) and SRS. The simulation results are in good agreement with our experiments.

Second-harmonic generation (SHG) from aperiodic optical superlattices in the regime of pump depletion is investigated, when the influence of typical fabrication errors, which can be introduced by the random fluctuation of the thickness for each domain in the simulation, is considered in accordance with the actual case. It is found that both the SHG conversion efficiencies calculated in the undepleted pump approximation (UPA) and an exact solution decrease when the fluctuation increases; however, the decreasing degree is related to the wavelength of the fundamental wave (FW), and the longer the FW wavelength, the less the decrease in the corresponding conversion efficiency. A relative tolerance with respect to SHG conversion efficiency calculated in the UPA and exact solution is defined as previously [Opt. Express21, 17592 (2013)], in which a typical model based on the relative tolerance curves was proposed to estimate the SHG conversion efficiency. The simulation results show that the relative tolerance curves are basically coincident with the standard curve when the random fluctuation is very small (typically below 1%); however, as the fluctuation increases, the relative tolerance curves exhibit a large deviation from the standard curve, and the deviation is also determined by the wavelength of the FW.

Based on the general mechanism of the coherent population oscillations, we propose the fundamental -harmonic fractional delay (FHFD) to evaluate the superluminal and slow light propagation in semiconductor optical amplifier (SOA). The sinusoidal and square-wave signals in SOA are investigated with the propagation equations. It is shown that the superluminal and slow light always accompany the signal distortion, and FHFD depends on the signal distortion as well as the incident power, the modulation frequency, and the optical gain.

We present a theory to investigate the existence and the propagation properties of incoherently coupled single-hump and dipole soliton pairs in self-defocusing media with parity-time symmetric lattice. These soliton pairs can exist provided that they are composed of two optical beams with the same polarization and wavelength. It is found that single-hump soliton pairs are always stable when the components copropagate in the lattice, whereas high-power dipole soliton pairs are unstable. If one of the components is absent, the propagation behavior of the other one is also studied.

In this Letter, the effects of the iron (Fe) dopant concentration on the nonlinear optical properties of iron-doped ferroelectric X-cut LiNbO3 crystals plates are studied by using the Z-scan technique with a cw laser at the wavelength of 532 nm. The amount of iron in the compound is varied from 0 to 0.15 mol%. Measurements of nonlinear refractive index n2 and the nonlinear absorption coefficient β are determined. The sign of the nonlinear refractive index is found to be negative and the magnitude is on the order of 10 8 cm2/W. This nonlinear effect increases as the concentration increases from 0 to 0.15 mol%. A good linear relationship is obtained between nonlinear refractive index, nonlinear absorption coefficient, and concentration.

The characteristics of stimulated Brillouin scattering (SBS) in perfluorinated amine media and the experimental structure used in hundreds of picoseconds pulse compression at 532 nm are demonstrated. A two-stage SBS pulse compression structure is adopted for this work. The compact double-cell SBS compression structure and the scattering media FC-70 are chosen to compress the incident light from 9.5 to about 1 ns in the first stage. Then, the light is used as the pumping source for the second pulse compression. In the second stage, using a single-cell SBS structure in a pulse compression system, perfluorinated amine media with different phonon lifetimes, such as FC-3283, FC-40, FC-43, and FC-70, are chosen to run the comparative experimental study. The narrowest compressed pulse times obtained are 294, 274, 277, and 194 ps; they respectively correspond to the above listed media. The average width of the compressed pulse width is 320 ps for FC-3283, with a fluctuation range of 87 ps. For FC-40, the average pulse width is 320 ps, with a fluctuation range of 72 ps. And for FC-43, the average pulse width is 335 ps, with a fluctuation range of 88 ps. However, the average pulse width is only 280 ps for FC-70, with a fluctuation range of 57 ps. The highest energy reflectivity is more than 80% for all of the media. The experimental results show that a two-stage SBS pulse compression system has lower pump energy requirements, thus making it easier to achieve a compressed pulse waveform. The results also show that the shorter the phonon lifetime of the medium, the narrower the obtained compressed pulse width.

We report on the rich dynamics of two-dimensional fundamental solitons coupled and interacting on the top of an elliptical shaped potential in a two-dimensional Ginzburg–Landau model. Under the elliptical shaped potential, the solitons display unique and dynamic properties, such as the generation of straight-line arrays, emission of either one elliptical shaped soliton or several elliptical ring soliton arrays, and soliton decay. When changing the depth and sharpness of the external potential and fixing other parameters of the potential, various scenarios of soliton dynamics are also revealed. These results suggest some possible applications for all-optical data-processing schemes, such as the routing of light signals in optical communication devices.

Pulsed collimated blue light at 420.3 nm is generated in hot Rb vapor by upconverting the 778.10 nm pumping beam through four wave mixing process. The energy conversion efficiency exceeds 1% when a 45 cm-long, 170°C heated Rb cell is used. The influence of cell temperature, wavelength, and energy of a pumping laser are fully examined. The efficiency of the photon conversion is found to be more sensitive to the blue detuning of the pump light and less sensitive to the red detuning of the pump light. This phenomenon can be explained by stimulated hyper-Raman scattering involved in the four-wave mixing process.

In this Letter, we investigate a method for controlling the intensity of a light by another light in a periodically poled MgO-doped lithium niobate (PPMgLN) crystal with a transverse applied external electric field. The power of the emergent light can be modulated by the power ratio of the incident ordinary and extraordinary beams. The light intensity control is experimentally demonstrated by the Mach–Zehnder interference configuration, and the results are in good agreement with the theoretical predictions.

The influence of group velocity dispersion (GVD) on the self-focusing of femtosecond laser pulses is investigated by numerically solving the extended nonlinear Schr?dinger equation. By introducing the GVD length LGVD into the semi-empirical, self-focusing formula proposed by Marburger, a revised one is proposed, which can not only well explain the influence of GVD on the collapse distance, but also is in good agreement with the numerical results, making the self-focusing formula applicable for more cases.

We experimentally demonstrate a femtosecond optical parametric oscillator (OPO) synchronously pumped by a home-made solid-state mode-locking Yb:YCOB laser, which is capable of laser pulse as short as 102 fs and average power of 620 mW at the central wavelength of 1052 nm. By using a periodically poled lithium niobate with tuning of the grating periods from 28.5 to 31.5 μm as the nonlinear gain crystal, tunable femtosecond pulses from 1444 to 1683 nm are realized by conveniently adjusting the OPO cavity length with 76.8 MHz repetition rate. The maximum average output power is 152 mW at 1568 nm, corresponding to an idler power of 75 mW at 3197 nm as well as 36.6% total extraction efficiency.

We investigate the ultrafast nonlinear phenomena of picosecond chirped non-ideal hyperbolic secant pulse evolution in silicon photonic nanowire waveguides with sum frequency generation cross-correlation frequency-resolved optical gating and nonlinear Schr?dinger equation modeling. Pulse broadening and spectral blue shifts are observed experimentally, and they show remarkable agreements with numerical predictions. Nonlinear losses dominate the pulse broadening and limit the spectral bandwidth broadening induced by self-phase modulation. The initial chirp results in noticeable bandwidth compression and aggravation of blue shifts in the presence of nonlinear losses, whereas it plays a negligible role in the output pulse temporal intensity distribution.

Nonlocality control is investigated in nonlinear media using material combination with self-focusing and self-defocusing media, and the controlling effects on the propagation and interaction of the spatial solitons are analyzed by numerical simulation. The propagation is stabilized, the interaction is suppressed by the proper material combination, and the effects of the control depend strictly on material maps.

We generate a flat temporal-phase distribution optical pulse by 1.3-mm-long photonic crystal waveguide. The effect of coupled pulse energy on the temporal-phase distribution of the output pulse is analyzed by numeral simulating. Simulation results indicate that the root mean square of the output pulse phase decreases to 0.0095 with the optimum coupled pulse energy, which is about 30 pJ, and the narrowest output pulse width is 418 fs. The generation of a flat temporal-phase distribution optical pulse on-chip scale results in potential application prospect in optical communication, pulse compression, pulse shaping and other nonlinear optical application fields.

We report on the existence and stability of defect solitons in two-dimensional optical Bessel potentials. It is found that for zero defect, defect solitons are stable in the entire existence domain. For negative defects, defect solitons are unstable in the moderate power region. It is worth emphasizing that for deep enough defects, another unstable domain will emerge in the high power region.

For linear electrooptic (EO) effect of femtosecond laser pulses that propagates along the direction of non-optical axis in linear-chirped and periodically poled MgO:LiNbO3 (LCPPLMN), we explore the compensation scheme for the phase mismatch and the group-velocity mismatch between the o- and e-light, and discuss the effect of the linear chirp parameter of the LCPPLMN on the waveform of the output o- and e-light. It is found that, for any input pulse duration, the phase mismatch and the group-velocity mismatch can be simultaneously compensated by the optimization of the linear chirp parameter. As a result, high conversion efficiency of linear EO effect can be performed. In addition, we discuss the influence of the linear chirp parameter on the temporal evolutions of the output o- and e-light pulse.

Dynamics of two-component vector dark solitons are investigated by the variational approach in the defocusing nonlinear media, and effects of the weak nonlocality on the soliton propagation and interaction are analyzed. The nonlocality degree determines the intensity distribution of the dark solitons in the stationary states, enhances the intensity transfer between two vector solitons, and affects the propagation and interaction. The numerical results confirm the theoretical findings.

A ring-cavity synchronously-pumped optical parametric oscillator (OPO) is investigated based on periodically poled KTi:OPO4 (PPKTP). The wavelength of the signal wave covers from 1 000 to 1 500 nm, the output power is 32.3 mW, and idler wave spectrum range from 1 800 to 2 500 nm is detected. By inserting a BBO or BIBO crystal respectively, a stable and adjustable range from 450 to 650 nm light is obtained. Three to six wavelengths can be output simultaneously.

We propose AND, NOR, and XNOR logic gates realized simultaneously for 40-Gb/s networks, in which the realization of NOR and XNOR logic gates using only MgO-doped periodically poled lithium niobate (MgO: PPLN) is reported. In our configuration, we exploit broadband quasi-phase matching (QPM) cascaded second harmonic and difference-frequency generation (cSHG/DFG), cascaded sum-frequency and difference-frequency generation (cSFG/DFG) in one MgO:PPLN, and the narrow band QPM sum-frequency generation (SFG) in another MgO:PPLN. The performance, including the quality-factor (Q-factor) and extinction ratio (ER), of the proposed multifunctional logic device is also simulated.

High quality z-cut LiNbO3 nonlinear photonic crystals with two-dimensional (2D) dodecagonal and fractal superlattices are successfully fabricated by applying the high voltage pulses. By collinear quasi-phase matching technique, second-harmonics at five wavelengths and ten wavelengths are observed simultaneously in one poled crystal, respectively. The same results can be obtained by rotating around the z axis by integrals of 30o in quasi-periodically poled crystal. In fractal nonlinear photonic crystal, the normalized conversion efficiency can be as high as 0.53%/mW for 499-nm second-harmonic laser spot.

Frequency doubled diode oscillator and fiber amplifier (DOFA) is frequency-stabilized to a hyperfine line of molecular iodine for calcium spectroscopy. The frequency doubling of DOFA is demonstrated using an Mg-doped periodically poled stoichiometric lithium tantalate crystal for generating a 544-nm beam. Saturated absorption spectroscopy is performed on the molecular iodine at 544 nm to find a hyperfine line for stabilizing the 272-nm beam to the calcium-48 transition. Stabilization of the 272-nm beam using the stabilization of the 544-nm beam to the corresponding 127I2 line is discussed.

The initial carrier-envelope phase dependence of dynamic process for an ultrashort laser pulse propagating non-resonantly in para-nitroaniline (pNA) molecule medium is investigated theoretically, by solving the full Maxwell-Bloch equations. The results show that when the laser pulse propagates with carrier frequency equal to the half of exciting frequency for the molecule’s charge-transfer state, the laser pulse is modulated severely and the population distribution exceeds 1/2 because the two-photon absorption is a resonant process for the interaction of the laser pulse and the dipolar molecule medium. Higher and lower frequencies than carrier frequency occur in the spectrum and even high-order harmonic components approaching to 7th harmonic are produced, forming a continuous spectrum. The sensitivities of the carrier wave reshaping, the high-order harmonic spectrum and the temporal evolution of excited state population to the initial carrier-envelope phase are discussed in detail. It is found that for given pulse width, the phase dependence increases as the laser field grows intense; while for given laser intensity, it decreases when the pulse width becomes narrow. Due to the extra nonlinear effects introduced by the permanent dipole moments of the pNA molecule, the phase sensitivity in this molecule medium is more distinct than in the medium composed of pseudo-molecules without permanent dipole moments.

An athermal design for Solc-type filter based on periodically poled KTP (PPKTP) is proposed. The athermal static phase retardation (ASPR) direction in KTP along which the quasi-phase-matched (QPM) condition is insensitive to the temperature is found. The optimal design for Solc-type filter along ASPR direction is obtained. The coupling theory of QPM linear electro-optic effect is employed to study the polarization coupling in PPKTP. The study results demonstrate that the central passing wavelength of Solc-type filter keep unchanged with an almost 100% output when the temperature varies from 268 to 328 K.

The influence of diffusive nonlinearity on mobility of photovoltaic lattice solitons is demonstrated. The dynamical evolution of collision between photovoltaic lattice solitons and nonlinear lattices are simulated numerically. The results show the lattice solitons with a transverse velocity have complicated behaviors and will not propagate with an oblique trajectory. When considering the diffusive nonlinearity, we find that diffusive nonlinearity can introduce a nonlinear chirped phase to lattice soliton and the lattice soliton with a special incident angle can become a "tilted soliton".

Using 300-fs 1039-nm Yb-doped fiber laser, we experimentally demonstrate blue light generation in a high-\Delta and high nonlinear photonic crystal fiber (PCF). The zero dispersion wavelength of PCF is 793 nm, detuning 245.8 nm from the pump wavelength. PCF allows a frequency conversion exceeding the octave of pump wavelength. The visible component of the measured output spectrum occurs in the fundamental mode and spans from 391.3 to 492.3 nm. The peak wavelength of 441.8 nm has a frequency detuning of 390 THz from the pump wavelength of 1039 nm.

We use a transient-grating (TG) process in a Kerr bulk medium to clean a femtosecond laser pulse. Using the technique, the temporal contrast of the generated TG signal is improved by more than two orders of magnitude in comparison with the incident pulse in a 0.5-mm-thick fused silica plate. The laser spectrum is smoothed and broadened, and the pulse duration is shortened simultaneously. We expect to extend this technique to a clean pulse with broadband spectral bandwidth at a wide spectral range because it is a phase-matched process.

We propose a novel terahertz-wave source through the four-wave mixing effect in a conventional single-mode optical fiber pumped by a dual-wavelength laser whose difference frequency lies in the terahertz range. Surface-emitted geometry is employed to decrease absorption loss. A detailed derivation of the terahertz-wave power expression is presented using the coupled-wave theory. This is a promising way for realizing a reasonable narrow-band terahertz-wave source.

Red frequency-upconversion fluorescence emission is observed in europium (III) complex with encapsulating polybenzimidazole tripodal ligands, pumped with 930- and 1070-nm picosecond laser pulses. The luminescence of transition 5D0->7F2 (612 nm) is induced by two-photon absorption of hypersensitive transitions 7F0->5D2 (465 nm) and 7F1->5D1 (535 nm). Analysis results suggest that the two-photon excitation strength of these hypersensitive transitions is increased dramatically owing to the C3 symmetry of the coordination field.

Based on the second-order nonlinearity, we present a bidirectional tunable all-optical switch at C-band by introducing backward quasi-phase-matching technique in Mg-doped periodically poled lithium niobate (MgO:PPLN) waveguide with a nano-structure called multiple resonators. Two injecting forward lights and one backward propagating light interact with difference frequency generations. The transmission of forward signal and backward idler light can be modulated simultaneously with the variation of control light power based on the basic “phase shift” structure of a single resonator. In this scheme, all the results come from our simulation. The speed of this bidirectional optical switch can reach to femtosecond if a femtosecond laser is used as the control light.

We theoretically propose a new method for generating intense isolated attosecond pulses during high-order harmonic generation (HHG) process by accurately controlling electron motion with a two-color laser field, which consists of an 800-nm, 4-fs elliptically polarized laser field and a 1400-nm, ~43-fs linearly polarized laser field. With this method, the supercontinua with a spectral width above 200 eV are obtained, which can support a ~15-as isolated pulse after phase compensation. Classical and quantum analyses explain the controlling effects well. In particular, when the pulse duration of the 800-nm laser field increases to 20-fs, sub-100-as isolated pulses can be obtained even without any phase compensation.

Optic parameters, such as the probabilities of radiative and non-radiative transition and the crossrelaxation probability between Yb3+ and Ho3+ ions in Yb,Ho:YAG crystal, are calculated on the basis of Judd-Ofelt and Dexter theories. The energy up-conversion process is analyzed by solving the transition rate-equations. The results show that (1) the intensity of the green fluorescence relates to the square of the concentration of the active ions; (2) the intensity increases with the concentration of sensitive ions as well, but the increasing rate goes rather too slow; (3) the efficiency of the energy up-conversion relates with the speed of the energy up-conversion and the quantum efficiency of the transiting from upper levelto lower level.