Photons, the individual quanta of the light field, are what the science of quantum photonics is dedicatedly investigating. The manipulation and coherent control of photons in quantum photonics enables the exploration of various quantum phenomena of high fundamental interest. In the meantime, due to the fast speed and a lack of the interaction with the environment, photons are now regarded as a promising platform for the emerging quantum information processing (QIP) studies. Therefore, there is a growing number of works on quantum computing, quantum communication, and quantum metrology that are solidly based on the techniques of quantum photonics.

The semiconductor UV photonics research has emerged as one of the most heavily invested areas among semiconductor photonics research due to numerous crucial applications such as sterilization, sensing, curing, and communication. The feature issue disseminates nine timely original research and two review papers from leading research groups and companies, covering most frontiers of the semiconductor UV photonics research, from epitaxy, device physics and design, nanostructures, fabrication, packaging, reliability, and application for light-emitting diodes, laser diodes, and photodetectors.

We demonstrate high-resolution and high-quality terahertz (THz) in-line digital holography based on the synthetic aperture method. The setup is built on a self-developed THz quantum cascade laser, and a lateral resolution better than 70 μm (～λ) is achieved at 4.3 THz. To correct intensity differences between sub-holograms before aperture stitching, a practical algorithm with global optimization is proposed. To address the twin-image problem for in-line holography, a sparsity-based phase retrieval algorithm is applied to perform the high-quality reconstruction. Furthermore, a new autofocusing criterion termed “reconstruction objective function” is introduced to obtain the best in-focus reconstruction distance, so the autofocusing procedure and the reconstruction are unified within the same framework. Both simulation and experiment prove its accuracy and robustness. Note that all the methods proposed here can be applied to other wavebands as well. We demonstrate the success of this THz synthetic aperture in-line holography on biological and semiconductor samples, showing its potential applications in bioimaging and materials analysis.

The scattering medium is usually thought to have a negative effect on the imaging process. In this paper, it is shown that the imaging quality of reflective ghost imaging (GI) in the scattering medium can be improved effectively when the binary method is used. By the experimental and the numerical results, it is proved that the existence of the scattering medium is just the cause of this phenomenon, i.e., the scattering medium has a positive effect on the imaging quality of reflective GI. During this process, the effect from the scattering medium behaves as the random noise which makes the imaging quality of binary ghost imaging have an obvious improvement.

We report a frequency-multiplexing method for multi-beam photon-counting light detection and ranging (LiDAR), where only one single-pixel single-photon detector is employed to simultaneously detect the multi-beam echoes. In this frequency-multiplexing multi-beam LiDAR, each beam is from an independent laser source with different repetition rates and independent phases. As a result, the photon counts from different beams could be discriminated from each other due to the strong correlation between the laser pulses and their respective echo photons. A 16-beam LiDAR system was demonstrated in three-dimensional laser imaging with 16 pulsed laser diodes at 850 nm and one single-photon detector based on a Si-avalanche photodiode. This frequency-multiplexing method can greatly reduce the number of single-photon detectors in multi-beam LiDAR systems, which may be useful for low-cost and eye-safe LiDAR applications.

The quantum multimode of correlated fields is essential for future quantum-correlated imaging. Here we investigate multimode properties theoretically and experimentally for the parametric amplified multiwave mixing process. The multimode behavior of the signals in our system stems from spatial phase mismatching caused by frequency resonant linewidth. In the spatial domain, we observe the emission rings with an uneven distribution of photon intensity in the parametric amplified four-wave mixing process, suggesting different spatial modes. The symmetrical distribution of spatial spots indicates the spatial correlation between the Stokes and anti-Stokes signals. While in the frequency domain, the multimode character is reflected as multiple peaks splitting in the signals’ spectrum. A novelty in our experiment, the number of multimodes both in the spatial and frequency domains can be controlled by dressing lasers by modifying the nonlinear susceptibility. Finally, we extend the multimode properties to the multiwave mixing process. The results can be applied in quantum imaging.

This work reports the real-time observation of the interlayer lattice vibrations in bilayer and few-layer PtSe2 by means of the coherent phonon method. The layer-breathing mode and standing wave mode of the interlayer vibrations are found to coexist in such a kind of group-10 transition metal dichalcogenides (TMDCs). The interlayer breathing force constant standing for perpendicular coupling (per effective atom) is derived as 7.5 N/m, 2.5 times larger than that of graphene. The interlayer shearing force constant is comparable to the interlayer breathing force constant, which indicates that PtSe2 has nearly isotropic interlayer coupling. The low-frequency Raman spectroscopy elucidates the polarization behavior of the layer-breathing mode that is assigned to have A1g symmetry. The standing wave mode shows redshift with the increasing number of layers, which successfully determines the out-of-plane sound velocity of PtSe2 experimentally. Our results manifest that the coherent phonon method is a good tool to uncover the interlayer lattice vibrations, beyond the conventional Raman spectroscopy limit. The strong interlayer interaction in group-10 TMDCs reveals their promising potential in high-frequency (～terahertz) micro-mechanical resonators.

The prominent third-order nonlinear optical properties of WTe2 films are studied through the Z-scan technique using a femtosecond pulsed laser at 1030 nm. Open-aperture (OA) and closed-aperture (CA) Z-scan measurements are performed at different intensities to investigate the nonlinear absorption and refraction properties of WTe2 films. OA Z-scan results show that WTe2 films always hold a saturable absorption characteristic without transition to reverse saturable absorption. Further, a large nonlinear absorption coefficient β is determined to be 3.37×103 cm/GW by fitting the OA Z-scan curve at the peak intensity of 15.603 GW/cm2. In addition, through the slow saturation absorption model, the ground state absorption cross section, excited state absorption cross section, and absorber’s density were found to be 1.4938×10 16 cm2, 1.2536×10 16 cm2, and 6.2396×1020 cm 3, respectively. CA Z-scan results exhibit a classic peak–valley shape of the CA Z-scan signal, which reveals a self-defocusing optical effect of WTe2 films under the measured environment. Furthermore, a considerable nonlinear refractive index value n2 can be obtained at 1.629×10 2 cm2/GW. Ultimately, the values of the real and imaginary parts of the third-order nonlinear s

Semiconductor heterostructures based on layered two-dimensional transition metal dichalcogenides (TMDs) interfaced to gallium nitride (GaN) are excellent material systems to realize broadband light absorbers and emitters due to their close proximity in the lattice constants. The surface properties of a polar semiconductor such as GaN are dominated by interface phonons, and thus the optical properties of the vertical heterostructure are influenced by the coupling of these carriers with phonons. The activation of different Raman modes in the heterostructure caused by the coupling between interfacial phonons and optically generated carriers in a monolayer MoS2–GaN (0001) heterostructure is observed. Different excitonic states in MoS2 are close to the interband energy state of intraband defect state of GaN. Density functional theory (DFT) calculations are performed to determine the band alignment of the interface and revealed a type-I heterostructure. The close proximity of the energy levels and the excitonic states in the semiconductors and the coupling of the electronic states with phonons result in the modification of carrier relaxation rates. Modulation of the excitonic absorption states in MoS2 is measured by transient optical pump-probe spectroscopy and the change in emission properties of both semiconductors is measured by steady-state photoluminescence (PL) emission spectroscopy. There is significant red-shift of the C excitonic band and faster dephasing of carriers in MoS2. However, optical excitation at energy higher than the bandgap of both semiconductors slows down the dephasing of carriers and energy exchange at the interface. Enhanced and blue-shifted PL emission is observed in MoS2. GaN band-edge emission is reduced in intensity at room temperature due to increased phonon-induced scattering of carriers in the GaN layer. Our results demonstrate the relevance of interface coupling between the semiconductors for the development of optical and electronic applications.

Polarizers have been widely used in various optical systems to reduce polarization cross talk. The polarizers based on the silicon nanowire waveguide can provide chip-scale device size and a high polarization extinction ratio. However, the working bandwidth for the on-chip silicon polarizers is always limited (～100 nm) by the strong waveguide dispersion. In this paper, an on-chip all-silicon polarizer with an extremely broad working bandwidth is proposed and demonstrated. The device is based on a 180° sharp waveguide bend, assisted with anisotropic subwavelength grating (SWG) metamaterial cladding to enhance the polarization selectivity. For TE polarization, the effective refractive index for SWG is extraordinary, so the incident TE mode can propagate through the sharp waveguide bend. For TM polarization, the effective refractive index for SWG is ordinary, so the incident TM mode will be coupled into the radiation mode regardless of the wavelength. The fabricated polarizer shows low loss (1 dB) and high polarization extinction ratio (>20 dB) over a >415 nm bandwidth from 1.26 to 1.675 μm, which is at least fourfold better than what has been demonstrated in all previous works. To the best of our knowledge, such a device is the first all-silicon polarizer that covers O-, E-, S-, C-, L-, and U-bands.

A depletion layer played by aqueous organic liquids flowing in a platform of microfluidic integrated metamaterials is experimentally used to actively modulate terahertz (THz) waves. The polar configuration of water molecules in a depletion layer gives rise to a damping of THz waves. The parallel coupling of the damping effect induced by a depletion layer with the resonant response by metamaterials leads to an excellent modulation depth approaching 90% in intensity and a great difference over 210° in phase shift. Also, a tunability of slow-light effect is displayed. Joint time-frequency analysis performed by the continuous wavelet transforms reveals the consumed energy with varying water content, indicating a smaller moment of inertia related to a shortened relaxation time of the depletion layer. This work, as part of THz aqueous photonics, diametrically highlights the availability of water in THz devices, paving an alternative way of studying THz wave–liquid interactions and developing active THz photonics.

Surface-enhanced Raman spectroscopy (SERS) with high-sensitivity performance is a very necessary detection technology. Here, we present a method for increasing the performance of SERS based on silver triangular nanoprism arrays (ATNAs) vertically excited via a focused azimuthal vector beam (AVB). The ATNA substrates with different structural parameters are prepared based on the traditional self-assembled and modified film lift-off technique. Based on a theoretical model established adopting the structural parameters of the ATNA substrates, theoretical simulation results show that AVB excitation can achieve greater electric-field enhancement than linearly polarized beam (LPB) excitation. Experimental result indicates that SERS sensitivity obtained via AVB excitation is 10 13 M (1 M = 1 mol/L) using rhodamine 6G (R6G) as the target analyte, which is 2 orders of magnitude lower than that of LPB excitation (10 11 M). Meanwhile, the uniformity and reproducibility of the ATNA substrates are examined using Raman mapping and batch-to-batch measurement, respectively, and the Raman enhancement factor is calculated to be ～3.3×107. This method of vector light field excitation may be used to improve the SERS performance of the substrates in fields of ultra-sensitive Raman detection.

Metasurfaces have pioneered a new avenue for advanced wave-front engineering. Among the various types of metasurfaces, Huygens’ metasurfaces are thought to be a novel paradigm for flat optical devices. Enabled by spectrally overlapped electric resonance and magnetic resonance, Huygens’ metasurfaces are imparted with high transmission and full phase coverage of 2π, which makes them capable of realizing high-efficiency wave-front control. However, a defect of Huygens’ metasurfaces is that their phase profiles and transmissive responses are often sensitive to the interaction of neighboring Huygens’ elements. Consequently, the original assigned phase distribution can be distorted. In this work, we present our design strategy of transmissive Huygens’ metasurfaces performing anomalous refraction. We illustrate the investigation of Huygens’ elements, realizing the overlapping between an electric dipole and magnetic dipole resonance based on cross-shaped structures. We find that the traditional discrete equidistant-phase design method is not enough to realize a transmissive Huygens’ surface due to the interaction between neighboring Huygens’ elements. Therefore, we introduce an extra optimization process on the element spacing to palliate the phase distortion resulting from the element interaction. Based on this method, we successfully design unequally spaced three-element transmissive metasurfaces exhibiting anomalous refraction effect. The anomalous refractive angle of the designed Huygens’ metasurface is 30°, which exceeds the angles of most present transmissive Huygens’ metasurfaces. A transmissive efficiency of 83.5% is numerically derived at the operating wavelength. The far-field electric distribution shows that about 93% of transmissive light is directed along the 30° refractive direction. The deflection angle can be tuned by adjusting the number of Huygens’ elements in one metasurface unit cell. The design strategies used in this paper can be inspiring for other functional Huygens’ metasurface schemes.

We propose a simple and efficient method to optimize two-color chirped laser pulses by forming a “temporal gate” for the generation of isolated attosecond pulses (IAPs) in soft X rays. We show that the generation process for higher and cutoff harmonics can be effectively limited within the temporal gate, and the harmonic emission interval can be further reduced with the help of phase-matching by only selecting the contribution from short-trajectory electrons. This two-color gating mechanism is verified by increasing the pulse duration, raising the gas pressure, and extending the target cutoff. Compared to the five-color waveform in Phys. Rev. Lett.102, 063003 (2009)PRLTAO0031-900710.1103/PhysRevLett.102.063003, our waveform can be used to generate the IAP in the long-duration laser pulse while the cutoff energy is higher without the reduction of harmonic yields. Our work provides an alternative temporal gating scheme for the generation of IAPs by simultaneously improving the harmonic conversion efficiency, thus making the attosecond soft X rays an intense and highly time-resolved tabletop light source for future applications.

A method is presented for one-to-many information encryption transmission by using temporal ghost imaging and code division multiple access. In the encryption transmission process, code division multiple access technologies combine multiple information sources, and the chip sequence corresponding to each set of information is used as the first key. The transmission end loads the transmission information onto a series of temporal random patterns of temporal ghost imaging and transmits it to the receivers. A series of temporal random patterns is the second key. During the decryption, each receiver can get the same encrypted information and use the second key to obtain the transmitted information. Finally, each receiver uses the unique chip sequence to get corresponding information. This encryption transmission method realizes one-to-many information encryption transmission at the same time over the same channel. Double encryption ensures the security of information. Simulation and experiment results verify the effectiveness and security of the method. The method has strong antinoise ability and can effectively resist various attack modes. At the same time, this method solves the problem that the use of code division multiple access enlarges the signal bandwidth, and ensures that no cross talk occurs between various sources of information.

For space missions, there is a need for fiber lasers of minimum power consumption involving stabilized frequency combs. We exploit the extreme sensitivity of the polarization state of circularly polarized light sent through polarization-maintaining (PM) fibers to power and temperature variations. Low-power nonlinear transmission is demonstrated by terminating a PM fiber by an appropriately oriented polarizer. The strong correlation between the power sensitivity of the polarization state and the temperature dependence of the birefringence of the PM fiber can be exploited for optical length stabilization in fiber lasers and interferometers.

In this paper, an integrated compact four-channel directly modulated analog optical transceiver is proposed and fabricated. The 3 dB bandwidth of this optical transceiver exceeds 20 GHz, and the measured spurious-free dynamic range is up to 91.2 dB·Hz2/3. The optical coupling efficiency (CE) is improved by using a precise submicron alignment technique for lens coupling in a transmitter optical subassembly, and the highest CE is achieved when the oblique angle of the arrayed waveguide grating using a silica-based planar lightwave circuit (AWG-PLC) in receiver optical sub assembly is set to 42°. Based on the proposed optical transceiver, we have experimentally demonstrated a 6.624 Gbit/s 4×4 multi-input multioutput (MIMO) 16-quadrature amplitude modulation orthogonal frequency division multiplexing (16QAM-OFDM) radio signal over 15.5 km standard single mode fiber, together with 1.2 m wireless transmission in both an uplink and a downlink. To cope with the channel interference and noise of the fiber-wireless transmission system, a low-complexity MIMO demodulation algorithm based on lattice reduction zero-forcing (LR-ZF) is designed. In our experiment, 1.6 dB power penalty is achieved by using the proposed LR-ZF algorithm, compared to the commonly used zero-forcing algorithm. Moreover, this LR-ZF algorithm has much less complexity than the optimal maximum-likelihood sequence estimation (MLSE) at a given transmission performance. These results not only demonstrate the feasibility of the integrated optical transceiver for MIMO fiber-wireless application but also validate that the proposed LR-ZF algorithm is effective to eliminate the interference for hybrid fiber-wireless transmission.

We demonstrate a passively mode-locked all-fiber laser generating cylindrical vector beams (CVBs) only using a symmetric two-mode fiber optical coupler (TMF-OC) for both high-order mode excitation and splitting. Theoretical analyses show that for a symmetric TMF-OC with appropriate taper diameter, the second-order mode can be excited and coupled into output tap with high purity due to the effective index difference of different modes. Based on the fabricated TMF-OC, the passively mode-locked fiber laser delivers pulsed CVBs at a center wavelength of 1564.4 nm with 3 dB linewidth of 11.2 nm, pulse duration of 2.552 ps, and repetition rate of 3.96 MHz. The purity of both radially and azimuthally polarized beams is estimated to be over 91%. Due to simple fabrication method of the TMF-OC and high purity of the generated CVBs, this mode-locked CVB fiber laser with all-fiber configuration has potential applications in optical trapping, optical communications, material processing, etc.

High-efficiency superconducting nanowire single-photon detectors (SNSPDs), which have numerous applications in quantum information systems, function by using the optical cavity and the ultrasensitive photon response of their ultra-thin superconducting nanowires. However, the wideband response of superconducting nanowires is limited due to the resonance of the traditional optical cavity. Here, we report on a supercontinuum SNSPD that can efficiently detect single photons over an ultra-broad spectral range from visible to mid-infrared light. Our detection approach relies on using multiple cavities with well-separated absorbed resonances formed by fabricating multilayer superconducting nanowires on metallic mirrors with silica acting as spacer layers. Thus, we are able to extend the absorption spectral bandwidth while maintaining considerable efficiency, as opposed to a conventional single-layer SNSPD. Our calculations show that the proposed supercontinuum SNSPD exhibits an extended absorption bandwidth with increased nanowire layers. Its absorption efficiency is greater than 70% over the entire range from 400 to 2500 nm (or 400 to 3000 nm), when using two-layer (or three-layer) nanowires. As a proof of principle, the SNSPD with bilayer nanowires is fabricated based on the proposed detector architecture with simplified geometrical parameters. The detector achieves broadband detection efficiency over 60% from 950 to 1650 nm. This type of detector may replace multiple narrow band detectors in a system and find uses in the emerging and rapidly advancing field of atomic and molecular broadband spectroscopy.

Based on the dispersive interaction between a high quality factor microcavity and nano-objects, whispering-gallery-mode microcavities have been widely used in highly sensitive sensing. Here, we propose a novel method to enhance the sensitivity of the optical frequency shift and reduce the impact of the laser frequency noise on the detection resolution through Brillouin cavity optomechanics in a parity-time symmetric system. The optical spring effect is sensitive to the perturbation of optical modes around the exceptional point. By monitoring the shift of the mechanical frequency, the detection sensitivity for the optical frequency shift is enhanced by 2 orders of magnitude compared with conventional approaches. We find the optical spring effect is robust to the laser frequency noise around the exceptional point, which can reduce the detection limitation caused by the laser frequency instability. Thus, our method can improve the sensing ability for nano-object sensing and other techniques based on the frequency shift of the optical mode.

A large number of different types of second-order non-Hermitian degeneracies called exceptional points (EPs) were found in various physical systems depending on the mechanism of coupling between eigenstates. We show that these EPs can be hybridized to form higher-order EPs, which preserve the original properties of the initial EPs before hybridization. For a demonstration, we hybridize chiral and supermode second-order EPs, where the former and the latter are the results of intra-disk and inter-disk mode coupling in an optical system comprised of two Mie-scale microdisks and one Rayleigh-scale scatterer. The high sensitivity of the resulting third-order EP against external perturbations in our feasible system is emphasized.

A coherence vortex (CV) carrying topological-charge information in its correlation dimension is a new option for optical manipulation and communication. CV generation by directly modulating the correlation function enables a way to control the light field in this dimension. However, few experimental realizations on this issue have been reported because of the difficulty in phase modulation when the light arrays are of low coherence. In this paper, we propose a method for generating a CV by utilizing partially coherent light arrays. A proper design of random arrays at the input plane leads to a complex CV field at the output plane after free-space propagation. This generation mechanism works well for beamlets of low coherence.