Photoacoustic microscopy (PAM) has quickly developed into a noninvasive biomedical imaging technique to achieve detection, diagnosis, and monitoring. Compared with Q-switched neodymium-doped yttrium aluminum garnet or optical parametric oscillator lasers, a low-cost and small-size laser diode (LD) used as an alternative light source is conducive to achieving the miniaturization and integration for preclinical transformation. However, the LD’s low peak output power needs the high numerical aperture objective to attain tight focus, which limits the working distance (WD) of the system in only 2–3 mm, resulting in not achieving the backward coaxial confocal mode. Here, we present a compact visible LD-based PAM system with a reflective objective to achieve a 22 mm long WD and a 10 µm lateral resolution. Different depth subcutaneous microvascular networks in label-free mouse ears have successfully reappeared in vivo with a signal-to-noise ratio up to 14 dB by a confocal alignment. It will be a promising tool to develop into a handy tool for subcutaneous blood vessel imaging.

We proposed a nonlinear photoacoustic (PA) technique as a new imaging contrast mechanism for tissue thermal-nonlinearity characterization. When a sine-modulated Gaussian temperature field is introduced by a laser beam, in view of the temperature dependence of the thermal diffusivity, the nonlinear PA effect occurs, which leads to the production of second-harmonic PA (SHPA) signals. By extracting the fundamental frequency PA and SHPA signal amplitudes of samples through the lock-in technique, a parameter that only reflects nonlinear thermal-diffusivity characteristics of the sample then can be obtained. The feasibility of the technique for thermal-nonlinearity characterization has been studied on phantom samples. In vitro biological tissues have been studied by this method to demonstrate its medical imaging capability, prefiguring great potential of this new method in medical imaging applications.

Aperture synthesis is an important approach to improve the lateral resolution of digital holography (DH) techniques. The limitation of the accuracy of registration positions between sub-holograms affects the quality of the synthesized image and even causes the failure of aperture synthesis. It is a major issue in aperture synthesis of DH. Currently intensity images are utilized to find the registration positions of sub-holograms in aperture synthesis. To improve the accuracy of registration positions, we proposed a method based on similarity calculations of the phase images between sub-holograms instead of intensity images. Furthermore, a quantitative indicator, degree of image distortion, was applied to evaluate the synthetic results. Experiments are performed and the results verify that the proposed phase-image-based method is better than the state-of-the-art intensity-image-based techniques in the estimation of registration positions and provides a better synthesized final three-dimensional shape image.

As a promising spectral window for optical communication and sensing, it is of great significance to realize on-chip devices at the 2 µm waveband. The development of the 2 µm silicon photonic platform mainly depends on the performance of passive devices. In this work, the passive devices were fabricated in the silicon photonic multi-project wafer process. The designed micro-ring resonator with a 0.6 µm wide silicon ridge waveguide based on a 220 nm silicon-on-insulator platform achieves a high intrinsic quality factor of 3.0×105. The propagation loss is calculated as 1.62 dB/cm. In addition, the waveguide crossing, multimode interferometer, and Mach–Zehnder interferometer were demonstrated at 2 µm with good performances.

In this paper, we demonstrated a series of short-living mode-locking (ML) states (each lasting a few to a hundred microseconds) that happened before a fiber laser reached a steady ML state. With time-stretched dispersion Fourier transform spectroscopy, a rich diversity of transient multi-pulse dynamics were revealed spectrally and temporally. As a result, we found that the formation of the short-living ML states was related to abundant pump power, and their decaying evolution dynamics were possibly governed by gain depletion and recovery. Our results revealed unexpected transient lasing behaviors of a soliton laser and thus might be useful to understand the complex dynamics of mode-locked lasers.

End-pumped by a 976 nm diode laser, a high-repetition-rate Er:Yb:YAl3(BO3)4 microchip laser passively Q-switched by a Co2+:MgAl2O4 crystal is reported. At a quasi-continuous-wave pump power of 20 W, a 1553 nm passively Q-switched laser with the repetition rate of 544 kHz, pulse duration of 8.3 ns, and pulse energy of 3.9 μJ was obtained. To the best of our knowledge, the 544 kHz is the highest reported value for the 1.5 μm passively Q-switched pulse laser. In the continuous-wave pumping experiment, the maximum repetition rate of 144 kHz with the pulse duration of 8.0 ns and pulse energy of 1.7 μJ was obtained at the incident pump power of 6.3 W.

In modern optics, particular interest is devoted to the phase singularities that yield complicated and twisted phase structures by photons carrying optical angular momentum. In this paper, the traditional M-line method is applied to a vortex beam (VB) by a symmetric metal cladding waveguide chip, which can host numerous oscillating guided modes via free space coupling. These ultrahigh-order modes (UOMs) result in high angular resolution due to the high finesse of the resonant chip. Experiments show that the reflected pattern of a VB can be divided into a series of inner and outer rings, whilst both of them are highly distorted by the M-lines due to the UOMs’ leakage. Taking the distribution of the energy flux into account, a simple ray-optics-based model is proposed to simulate the reflected pattern by calculating the local incident angle over the cross section of the beam. The theoretical simulations fit well with the experimental results, and the proposed scheme may enable new applications in imaging and sensing of complicated phase structures.

We demonstrate a three-nanosecond equidistant sub-pulse multi-step Q-switched Nd:Y3Al5O12 (Nd:YAG) laser. In the time interval of 100–1000 ns, three pulses with the same nanosecond interval and the same peak power are obtained at the pulse width of 24 ns, 28 ns, and 36.6 ns, respectively. The energy is 32.5 mJ, and the optical efficiency is 10.8%. The multi-step Q-switched method does not require the insertion of other optical elements into the traditional Q-switched laser, and it is very suitable to obtain pulse group output with several nanosecond pulse intervals.

We propose and demonstrate the cascaded multi-wavelength mode-locked erbium-doped fiber laser (EDFL) based on ultra-long-period gratings (ULPGs) for the first time, to the best of our knowledge. Study found that the ULPG can be used as both a mode-locker for pulse shaping and a comb filter for multi-wavelength generation simultaneously. Using the dual-function of ULPG, three-, four-, five-, six-, and seven-wavelength mode-locked pulses are obtained in EDFL, seven of which are the largest number of wavelengths up to now. For the four-wavelength soliton pulses, their pulse width is about 7.8 ps. The maximum average output power and slope efficiency of these pulses are 8.4 mW and 2.03%, respectively. Besides the conventional pulses, hybrid soliton pulses composed of a four-wavelength pulse and single soliton are also observed. Finally, the effect of cavity dispersion on the multi-wavelength mode-locked pulses is also discussed. Our findings indicate that apart from common sensing and filtering, the ULPG may also possess attractive nonlinear pulse-shaping property for ultrafast photonics application.

We show that inhomogeneous waveguides of slowly varied parity-time (PT) symmetry support localized optical resonances. The resonance is closely related to the formation of exceptional points separating exact and broken PT phases. Salient features of this kind of non-Hermitian resonance, including the formation of half-vortex flux and the discrete nature, are discussed. This investigation highlights the unprecedented uniqueness of field dynamics in non-Hermitian systems with many potential adaptive applications.

In this paper, we experimentally demonstrate ultrafast optical control of slow light in the terahertz (THz) range by combining the electromagnetically induced transparency (EIT) metasurfaces with the cut wire made of P+-implanted silicon with short carrier lifetime. Employing the optical-pump THz-probe spectroscopy, we observed that the device transited from a state with a slow light effect to a state without a slow light effect in an ultrafast time of 5 ps and recovered within 200 ps. A coupled oscillator model is utilized to explain the origin of controllability. The experimental results agree very well with the simulated and theoretical results. These EIT metasurfaces have the potential to be used as an ultrafast THz optical delay device.

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 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.

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.

A fiber-based source that can be exploited in a stimulated emission depletion (STED) inspired nanolithography setup is presented. Such a source maintains the excitation beam pulse, generates a ring-shaped depletion beam, and automatically realizes dual-beam coaxial alignment that is critical for two beam nanolithography. The mode conversion of the depletion beam is realized by using a customized vortex fiber, which converts the Gaussian beam into a donut-shaped azimuthally polarized beam. The pulse width and repetition frequency of the excitation beam remain unchanged, and its polarization states can be controlled. According to the simulated point spread function of each beam in the focal region, the full width at half-maximum of the effective spot size in STED nanofabrication could decrease to less than 28.6 nm.

Heterodyne detectors as phase-insensitive (PI) devices have found important applications in precision measurements such as space-based gravitational-wave (GW) observation. However, the output signal of a PI heterodyne detector is supposed to suffer from signal-to-noise ratio (SNR) degradation due to image band vacuum and imperfect quantum efficiency. Here, we show that the SNR degradation can be overcome when the image band vacuum is quantum correlated with the input signal. We calculate the noise figure of the detector and prove the feasibility of heterodyne detection with enhanced noise performance through quantum correlation. This work should be of great interest to ongoing space-borne GW signal searching experiments.

The spiderlike structures in the photoelectron momentum distributions of ionized electrons from the hydrogen atom are numerically simulated by using a semiclassical rescattering model (SRM) and solving the time-dependent Schr?dinger equation (TDSE), focusing on the role of the phase of the scattering amplitude. With the SRM, we find that the spiderlike legs shift to positions with smaller transverse momentum values while increasing the phase. The spiderlike patterns obtained by SRM and TDSE are in good agreement upon considering this phase. In addition, the time differences in electron ionization and rescattering calculated by SRM and the saddle-point equations are either in agreement or show very similar laws of variation, which further corroborates the significance of the phase of the scattering amplitude.

Carbon fiber (CF)/pyrolytic graphite (PG) composites are promising structural materials for molten salt reactors because of their superior performance. Due to the minor density difference between CF and PG, existing methods are impractical for efficient three-dimensional characterization of CF/PG composites. Therefore, in this study, a method based on in-line phase-contrast X-ray microtomography was developed to solve the aforementioned problem. Experimental results demonstrate that the method is suitable for comprehensive characterization of CF/PG composites. The relationship between the microporous defects and fiber orientations of such composites was also elucidated. The findings can be useful for improving the manufacturing process of CF/PG composites.

The pure-silica hollow-core fiber (HCF) has excellent thermostabilities that can benefit a lot of high-temperature sensing applications. The air-core microstructure of the HCF provides an inherent gas container, which can be a good candidate for gas or gas pressure sensing. This paper reviews our continuous efforts to design, fabricate, and characterize the high-temperature and high-pressure sensors with HCFs, aiming at improving the sensing performances such as dynamic range, sensitivity, and linearity. With the breakthrough advances in novel anti-resonant HCFs, sensing of high temperature and high pressure with HCFs will continuously progress and find increasing applications.

Radio frequency (RF) self-interference is a key issue for the application of in-band full-duplex communication in beyond fifth generation and sixth generation communications. Compared with electronic technology, photonic technology has the advantages of wide bandwidth and high tuning precision, exhibiting great potential to realize high interference cancellation depth over broad band. In this paper, a comprehensive overview of photonic enabled RF self-interference cancellation (SIC) is presented. The operation principle of photonic RF SIC is introduced, and the advances in implementing photonic RF SIC according to the realization mechanism of phase reversal are summarized. For further realistic applications, the multipath RF SIC and the integrated photonic RF SIC are also surveyed. Finally, the challenges and opportunities of photonic RF SIC technology are discussed.