Silicon photonics (SiPh) has emerged as the predominant platform across a wide range of integrated photonics applications, encompassing not only mainstream fields such as optical communications and microwave signal processing but also burgeoning areas such as artificial intelligence and quantum processing. A vital component in most SiPh applications is the optical phase shifter, which is essential for varying the phase of light with minimal optical loss. Historically, SiPh phase shifters have primarily utilized the thermo-optic coefficient of silicon for their operation. Thermo-optic phase shifters (TOPSs) offer significant advantages, including excellent compatibility with complementary metal–oxide–semiconductor technology and the potential for negligible optical loss, making them highly scalable. However, the inherent heating mechanism of TOPSs renders them power-hungry and slow, which is a drawback for many applications. We thoroughly examine the principal configurations and optimization strategies that have been proposed for achieving energy-efficient and fast TOPSs. Furthermore, we compare TOPSs with other electro-optic mechanisms and technologies poised to revolutionize phase shifter development on the SiPh platform.

Optical-resolution photoacoustic microscopy (OR-PAM) has rapidly developed and is capable of characterizing optical absorption properties of biological tissue with high contrast and high resolution (micrometer-scale lateral resolution). However, the conventional excitation source of rapidly diverging Gaussian beam imposes limitations on the depth of focus (DOF) in OR-PAM, which in turn affects the depth-resolving ability and detection sensitivity. Here, we proposed a flexible DOF, depth-invariant resolution photoacoustic microscopy (FDIR-PAM) with nondiffraction of Airy beams. The spatial light modulator was incorporated into the optical pathway of the excitation source with matched switching phase patterns, achieving the flexibly adjustable modulation parameters of the Airy beam. We conducted experiments on phantoms and intravital tissue to validate the effectiveness of the proposed approach for high sensitivity and high-resolution characterization of variable topology of tissue, offering a promising DOF of 926 μm with an invariant lateral resolution of 3.2 μm, which is more than 17-fold larger compared to the Gaussian beam. In addition, FDIR-PAM successfully revealed clear individual zebrafish larvae and the pigment pattern of adult zebrafishes, as well as fine morphology of cerebral vasculature in a large depth range with high resolution, which has reached an evident resolving capability improvement of 62% mean value compared with the Gaussian beam.

Sensitive mid-infrared (MIR) detection is in high demand in various applications, ranging from remote sensing, infrared surveillance, and environmental monitoring to industrial inspection. Among others, upconversion infrared detectors have recently attracted increasing attention due to their advantageous features of high sensitivity, fast response, and room-temperature operation. However, it remains challenging to realize high-performance passive MIR sensing due to the stringent requirement of high-power continuous-wave pumping. Here, we propose and implement a high-efficiency and low-noise MIR upconversion detection system based on pumping enhancement via a low-loss optical cavity. Specifically, a single-longitudinal-mode pump at 1064 nm is significantly enhanced by a factor of 36, thus allowing for a peak conversion efficiency of up to 22% at an intracavity average power of 55 W. The corresponding noise equivalent power is achieved as low as 0.3 fW/Hz1/2, which indicates at least a 10-fold improvement over previous results. Notably, the involved single-frequency pumping would facilitate high-fidelity spectral mapping, which is particularly attractive for high-precision MIR upconversion spectroscopy in photon-starved scenarios.

The ultimate goal of artificial intelligence (AI) is to mimic the human brain to perform decision-making and control directly from high-dimensional sensory input. Diffractive optical networks (DONs) provide a promising solution for implementing AI with high speed and low power-consumption. Most reported DONs focus on tasks that do not involve environmental interaction, such as object recognition and image classification. By contrast, the networks capable of decision-making and control have not been developed. Here, we propose using deep reinforcement learning to implement DONs that imitate human-level decision-making and control capability. Such networks, which take advantage of a residual architecture, allow finding optimal control policies through interaction with the environment and can be readily implemented with existing optical devices. The superior performance is verified using three types of classic games: tic-tac-toe, Super Mario Bros., and Car Racing. Finally, we present an experimental demonstration of playing tic-tac-toe using the network based on a spatial light modulator. Our work represents a solid step forward in advancing DONs, which promises a fundamental shift from simple recognition or classification tasks to the high-level sensory capability of AI. It may find exciting applications in autonomous driving, intelligent robots, and intelligent manufacturing.

Fluorescent nanothermometers for remote temperature measurement at the micro/nanoscale have stimulated growing efforts in developing efficient temperature-responsive materials and detection procedures. However, the efficient collection and transmission of optical signals have been a tremendous challenge for practical applications of these nanothermometers. Herein, we design an all-fiberized thermometry based on a fiber-coupled microsphere cavity coated with thermo-sensitive NaYF4:20%Yb3+,2%Er3+@NaYF4 nanocrystals (NCs), allowing for spatial temperature sensing with resolution down to the few-micrometer scale. In our design, the microsphere efficiently excites the NCs and collects their upconversion emissions, and the use of a fiber splitter coupled with the microsphere allows for lossless routing of excitation and emitted light. We demonstrate the use of this all-fiber temperature sensor in diverse environments, especially in strongly acidic and alkaline conditions. Leveraging the high flexibility of commercial silica fiber, this all-fiber temperature sensor was employed for stable fixed-point real-time temperature measurement and multipurpose temperature recording/mapping in opaque environments, microscale areas, various solutions, and complicated bent structures. Thus, the demonstrated design could have strong implications for the practical use of nanothermometers in various possible scenarios, especially monitoring temperatures in diverse physiological settings.

The optoelectronic oscillator (OEO) is a typical time-delay system with rich nonlinear dynamical characteristics. Most of the previous research on OEOs has been focused on analyzing the properties of OEOs with a long time delay, which makes it difficult to realize mode locking without additional phase-locking mechanisms. We have achieved, for the first time to our knowledge, a self-mode-locking OEO and generated stable microwave frequency combs by analyzing the characteristics of OEOs with an ultrashort time scale. In the experiment, the self-mode-locking OEOs with fundamental mode, second-order harmonic, and sixth-order harmonic were realized by adjusting the system parameters, all of which produced uniform square wave signals with tunable duty cycles, steep rising and falling edges, and periods of less than 20 ns. The self-fundamental-mode-locking OEOs with different time delays were also implemented and experimentally realized. Furthermore, the experiment revealed the self-hybrid mode-locking OEO, which is the coexistence and synchronization of the three measured self-locking modes in one OEO cavity, demonstrating the complex nonlinear dynamical behaviors of the OEO system and enabling the generation of periodic nonuniform hybrid square wave signals. The realization of the self-mode-locking OEO and the generation of flexible and stable square wave signals at ultrashort time scales enrich the study of OEO nonlinear dynamics in the realm of complex microwave waveform generation, offering promising applications in areas such as atomic clocks, radars, communications, and optoelectronic neural networks.

Dual-comb ranging allows rapid and precise distance measurement and can be universally implemented on different comb platforms, e.g., fiber combs and microcombs. To date, dual-fiber-comb ranging has become a mature and powerful tool for metrology and industry, but the measurement speed is often at a kilohertz level due to the lower repetition rates. Recently, dual-microcomb ranging has given rise to a new opportunity for distance measurement, in consequence of its small footprint and high repetition rates, but full-comb stabilization is challenging. Here, we report a dual-hybrid-comb distance meter capable of ultrarapid and submicrometer precision distance measurement, which can not only leverage the advantage of easy locking inherited from the fiber comb but also sustain ultrarapid measurement speed due to the microcomb. The experimental results show that the measurement precision can reach 3.572 μm at 4.136 μs and 432 nm at 827.2 μs averaging time. Benefiting from the large difference between the repetition rates of the hybrid combs, the measurement speed can be enhanced by 196 folds, in contrast to the dual-fiber-comb system with about a 250 MHz repetition rate. Our work can offer a solution for the fields of rapid dimensional measurement and spectroscopy.

Achieving efficient and intense second-harmonic generation (SHG) in the terahertz (THz) spectrum holds great potential for a wide range of technical applications, including THz nonlinear functional devices, wireless communications, and data processing and storage. However, the current research on THz harmonic emission primarily focuses on inorganic materials, which often offers challenges in achieving both efficient and broadband SHG. Herein, the remarkable efficiency of organic materials in producing THz harmonics is studied and demonstrated, thereby opening up a new avenue for searching candidates for frequency-doubling devices in the THz band. By utilizing DAST, DSTMS, and OH1 crystals, we showcase their superior frequency conversion capabilities when pumped by the narrowband THz pulses centered at 2.4, 1.6, and 0.8 THz. The SHG spans a high-frequency THz domain of 4.8 THz, achieving an unprecedented conversion efficiency of ∼1.21% while maintaining a perturbative nonlinear response. The highly efficient SHG of these materials is theoretically analyzed by considering the combined effects of dispersion, phonon absorption, polarization, and the nonlinear susceptibility of organic crystals. This work presents a promising platform for efficient THz frequency conversion and generation across a wide range of frequencies, offering new opportunities for novel nonlinear THz applications in next-generation electronics and optics.