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.

Secure and high-speed optical communications are of primary focus in information transmission. Although it is widely accepted that chaotic secure communication can provide superior physical layer security, it is challenging to meet the demand for high-speed increasing communication rate. We theoretically propose and experimentally demonstrate a conceptual paradigm for orbital angular momentum (OAM) configured chaotic laser (OAM-CCL) that allows access to high-security and massive-capacity optical communications. Combining 11 OAM modes and an all-optical feedback chaotic laser, we are able to theoretically empower a well-defined optical communication system with a total transmission capacity of 100 Gb/s and a bit error rate below the forward error correction threshold 3.8×10−3. Furthermore, the OAM-CCL-based communication system is robust to 3D misalignment by resorting to appropriate mode spacing and beam waist. Finally, the conceptual paradigm of the OAM-CCL-based communication system is verified. In contrast to existing systems (traditional free-space optical communication or chaotic optical communication), the OAM-CCL-based communication system has three-in-one characteristics of high security, massive capacity, and robustness. The findings demonstrate that this will promote the applicable settings of chaotic laser and provide an alternative promising route to guide high-security and massive-capacity optical communications.

Since the concept of computational spectroscopy was introduced, numerous computational spectrometers have emerged. While most of the work focuses on materials, optical structures, and devices, little attention is paid to the reconstruction algorithm, thus resulting in a common issue: the effectiveness of spectral reconstruction is limited under high-level noise originating from the data acquisition process. Here, we fabricate a computational spectrometer based on a quantum dot (QD) filter array and propose what we believe is a novel algorithm, TKVA (algorithm with Tikhonov and total variation regularization, and the alternating direction method of multipliers), to suppress the impact of noise on spectral recovery. Surprisingly, the new TKVA algorithm gives rise to another advantage, i.e., the spectral accuracy can be enhanced through interpolation of the precalibration data, providing a convenient solution for performance improvement. In addition, the accuracy of spectral recovery is also enhanced via the interpolation, highlighting its superiority in spectral reconstruction. As a result, the QD spectrometer using the TKVA algorithm shows supreme spectral recovery accuracy compared to the traditional algorithms for complex and broad spectra, a spectral accuracy as low as 0.1 nm, and a spectral resolution of 2 nm in the range of 400 to 800 nm. The new reconstruction algorithm can be applied in various computational spectrometers, facilitating the development of this kind of equipment.

The detection of the state of polarization (SOP) of light is essential for many optical applications. However, cost-effective SOP measurement is a challenge due to the complexity of conventional methods and the poor transferability of new methods. We propose a straightforward, low-cost, and portable SOP measurement system based on the multimode fiber speckle. A convolutional neural network is utilized to establish the mapping relationship between speckle and Stokes parameters. The lowest root-mean-square error of the estimated SOP on the Poincaré sphere can be 0.0042. This method is distinguished by its low cost, clear structure, and applicability to different wavelengths with high precision. The proposed method is of great value in polarization-related applications.

A compact and high-resolution fiber-optic refractive index (RI) sensor based on a microwave photonic filter (MPF) is proposed and experimentally validated. The sensing head utilizes a cascaded in-line interferometer fabricated by an input single-mode fiber (SMF) tapered fusion with no-core fiber-thin-core fiber (TCF)-SMF. The surrounding RI (SRI) can be demodulated by tracing the passband’s central frequency of the MPF, which is constructed by the cascaded in-line interferometer, electro-optic modulator, and a section of dispersion compensation fiber. The sensitivity of the sensor is tailorable through the use of different lengths of TCF. Experimental results reveal that with a 30 mm length of TCF, the sensor achieves a maximum theoretical sensitivity and resolution of −1.403 GHz/refractive index unit (RIU) and 1.425×10−7 RIU, respectively, which is at least 6.3 times higher than what has been reported previously. Furthermore, the sensor exhibits temperature-insensitive characteristics within the range of 25°C−75°C, with a temperature-induced frequency change of only ±1.5 MHz. This value is significantly lower than the frequency change induced by changes in the SRI. The proposed MPF-based cascaded in-line interferometer RI sensor possesses benefits such as easy manufacture, low cost, high resolution, and temperature insensitivity.

Broad and safe access to ultrafast laser technology has been hindered by the absence of optical fiber-delivered pulses with tunable central wavelength, pulse repetition rate, and pulse width in the picosecond–femtosecond regime. To address this long-standing obstacle, we developed a reliable accessory for femtosecond ytterbium fiber chirped pulse amplifiers, termed a fiber-optic nonlinear wavelength converter (FNWC), as an adaptive optical source for the emergent field of femtosecond biophotonics. This accessory empowers the fixed-wavelength laser to produce fiber-delivered ∼20 nJ pulses with central wavelength across 950 to 1150 nm, repetition rate across 1 to 10 MHz, and pulse width across 40 to 400 fs, with a long-term stability of >2000 h. As a prototypical label-free application in biology and medicine, we demonstrate the utility of FNWC in real-time intravital imaging synergistically integrated with modern machine learning and large-scale fluorescence lifetime imaging microscopy.

Water photoacoustic microscopy (PAM) enables water absorption contrast mapping in deep biological tissue, which further allows a more detailed architecture analysis and facilitates a better understanding of metabolic and pathophysiological pathways. The strongest absorption peak of water in the near-infrared region occurs at 1930 nm, where the first overtone of the O-H bond lies. However, general light sources operating in this band hitherto still suffer from low optical signal-to-noise ratio and suboptimal pulse widths for photoacoustic signal generation. These lead to not only PAM contrast deterioration but also a high risk of sample photodamage. Consequently, we developed a hybrid optical parametrically-oscillating emitter (HOPE) source for an improved water PAM image contrast, leading to noninvasive and safer bioimaging applications. Our proposed source generates 1930 nm laser pulses with high spectral purity at a repetition rate of 187.5 kHz. The pulse width is flexibly tunable from 4 to 15 ns, and the maximum pulse energy is 700 nJ with a power stability of 1.79%. Leveraging these advancements, we also demonstrated high-contrast water PAM in multifaceted application scenarios, including tracking the dynamic of water distribution in a zebrafish embryo, visualizing the water content of a murine tumor xenograft, and mapping the fluid distribution in an edema mouse ear model. Finally, we showcased 1750-nm/1930-nm dual-color PAM for quantitative imaging of lipid and water distributions with reduced cross talk and imaging artifacts. Given all these results, we believe that our HOPE source can heighten water PAM’s relevance in both biological research and clinical diagnostics.

Terahertz (THz) radiation finds important applications in various fields, making the study of THz sources significant. Among different approaches, electron accelerator-based THz sources hold notable advantages in generating THz radiation with narrow bandwidth, high brightness, high peak power, and high repetition rate. To further improve the THz radiation energy, the bunching factor of the free electron bunch train needs to be increased. We propose and numerically reveal that, by adding an additional short-pulse drive beam before the main beam as the excitation source of nonlinear plasma wake, the bunching factor of the main beam can be further increased to ∼0.94, even though with a relatively low charge, low current, and relatively diffused electron beam. Two such electron beams with loose requirements can be easily generated using typical photoinjectors. Our work provides a way for a new THz source with enhanced radiation energy.