High-quality (Q) microcavities, which significantly enhance wave mixings through optical resonances, play an important role in the investigation of light–matter interaction. For second-harmonic generation (SHG) in quadratic microcavities, satisfying a double-resonance condition is crucial for enhancing the nonlinear conversion. However, directly revealing a precise double-resonance condition can be demanding and has seldom been implemented. In this work, we introduce a method to synchronize the fundament wave and its second harmonic during the transmission measurement, and directly distinguish the double-resonance detuning. We also investigate the impact of each detuning on the cavity-enhanced SHG spectral line shape and intensity. The approach provides a method to help deeper understand of the nonlinear coupling processes and benefit research on cavity-enhanced nonlinear optics.

The growing demand for cost-effective and miniaturized spectrometers is driven by advances in fields such as analytical chemistry, biochemical sensing, and materials science. Speckle-based spectrometers using multi-mode fiber (MMF) as the interference medium offer a compelling platform for broadband, high-resolution spectral analysis. However, conventional near-infrared fiber spectrometers require bulky cameras for spectral reconstruction, hindering efforts toward miniaturization and cost reduction. In this work, we propose a novel approach that replaces the bulky imaging system with a silicon photonic chip functioning as the speckle detector. By employing six eigenmodes within a two-mode few-mode fiber (FMF) captured by the few-mode antenna and utilizing an on-chip reconfigurable unitary matrix to dynamically modulate the transmission matrix of the speckle spectrometer system, we enhanced the number of physical channels to compensate for the lower modal number of FMF compared to MMF. This strategy enables an ultra-high resolution and extends the operating range beyond the free spectral range limitations of FMFs. Our experimental results demonstrate that the proposed spectrometer achieves a resolution of 6 pm and a working range of 100 nm. Leveraging mature fiber and photonic chip technologies, our approach offers a scalable path toward compact, low-cost, and portable spectroscopic solutions.

Ultrasound imaging is a widely used technique for non-destructive testing and in vivo inspection. However, conventional ultrasound imaging relies on contact-based transducers to generate and detect signals, limiting the applicability of the ultrasound. In this study, we propose an approach to implement fully non-contact ultrasound imaging based on the sub-terahertz photoacoustic effect and laser Doppler vibrometry. Compared to infrared pulse-based laser ultrasound systems, the use of continuous sub-terahertz waves enables ultrasound generation through opaque materials, such as clothing and plastics, and offers further prospects of system integration based on solid-state electronics. We demonstrate that the system can detect minute changes in resonance frequency across different media and is capable of tracking multiple ultrasound reflections through inverse Fourier transform. Furthermore, we present a result of raster scan imaging, showcasing the capability of the system for non-contact ultrasound imaging.

Raman lasers are well known for wavelength shifting and beam clean up, but less so for linewidth narrowing. Recent advances in design and material choice have enabled stable single frequency operation and the opportunity to investigate their narrow linewidth properties. Here, we use modeling and experiments to investigate linewidth and frequency noise as a function of pump noise. The model reveals that the picosecond-scale phonon dephasing time, typical of most Raman materials, suppresses pump frequency noise by factors up to 108 at Fourier frequencies up to 1011 Hz. To support the model, frequency noise spectra were measured for the input and output beams of a singly-resonant Raman laser operating at 1178 nm, revealing noise suppression by more than 104 at offset frequencies above 1 MHz. The results show that Raman lasers offer orders-of-magnitude greater linewidth-narrowing than Brillouin lasers, even for cavities of moderate Q factor. This makes them a promising route toward ultra-narrow linewidth lasers across the spectrum with adaptability to a wide range of pumps.

We report on a novel method for optical microwave generation using a frequency comb based on difference-frequency generation, which passively eliminates the carrier–envelope offset frequency (fceo), with the repetition rate (frep) locked to an optical reference. We demonstrate the generation of ultra-low phase noise microwave signals by transferring the stability of the optical reference to 9.6 GHz, reaching noise levels of −147 dBc/Hz at 1 kHz offset. The optimization of pulse timing after interleaving and a scheme for additional long-term stabilization of the microwave signal to global positioning system standards are discussed. This work presents a new variant of highly stable radio frequency signal generation for precision applications, such as radar, atomic clock local oscillators, and optical quantum technologies.

Developing a high-quality, bright source of polarization-entangled photon pairs on CMOS-compatible integrated platforms, such as silicon or silicon nitride, remains a significant challenge. Here, a novel silicon chip-based nonlinear Sagnac interferometer, which mainly integrates a bidirectionally pumped microring resonator with a polarization-splitting rotator, was proposed to generate polarization-entangled photon pairs. By pumping the system with a pulsed laser at 1550 nm and a power of 78 μW, polarization-entangled photon pairs in the telecom C-band were produced through spontaneous four-wave mixing, achieving a state-of-the-art brightness of 1.6 × 107 pairs s−1 GHz−1 mW−2. Using an off-chip polarization analyzer, the visibility of polarization-correlation interference fringe was measured to be 96.6%. Quantum state tomography revealed a fidelity of 0.958 for the maximally entangled state produced by the source. These exceptional performance metrics, combined with the integrability of the device, make it highly suitable for advanced quantum applications, particularly in multi-user entanglement-based quantum communication networks.

Silicon photonics has emerged as a transformative solution to address the energy and bandwidth challenges of modern computing and communication systems. While integrating diverse materials with silicon has enhanced the functionality of photonic integrated circuits, these hybrid approaches often face challenges related to scalability, cost, and compatibility with CMOS processes. We provide a comprehensive perspective on advancing high-performance all-silicon photonic devices and systems. By fully leveraging the inherent potential of silicon, diverse functionalities have been demonstrated, including Raman lasers, modulators, photodiodes, and optical memories. This approach outlines a pathway toward fully integrated electronic–photonic circuits on the silicon platform, seamlessly aligned with the existing CMOS infrastructure.

We investigated the optothermal dynamics of optical microresonators operating on a chiral exceptional surface (ES). ES is formed via unidirectional coupling between the clockwise and counterclockwise modes of a microtoroid resonator using a fiber loop mirror placed at one end of the taper fiber waveguide used to couple light in and out of the resonator. By tuning the strength of unidirectional coupling and, hence, the chirality of the ES under various coupling conditions, we examine the impact of the dynamic optothermal response of the resonator on the transmission and reflection spectra. Our theoretical model, supported by numerical simulations and experiments, reveals that the transmission spectra deviate from the response of the same resonator operating at a diabolic point, while the reflection spectra broaden from a squared-Lorentzian profile. These findings emphasize the complex interplay between optothermal effects and non-Hermitian dynamics in optical resonators, offering valuable insights for designing thermally stable optical systems.

Laser irradiated nanostructures generate nanoplasmas with ultrahigh energy densities, yet retrieving their three-dimensional (3D) spatial distribution remains challenging due to ion expansion and loss of original spatial information. In this study, we introduce a momentum-to-position mapping method to retrieve lost 3D information in position space of the ions from intense laser irradiated nanosphere, supported by ion momentum tomographic reconstruction and molecular dynamics (MD) modeling. Our results demonstrate pronounced variations in localized nanoplasma across the nanosphere surface for different irradiated intensities, providing experimental evidence for the combined action of external and internal fields on nanoplasma formation. This work represents an advanced correlation between ion emission in both momentum and position space, offering an intuitive 3D visualization of localized nanoplasma and facilitating potential applications based on nanoplasma manipulation.

We report on the terahertz (THz) harmonic generation in an active graphene/metamaterial device by using powerful ultrafast table-top THz time-domain spectroscopic systems. Complex nonlinear transmission spectra, comprising even and odd harmonics, emerge when the devices are tested with intense ultrafast THz pulses with peak electric fields in the range of 1–150 kV/cm. The odd and even harmonic features show a positive correlation with carrier concentration, allowing for efficient frequency tuning on top of tunable group delay dispersion. Interestingly, before the onset of saturation, observed at 20–25 kV/cm, the main resonance and the harmonic features exhibit an anti-crossing trend that can be further exploited for active frequency tuning. These results report a key milestone for the fundamental investigation of the nonlinearity of 2D materials. At the same time, they represent an important advance in the design of future integrated THz optoelectronics by providing novel functionalities for THz light generation and manipulation.