Beam quality, serving as a crucial parameter for assessing laser divergence angle and mode purity, plays a significant role in the control and application of laser. The diamond Raman laser (DRL) combines the beam cleanup effect of stimulated Raman scattering with the excellent optical and thermal properties of the diamond crystal, demonstrating significant advantages in the generation of high-power lasers with high beam quality. In cascaded Raman conversion, the matching effectiveness among multiple modes within the resonator directly impacts the output beam quality and the cascaded conversion efficiency. In this work, a cascaded diamond Raman oscillator was pumped by a 1 & micro;m pump source with low beam quality. The beam quality and conversion efficiency of the cascaded Raman laser were investigated under different mode matching conditions. In the experiment, within a large cavity length adjustment range of 13 mm, the mode matching factor between the pump and the first Raman laser varied by nearly 0.5. Despite this, a 1.5 & micro;m second-order DRL output was achieved, with beam quality improved by a factor of 4.5 compared to the pump. The experimental results demonstrate that the DRL possesses a large mode matching acceptance range and high flexibility in resonator design for improving laser beam quality, further verifying the advantages of DRLs in achieving high-power, high beam-quality laser output with broad wavelength extensibility.

ditelluride (MoTe2), a novel transition metal chalcogenide, exhibits considerable potential for applications in optoelectronic devices. In this article, 1T' and 2H phases of MoTe2 were fabricated into saturable absorbers utilizing the liquid-phase exfoliation technique. The nonlinear saturable absorption characteristics of the 1T' &2H MoTe2 saturable absorbers were evaluated through the open-aperture Z-scan method. Furthermore, passively Q-switched Nd:YAG lasers were developed based on the 1T' &2H MoTe2 saturable absorbers. The results indicate that the 1T' phase of MoTe2 demonstrates superior performance compared to the 2H phase in terms of producing lasers with narrow pulse widths and high repetition rates.

We report a periodically tunable continuous-wave intracavity diamond Raman laser employing a rotating quartz birefringent filter (BRF). Experimental results reveal a counter-intuitive phenomenon where the insertion of a lossy BRF leads to higher Stokes output power compared to the free-running regime. This power enhancement is primarily attributed to the significant spectral narrowing of the fundamental field, which increases the effective Raman gain and overcompensates for the additional insertion losses. By utilizing BRFs with thicknesses of 0.5 and 1 mm, the system achieved stable tuning ranges of approximately 0.24 and 0.3 nm, respectively, with maximum Stokes output powers of 1.24 and 1.05 W. It should be noted that although the absolute wavelength span observed at extreme BRF rotation angles can extend to 2.06 and 1.77 nm, respectively, this is primarily due to intermittent outliers. Both the output wavelength and power exhibit a strict 180° periodicity relative to the BRF rotation angle.

Stimulated Brillouin Scattering (SBS) is one of the important methods for obtaining high-energy pulsed lasers. Its pulse compression characteristics enable the generation of high-peak-power, narrow-pulse-width lasers while maintaining good beam quality, making it promising for applications in fields such as ranging and medical treatment. Based on the stimulated Brillouin scattering (SBS) pulse compression effect, this paper theoretically calculates and simulates the SBS pulse compression performance under different pump pulse widths using liquid FC-770 as the SBS medium. Guided by these calculations, experiments were conducted using a laser with adjustable pulse width as the pump source to compare the influence of different pump pulse widths on pulse compression.This study obtained the variation patterns of output pulse width and energy conversion efficiency with pump energy under different pump pulse widths. This research provides valuable guidance for understanding the pulse compression behavior of SBS and the design of related structures.

To overcome the insufficient long-term stability of conventional Raman lasers, we demonstrate a single-frequency diamond Raman laser enabled by a pump-wavelength locking strategy. A three-mirror V-shaped cavity pumped by a 1064 nm fiber laser (6 kHz linewidth) provides single-longitudinal-mode (SLM) output at 1240 nm using Pound-Drever-Hall (PDH) stabilization. We further find that eliminating active temperature control of the diamond crystal suppresses rapid thermal perturbations and improves short-term stability. To compensate thermally induced slow cavity-length drifts, we implement a dual-loop feedback architecture that employs a fast PID loop acting on the pump wavelength and a slow PID loop driving a cavity piezoelectric transducer (PZT). Specifically, the fast loop tunes the pump wavelength via laser-current modulation, while the slow loop stabilizes the cavity length by regulating the PZT voltage. With 22 W pump power, the system produces 2.5 W SLM output with a linewidth of ∼2.9 kHz. The locked Raman laser exhibits an output power instability below 1.59% and a wavelength drift below 87 MHz, demonstrating markedly improved long-term operational stability. Unlike mainstream cavity-length locking techniques, our approach exploits the PDH error signal to directly lock the pump wavelength, providing a high-performance and wavelength-scalable route to highly stable single-frequency laser sources.

Highly coherent lasers are central to modern photonics. To date, high-coherence operation has been achieved predominantly in microcavity and fiber-based platforms. More recently, free-space Brillouin-laser experiments have revealed unusually strong noise suppression whose physical origin cannot be explained by conventional continuous-medium models developed for those platforms. In conventional continuous-medium models, the optical and acoustic fields are assumed to remain continuously coupled throughout the cavity evolution, whereas in free-space implementations the coupling is confined to the nonlinear medium and interrupted by passive propagation over the rest of the round trip. To describe this interaction-propagation separation, we develop a discrete-cavity model in which the short Brillouin interaction inside the gain medium and the subsequent free-space propagation are treated as two separate stages of the round-trip evolution. This separation introduces a temporal asymmetry between optical storage and acoustic relaxation, which effectively enhances acoustic damping at the cavity level and strongly reduces pump-noise transfer to the Stokes field. If the cavity round-trip time is much longer than the interaction time in the nonlinear medium, the noise-suppression ratio scales with the ratio of the total cavity length to the nonlinear-medium length. Our discrete-cavity model further provides quantitative predictions for the lasing threshold, output power, phase-noise transfer, and fundamental linewidth, in good agreement with experiment. These results identify the discrete interaction-propagation structure as the physical origin of the unusually strong noise suppression in free-space Brillouin lasers systems.

We propose a multifunctional terahertz (THz) metasurface based on a multilayer vanadium dioxide (VO 2 ) hybrid structure that achieves high-contrast switching between a perfect absorber and an off-axis vortex beam generator. In the metallic state, the metasurface operates as a high-performance absorber, exhibiting a near-perfect absorption exceeding 99% at 2.3 THz, with a fractional bandwidth of 43%. Conversely, in the insulating state, the structure acts as a precise geometric phase modulator; it successfully generates well-defined deflected vortex beams with customizable topological charges across multiple frequency channels (1.25, 2.1, and 3 THz). This integrated THz platform promises advancements in advanced communication, dynamic beam steering, and electromagnetic camouflage.

Stress in diamond crystals is a critical factor affecting Raman gain and the polarization state of the output Stokes. This study systematically investigates the multi-field coupling among residual-stress-induced birefringence in diamond, initial pump ellipticity, and stimulated Brillouin scattering (SBS). By combining theoretical simulations, division-of-focal-plane polarization imaging, and Raman laser experiments, we elucidate the mechanisms responsible for the stepwise evolution of the Stokes polarization direction. In high-stress regions, birefringence-induced phase distortion dominates the polarization dynamics, leading to abrupt polarization switching. In low-stress regions, SBS acts as a parasitic loss channel that forces polarization-mode transitions and limits power scaling. The results further confirm that even slight pump polarization perturbations can degrade the Raman output polarization state, reduce the effective gain, and increase the laser threshold. These theoretical and experimental findings provide practical guidance for overcoming performance bottlenecks in diamond Raman lasers, screening high-quality crystals, and developing efficient coherent light sources with precisely controllable polarization states.

Vortex particles carrying orbital angular momenta (OAMs) have found important applications in broad fields. However, the experimental verification of OAM transfer at the nuclear scale remains a great challenge. Here, we put forward a novel method to probe such OAM transfer through nuclear excitation via inelastic scattering of low-energy vortex electrons. We develop a Dirac distorted-wave Born approximation framework that incorporates the incident-electron OAM and a nonperturbative treatment of the Coulomb field, and apply it to ^229Th. We find that the vortex and non-vortex electrons yield opposite angular distributions, attributed to the OAM-modified selection rule and the Coulomb-induced redistribution of partial-wave strengths, providing an angle-resolved signature. Moreover, the vortex electron exhibits topological protection in the nuclear Coulomb field. Our method offers a route to probing nuclear-scale OAM transfer and deepens our understanding of the topological properties of vortex particles.

We propose and experimentally demonstrate a high-efficiency cascaded Raman structured-light laser based on a Ba(NO3)2 crystal. By combining a non-collinear V-shaped resonator with large-mode-area pumping and mode-size matching to high-order Stokes eigenmodes, the spatial overlap between the pump and target Stokes fields is improved, enabling enhanced Raman gain extraction and efficient cascaded conversion of high-order structured modes. High-quality one-dimensional Hermite-Gaussian (HG) modes up to the 20th order and two-dimensional HG modes are generated, together with direct intracavity generation of a first-order Laguerre-Gaussian (LG) mode and extracavity conversion to higher-order LG modes. At the second-Stokes wavelength of 1369 nm, the HG modes achieve a maximum pulse energy of 0.55 mJ with a conversion efficiency above 35%, while the directly generated LG mode reaches 0.64 mJ with a maximum conversion efficiency of 42%. The structured Raman output also preserves pulse compression, with durations 1-2 ns shorter than those of the pump. This work provides an effective route for high-efficiency wavelength extension of high-energy structured light.

Significance Single-frequency lasers with high spatial and temporal coherence are indispensable for coherent metrology, optical storage, precision spectroscopy, quantum optics, and information processing, as well as aerospace, defense, and deep-space sensing. As the demand for measurement sensitivity and detection precision continues to grow, modern applications increasingly require light sources that simultaneously deliver high power, narrow linewidth, low noise, and high beam quality. Conventional routes toward high-power, narrow-linewidth lasers, including single-oscillator architectures and master-oscillator power-amplifier systems, are often constrained by thermal effects, modal degradation, amplified spontaneous emission, and pump-noise transfer at high power. In addition, their accessible wavelength range is fundamentally limited by the energy-level structure of population-inversion gain media, making it difficult to realize high-power, narrow-linewidth laser emission at many unconventional wavelengths. As a typical third-order nonlinear optical process, stimulated Brillouin scattering (SBS) provides an attractive alternative for generating high-coherence, low-noise, single-frequency radiation, owing to its ultranarrow gain bandwidth, low quantum defect, and intrinsic noise-filtering capability. These features make Brillouin lasers especially promising for the development of high-power, narrow-linewidth sources beyond the limitations of conventional gain media. Progress In recent years, Brillouin lasers based on guided-wave platforms, including microcavities, integrated waveguides, and optical fibers, have made remarkable progress. Microcavity and integrated devices benefit from ultrahigh intracavity power density, small mode volume, and long effective interaction time, enabling dramatic linewidth narrowing and systematic studies of intrinsic linewidth, noise transfer, cascaded Stokes dynamics, and thermorefractive noise. However, their output power generally remains in the milliwatt-to-subwatt regime. Fiber-based Brillouin lasers can extend the output power to the watt and even tens-of-watt level by employing long interaction lengths, large-mode-area fibers, and hybrid gain schemes, but they often suffer from trade-offs among output power, conversion efficiency, amplified spontaneous emission, thermal noise, and spectral purity. As a result, it remains challenging for guided-wave Brillouin lasers to simultaneously achieve ultrahigh power, ultranarrow linewidth, and ultralow noise. Against this background, free-space Brillouin lasers have rapidly emerged as an important new direction. Compared with guided-wave systems, free-space platforms offer greater flexibility in resonator design, gain-medium selection, and thermal management, and are therefore more suitable for power scaling while preserving high spectral purity and single-frequency operation. Two major technical routes have gradually taken shape. The first is direct pumping, in which the pump and Brillouin-Stokes fields are simultaneously resonant in a free-space cavity. This route has evolved from early proof-of-principle demonstrations to high-power operation, with systematic investigations of linewidth narrowing, signal-to-noise ratio enhancement, phase-matching design, cascade suppression, and frequency-noise compression. In particular, recent free-space diamond Brillouin lasers have demonstrated watt-to tens-of-watt-level single-frequency output with kilohertz-level linewidths, high signal-to-noise ratios, and strong frequency-noise suppression, highlighting the unique capability of free-space systems to combine high output power and high coherence in a single architecture. The second route is Raman-mediated indirect pumping, in which a high-power intracavity Raman field serves as the effective Brillouin pump. This configuration enables efficient intracavity energy recycling and facilitates cascaded SBS generation, controllable Brillouin order selection, and Brillouin frequency comb formation. More importantly, because the Brillouin process is strongly coupled to the cavity-mode structure, this route also provides exceptional flexibility for transverse-mode control. Recent studies have demonstrated controllable cascaded Brillouin generation under Raman excitation, spatially programmable Brillouin structured light, and even single-frequency Brillouin vortex beams. These results show that free-space Brillouin lasers are not only a promising platform for high-power, narrow-linewidth operation, but also a unique system for joint control of spectral properties, cascade dynamics, and spatial modes. Conclusions and Prospects In summary, although free-space Brillouin lasers have emerged relatively recently, they have already shown strong potential for power scaling, efficient thermal management, linewidth narrowing, and spatial-mode engineering. In particular, the development of two complementary routes, direct pumping and Raman-mediated indirect pumping, has greatly expanded their functionality, enabling not only high-power single-frequency emission, but also cascaded frequency control, frequency comb generation, and structured-light output. Future advances will rely on progress in both physical understanding and system engineering. A key priority is to establish predictive models for noise transfer, linewidth evolution, and suppression mechanisms in free-space systems, especially since the exceptionally strong noise reduction observed in recent experiments cannot yet be fully explained by existing theories developed for fiber-based and microcavity platforms. At the same time, further efforts are needed to improve cavity locking, phase matching, cascade control, thermal stability, and long-term robustness under higher-power operation. With continued advances in materials, cavity design, and multiphysics control, free-space Brillouin lasers are expected to become a promising platform for high-power, narrow-linewidth, and low-noise laser sources with flexible spatial-mode control.

High pulse energy, high average power lasers with low spatial modulation have attracted many advanced applications in scientific research, and beam combination through stimulated Brillouin amplification (SBA) offers the potential for energy scaling with repetitive operation. However, attaining low-modulation beam combining output remains a significant challenge due to the near-field interference induced by the overlapping configuration of multiple pump beams with identical frequencies during the SBA process. Here, the method for reducing spatial modulation intensity via optimizing the structure parameters in the SBA beam combination system is proposed. In the proof-of-principle nine-channel SBA beam combination experiment, a low-modulation near-field output with an M-T value of 1.088 was demonstrated through adjustment, which is nearly identical to the M-T value of the incident Stokes seed. This work introduces a promising methodology for achieving high-pulse energy and high-average power lasers via SBA beam combining techniques, ensuring exceptional beam quality, which have profound implications for petawatt laser pumping, material processing, and laser-matter interaction.

Compared to black phosphorus (BP), violet phosphorus (VP) exhibits higher carrier mobility and better oxidation resistance, showing great promise in the field of passive Q-switching. In this work, two-dimensional (2D) VP nanosheets were successfully prepared using the liquid phase exfoliation (LPE) method. Their nonlinear optical properties and saturable absorption characteristics were investigated through non-degenerate pump-probe techniques, Z-scan and I-scan measurements, demonstrating that they meet the essential requirements for high-performance passive Q-switches. Furthermore, the prepared 2D VP nanosheets were employed as saturable absorbers (SAs) in all-solid-state lasers to achieve passive Q-switching operation. With slight adjustment, passive Q-switched pulse outputs were realized at working wavelengths of 1.0 mu m and 1.9 mu m, with repetition rates of 1.1 MHz and 124.1 kHz, respectively. This represents the highest pulse repetition frequency achieved to date for 2D VP nanosheets employed as passive Q-switches in the 1.0 mu m and 1.9 mu m, highlighting the excellent optical modulation performance of 2D VP nanosheets and further demonstrating their tremendous potential in the field of all-solid-state ultrafast photonics.

This paper investigates the nonlinear polarization evolution characteristics of diamond cascaded Raman lasers under complex pump polarization states. Both theoretical and experimental results show that the crystal tensor symmetry and stimulated Raman scattering jointly dominate the polarization selection effect of the beam. Even if the pump light contains a non-negligible elliptically polarized component, the cascaded system can still achieve polarization purification. Due to the presence of stress-induced birefringence and polarization-dependent loss, the Stokes light field is confined to the cavity’s low-loss eigenaxes. Moreover, while multi-order cascades can enhance polarization purity, at high-symmetry crystal orientations, gain competition can still induce macroscopic depolarization of the light field. This study profoundly elucidates the nonlinear competition mechanism between Raman tensor anisotropy and intrinsic cavity loss, providing essential theoretical basis and experimental support for the design and control of high-power, high-polarization-purity multi-order cascaded Raman lasers.

The existing intense laser-based approaches for nuclear excitation offer ultrafast temporal resolution and high efficiency compared to traditional accelerator probes. However, controlling nuclear properties such as spin and magnetic moment remains an unprecedented challenge. Here, we put forward a novel method for nuclear excitation and control induced by intense vortex lasers. We develop a theory incorporating the orbital angular momentum (OAM) of vortex laser within the nuclear hyperfine mixing framework. We find that intense vortex laser can effectively excite hydrogen-like thorium-229 nucleus and induce three-dimensional rotation of the nuclear magnetic moment. This rotation arises from the localized electromagnetic field and new transition channels excited by the vortex laser, and can be reconstructed through radiation spectrum analysis. Moreover, the OAM of vortex laser enables the chaotic system to exhibit topologically protected periodic patterns in nuclear excitation and radiation, facilitating precise experimental measurements. Our findings underscore the potential of vortex laser for high-precision nuclear control and imaging, deepening our understanding of nuclear properties and hyperfine structure, and advancing quantum information and nuclear technologies.

This study proposes and validates a systematic measurement and control analysis methodology for the quantitative characterization and optimization of frequency-locked Raman lasers. Focusing on the problems of system performance diagnostics and parameter optimization in measurement science, we employ a diamond Raman laser as the experimental platform. Through precise measurements of open-loop and closed-loop transfer functions as well as laser relative intensity noise, we achieve direct quantification and evaluation of the frequency-locking loop's stability, phase margin, and noise suppression capability. This method not only diagnoses the dynamic response of the system but also establishes a measurement-data-driven, universally applicable optimization procedure for frequency-locking parameters. Applying this methodology to a Pound-Drever-Hall (PDH) locked diamond Raman laser system, we realize a single-longitudinal-mode output of 3.3 W at 1240 nm and characterize and compress its linewidth to 2.3 kHz—more than a twofold reduction compared to the pump laser. The measurement and characterization framework developed in this work provides a validated optimization method for the precise control of various narrow-linewidth lasers, holding direct relevance for advancing the performance limits of precision optical instrumentation and sensing systems.

White-light interferometry is widely used in precision manufacturing and semiconductor fabrication due to its high precision and non-contact characteristics. However, noise in the interferometric signals significantly affects measurement accuracy. This paper proposes a denoising mechanism based on variational mode decomposition (VMD) combined with improved wavelet thresholding to suppress signal noise. The simulations demonstrate that this method can effectively improve the signal-to-noise ratio, thereby enhancing signal quality and surface morphology measurement accuracy. Compared to unprocessed measurements, the measurement error is reduced by 60%. The measurements on silicon wafers (repeatability error <5 nm) and gratings (height error 1.14%) demonstrate the algorithm's consistent performance under these tested conditions. Additionally, comparisons with existing denoising algorithms further validate the advantages of the proposed method.

Passive Q-switching and passive mode-locking are the key technologies for ultrafast laser to achieve ultrashort pulse and ultrahigh peak power. The transition metal dihalides (TMDs) turn to be promising saturable absorber (SA) materials due to its outstanding characteristics of wide-band absorption effect, third-order nonlinear magnetic susceptibility and high carrier mobility. The optical and physical properties of different TMDs are affected by the composition and structure. In this review, a variety of SA with different transition metals and chalcogens have been discussed. Also, the breakthrough research on defect regulation based on TMDs, combination of heterostructures were emphasized. Furthermore, the future prospects and challenge have also been addressed.