Free electron radiation sources feature unparalleled high brilliance and broad spectral range, serve as a cornerstone of contemporary science and technology. While pursuing cost and space affordable, the free-electron radiation sources are transitioning from kilometer-scale facilities to on-chip integrated architectures, driven by replacing conventional accelerator-undulator systems with engineered photonic quasiparticle interactions. This review highlights surface plasmon polaritons (SPPs) as a pivotal platform for compact light sources. We examine material-intrinsic SPPs for short-wavelength (X-ray to UV) generation and engineered spoof SPPs for longer wavelengths (THz to microwave). Key breakthroughs include threshold-less Cherenkov radiation, graphene-plasmon undulators, laser-plasma-driven SPP sources, and photonic flatband resonances that resolve transverse momentum mismatches. Finally, we outline critical frontiers: all-silicon free-electron lasers, hybrid plasmonic-photonic structures, and broadband on-chip beam-wave separation.

In this work, we report an oxygen p-doping and full contact structure to reduce the contact resistance $(\mathrm{R}_{\mathrm{c}})$ of monolayer p-type tungsten diselenide $(\text{WSe}_{2})$ and demonstrate high-performance ptype WSe ${ }_{2}$ field-effect transistors (FETs) with a scaled channel length $\left(\mathrm{L}_{\text{ch}}\right)$ of 16 nm. The controllable oxygen doping and the full contact structure greatly increase the contact quality. The co-optimized $\mathrm{R}_{\mathrm{c}}$ is as low as $352 \Omega \cdot \mu \mathrm{m}$ at 4.2 K, which is ∼two times lower than that of the control sample with top contact. Consequently, $16-\text{nm}$-channel $\text{WSe}_{2}$ pFETs deliver a record saturation current of $1532 \mu \mathrm{A} / \mu \mathrm{m}$ at a drain voltage of −1.0 V and a ballistic ratio of 81% at room temperature. This work presents a major breakthrough in p-type twodimensional (2D) transistors in terms of contact optimization and device performance, filling the research gap in 2D p-type transistors.

The ultrafast dynamics of exciton-polaritons (EPs) in single plasmonic nanocavities are of fundamental interest due to extreme mode confinement (V∼10^{-6}λ^{3}), which enables few-exciton strong coupling and strong nonlinearity of EPs. However, directly probing transient evolutions of EPs in a single nanocavity has remained a challenge. Here, we develop an EP-resonant enhanced ultrafast difference-frequency generation (DFG) microscopy for directly visualizing EP dynamics in a single WSe_{2}-Au nanocavity. By scanning the pump photon energy across the polariton resonances and recording the DFG intensity as a function of pump-probe delay, we construct two-dimensional maps that directly visualize the transient evolution of the high and low energy branches of EPs. The maps reveal that the EPs collapse suddenly and then revive within 2 ps with a saturation threshold below 2 pJ, indicating strong nonlinearity of EPs in the few-exciton saturation regime. This Letter provides a new approach for visualizing the nonequilibrium dynamics of EPs in a single plasmonic nanocavity, offering important experimental evidence for ultrafast manipulation of EPs.

Cooperative emission is a collective quantum optical process that requires macroscopic phase coherence among coupled emitters. Recent observations of cooperative emission in QD superlattices have renewed interest in how such coherence emerges in nanostructured solids. Meanwhile, theoretical studies have long discussed the relationship between electronic delocalization and coherence, particularly whether delocalized states necessarily give rise to cooperative emission. This study addresses this question through power-dependent steady-state PL and time-resolved PL decay measurements. The findings indicate that, although the quantum resonance peak exhibits delocalized excitonic characteristics, it shows no signatures of cooperative radiation. In particular, neither superlinear intensity scaling nor power-dependent emission delay was observed, indicating the absence of cooperative-radiation signatures. This can be understood from two disorder-related aspects. Temperature-dependent spectroscopy reveals pronounced inhomogeneous broadening and low-temperature dark-exciton participation, pointing to intra-domain static disorder and exciton-state mixing. These effects collectively hinder the establishment of macroscopic coherence. The temperature dependence of the quantum resonance peak decay lifetime is consistent with two-dimensional exciton dynamics. This work provides direct experimental evidence that electronic delocalization can be decoupled from cooperative coherence in CdSe quantum dot superlattices.

Emerging nonlinear van der Waals (vdW) crystals show strong nonlinearity for various applications in nonlinear optics. Here, we investigate the characteristics of second-harmonic generation in vdW semiconductors under transmission loss conditions using 3R-stacked MoS2 as a model system. Incorporating dissipation terms and Fabry-Pérot interference effects, we establish a nonlinear transmission model applicable to finite-thick lossy van der Waal materials. Our analysis reveals that strong second-harmonic absorption leads to rapid intensity saturation, rendering conventional phase-matching strategies ineffective. To overcome this limitation, we propose a quasi-phase-matching scheme based on multilayer stacking with controllable orientation angles, enabling effective enhancement of second-harmonic signals even under finite-loss conditions by adjusting the thickness and lattice orientation of each layer. This work uncovers the loss mechanisms and quasi-phase-matching behavior in nonlinear-optical processes of two-dimensional materials, important for high-efficiency ultrathin nonlinear devices, integrated photonic chips, and quantum photonic technologies based on vdW structures.

Absorption spectroscopy plays a critical role in applications such as atmospheric pollution monitoring and healthcare. Particularly with the increasing demand for trace-level detection, the need for enhancing absorption has become urgent. While micro/nano-fabricated array structures can effectively enhance signals, their practical implementation faces significant limitations including fixed operational wavelengths, poor multi-band adaptability, and high fabrication costs. To overcome these challenges, we developed a continuously tunable Fabry-Perot (FP) microcavity platform capable of broadband signal enhancement. It features a tunable length down to 10 mu m with high precision (up to 400 nm), while maintaining excellent mechanical stability. Experimental validation through gas detection demonstrate 6 x and 4 x sensitivity enhancement for methane and acetylene molecules. Both the simulation and experimental results reveal that as the cavity length decreases, the detection sensitivity enhancement becomes higher. For molecular fingerprint recognition, our FP cavity platform not only facilitates real-time monitoring but also enables simultaneous detection across multiple characteristic spectral regions, demonstrating exceptional versatility for practical sensing applications.

Acoustic metasurfaces have been widely employed for manipulating audible sound and low-frequency ultrasound in the kHz range. However, their application to underwater high-frequency acoustic fields in the MHz range remains limited due to structural complexity and subwavelength feature constraints. Herein, a simple, broadly applicable, and fabrication-friendly acoustic metasurface is proposed to achieve transmission-based phase modulation of MHz-frequency acoustic waves, enabling continuous phase shifts spanning a 0-π range. By introducing a universal dimensionless area ratio γ as the core control parameter, the metasurface avoids reliance on complex subwavelength structures and realizes precise phase manipulation via a macroscopically tunable parameter. Polymethyl methacrylate (PMMA) is utilized as the substrate material, allowing for low-cost, scalable fabrication via CNC machining with sub-millimeter precision. Finite element analysis (FEA) and experimental results demonstrate a full width at half maximum (FWHM) of 1.3λ for the focal pressure profile, with focal lengths tunable from 0.5 to 5 mm. These findings confirm the metasurface's suitability for manipulating underwater high-frequency acoustic fields in the 1-10 MHz range. The proposed design balances physical mechanism innovation with practical engineering feasibility, offering a readily implementable solution for applications such as biomedical imaging and targeted acoustic therapy, where precise wavefront manipulation in underwater high-frequency regimes is essential.

Interfacial excitons (IXs) in CuPc/CdSe nanowire (NW) heterostructures (HS) show enhanced nonlinear optoelectronic properties through hybridization, but their resonant dynamic behavior is still not well understood. Our combined experimental and theoretical study shows the resonant excitonic nature of these hybrid states: enhancement of second-harmonic generation (SHG) around 725 nm (B-exciton) and 835 nm (A-exciton) demonstrates the coexistence of IXs (760 nm) with efficient charge transfer, increasing the SHG intensity by about more than 14 times in CuPc/CdSe NW interface. First-principles DFT calculations confirm the type-II band alignment and hybridized interfacial states (1.62 eV) resulting from orbital interactions between the 2D-organic (CuPc) and 1D-inorganic (CdSe) components. However, transient reflection (TR) spectroscopy confirms the modified ultrafast charge transfer at the CuPc/CdSe NW interface, driven by type-II band alignment. TR spectroscopy supports intralayer A- and B- excitons resonance, attributed to Forster resonance energy transfer. On the other hand, the IXs peak shows a faster decay than bare CdSe NW, enhancing excitonic resonance signatures due to strong interlayer coupling. These findings advance our understanding of many-body exciton physics in 2D-organic/1D-inorganic hybrid systems, underscoring how nonlinear optics can probe charge-transfer-mediated SHG enhancements- a finding supported by traditional TR spectroscopy that offers valuable insights for designing optoelectronic devices.

van der Waals (vdW) materials offer a highly tunable and efficient platform at nanoscale for nonlinear and quantum optics. Twist-stacked vdW heterostructures enable elegant control of symmetry and interlayer coupling. Prior studies mainly focus on planar twisted interfaces, while neglecting the naturally formed and mandatory defects in such vdW heterostructures. Here, we demonstrate nonlinear singular optics with topologically configurable nonlinear vortex generation at the corner singularity of vdW heterostructures. By tailoring azimuthally discrete second-harmonic phase gradients at each interface, we obtain programmable nonlinear vortex emitters with dominant target OAM components. Nonlinear OAM beams with topological charge ℓ = 1 and ℓ = -2 are experimentally realized, respectively. Our work unlocks the untapped potentials of nonlinear singular optics in twisted vdW materials as a reconfigurable and lithography-free platform for nonlinear structured light generation, important in quantum nonlinear optics and related fields.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have the advantages of strong optical nonlinear response, negligible harmonic absorption, and easy fulfillment of the phase-matching condition and are considered as attractive materials for high-harmonic generation (HHG). However, the high-harmonic yield is limited by the short light–matter interaction length. Here, we demonstrate the enhancement of HHG in 2D TMDs by a semi-open structure that couples the 2D TMDs with a distributed Bragg reflector (DBR). By using this scheme, the electric field of the driving laser pulse can be enhanced in the full area of the light–TMD interaction. The semi-open-cavity structure avoids the absorption of harmonics by the cavity. Experimental results show that the HHG of TMDs on the DBR structure is enhanced by more than two orders of magnitude compared with that of TMDs on the normal substrate. Our work paves the way for optimizing nanoscale solid-state HHG light sources.

The strong coupling between photons and phonons in polar materials gives rise to phonon-polaritons that encapsulate a wealth of physical information, offering crucial tools for the ultrafast terahertz sources and the topological engineering of terahertz light. However, it is still quite challenging to form and manipulate the terahertz phonon-polaritons under the ultrastrong coupling regime till now. In this work, we demonstrate the ultrastrong coupling between the phonon (at 0.95 THz) in a MaPbI3 film and the metallic bound states in the continuum (BICs) in Au metasurfaces. The Rabi splitting can be continuously tuned from 28% to 48.4% of the phonon frequency by adjusting the parameters (size, shape and period) of Au metasurfaces, reaching the ultrastrong coupling regime. By introducing wavelet transform, the mode evolution information of the terahertz phonon-polariton is successfully extracted. It indicates that the phonon radiation intensity of the MaPbI3 film is enhanced as the coupling strength is increased. This work not only establishes a new platform for terahertz devices but also opens new avenues for exploring the intricate dynamics of terahertz phonon-polaritons.

Strong coupling, as a unique paradigm of cavity quantum electrodynamics, provides an important foundation for exploring novel quantum phenomena. Under ambient conditions, exciton-plasmon strong coupling systems have developed rapidly with various exquisite nanocavity structures proposed and refined. Nanocavities based on metal nanoparticles, such as single-particle and nanoparticle-on-mirror (NPoM), offer excellent compatibility with exciton materials, making them significant platforms for studying plasmon-exciton strong coupling. However, the intrinsic losses of metallic materials severely limit the exploration of novel quantum phenomena such as low-threshold lasing and nonlinear properties. In this work, we propose and realize a plasmonic nanocavity based on high-refractive-index GaAs nanowires positioned on a Au substrate, forming a nanowire-on-mirror (NWoM) architecture. The NWoM nanocavities exhibit strong local field confinement and a significantly enhanced damage threshold, reaching 5.6×106 W/cm2. Furthermore, by integrating the nanocavity with monolayer WSe2, we successfully realized the strong coupling at room temperature. By tuning the diameters of the dielectric nanowires, we observe the characteristic anticrossing behavior and achieve a Rabi splitting of 140 meV. This study lays the foundation for future investigations into more efficient quantum behaviors and provides a new approach to designing low-loss strong coupling systems.

The direct mapping of the intrinsic topology in a leaky photonic band is crucial and challenging in topological photonics. For instance, observables in bound states in the continuum (BICs) feature complex topological textures such as a polarization vortex in momentum space, which nonetheless is difficult to be characterized in far-field scattering, especially considering the dominant direct channel. Here, we propose and experimentally demonstrate a hybrid nonlinear metasurface that enables a direct visualization of the intrinsic topology in BICs via second-harmonic generation (SHG). The enhanced local-source of SHG from the ultrathin indium tin oxide can effectively excite the emissions from the eigenmodes of a TiO2 photonics crystal slab, achieving three-order enhancement of SHG magnitudes. Importantly, these enhanced SH emissions carry topological polarization textures of BICs to the far field. With this, we can directly construct polarization vector maps of symmetry-protected BICs and chiral symmetry-broken quasi-BICs, clearly visualizing the winding structure around V points, the generation and evolution of chiral C points. This work provides a universal approach for characterizing topological photonic systems via coherent nonlinearity processes, opening new avenues for studying topological phenomena in non-Hermitian photonic systems.

On-chip electronic devices driven by ultrafast light represent a promising approach to surpass traditional information processing speeds. However, practical implementation has been limited by the requirement for material with complex heterostructure and femtosecond lasers with high pulse energy, carrier-envelope phase stability, and few-cycle durations. To address this limitation, an on-chip logic gate is developed on a metallic material platform based on the photothermoelectric effect (PTE) and plasma resonance absorption of gold. By manipulating the light polarization, hot carrier migration is controlled, achieving a high polarization ratio and bipolar response. Meanwhile, time-resolved transient absorption spectroscopy demonstrates that the switching time is on the sub-picosecond scale. The logic gate used two picojoule-level laser pulses as inputs, outputting nanoampere-level currents with controllable polarity. This design provides a convenient fabrication process, promising for large-scale high speed logic computing devices.

Investigating interfacial charge carrier dynamics is important for improving the efficiency of photovoltaic devices with organic and inorganic heterostructures to exceed the Shockley-Queisser limit. Charge transfer dynamics at the organic/inorganic interfaces with two different types of excitons are still unclear. In this work, we reveal the photogenerated charge carrier dynamics at the interface of CuPc/CdSe nanoflakes using steady-state and transient reflection spectroscopy. The CuPc layer deposited on CdSe effectively modifies the charge carrier dynamics, reducing the fast electron lifetime from 10.96 to 3.12 ps. Following photonic interaction with ZB-CdSe, the photogenerated electrons are transferred to CuPc, forming a singlet charge transfer state (1CT). Rapid intersystem crossing converts this into a triplet state (3CT), preventing electrons return to CdSe and enabling efficient exciton dissociation into long-lived polarons in CuPc with longer lifetimes. The results show that an enhanced red-shift around 20 nm, caused by a decrease in the bandgap, ultimately improves the overall charge transfer efficiency, eta similar to 70% at CuPc/ZB-CdSe interfaces, comparable to that achieved in quantum dot systems. Our work demonstrates an effective pathway for improving the photoelectric performance of CuPc/CdSe composites to get electrons out from CdSe, which involves transporting them across CuPc/CdSe interfaces.

The canalization effect of phonon polaritons (PhPs) shows highly directional and diffraction-less propagation characteristics in van der Waals (vdW) materials, offering new opportunities to mold the light flow at nanoscale for near-field energy, information, and thermal management. Previously, canalized PhPs have only been experimentally realized in the hexagonal boron nitride metasurface, heterostructures of twisted alpha-phase molybdenum trioxide (alpha-MoO3) crystal flakes, or the hybridized system. However, these systems typically have complex structures, and require strict operational conditions, such as fine structural parameters, a specific photonic magic angle, or a doping level of graphene, for realizing polariton canalization with a modest performance. Here, we demonstrate the high-quality PhPs canalization in a single natural -alpha-MoO3 crystal flake. The canalized PhPs exhibit the highly directional and diffraction-free propagation features associated with lateral confinement ratios up to lambda(0)/80 (where lambda(0) is the free-space wavelength of the incident laser). We believe this work is important to effectively manipulate PhPs in natural vdW materials, with potential applications in nanoimaging, directional energy transfer, and enhanced nonlinearity at the deep subwavelength scale. (c) 2025 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement

Laser probes have tremendous potential in biological aerosol, and laser-induced plasma probes (LIPP) underpin the recent development of real-time biological aerosol detection, enabling the tracing of aerosol species information. However, laser probes suffer from low hit rates and accuracy due to the weak signals of aerosols and their susceptibility to interference. Specifically, LIPP analyzes aerosols by breaking them down to obtain elemental information, often ignoring the inherent surface information. Herein, optical trapping-assisted second harmonic generation (SHG) was utilized to investigate small amounts of aerosols. The results demonstrate that optical trapping effectively controlled the aerosol count, from a few tens to single particles. Additionally, the adsorption free energy of trans-4-[(4-dimethylamino)styryl]-1-methylpyridinium iodide molecules on the bioaerosol surface was determined. Furthermore, optical trapping-assisted LIPP detected principal elements (K, Ca, Na, and Mg) in the bioaerosol. The homologous heterogeneous information (spectra, sound (shock wave images), and plasma images) of the plasma was analyzed, and multiple signals were complementarily corrected to enhance the classification accuracy of LIPP analysis. Finally, to enhance LIPP and SHG data mining, we proposed an artificial intelligence (AI)-driven adaptive multimodal attention fusion network, which improved the classification accuracy of 13 bioaerosols from 83% to 96%. This work establishes a highly sensitive laser probe detection platform that synergistically analyzes surface adsorption and internal element components, paving the way for future single-bioaerosol detection and alarm systems.

Difference frequency generation (DFG), underlying on second‐order nonlinear optical effect, is a parametric down‐conversion process that is widely used to generate and amplify broadband signals. It leads to the wide applications in tunable laser systems and infrared sources, as well as quantum light sources. However, it is still challenging to realize the widely tunable, ultra‐compact, and efficient DFG light sources in nanoscales. Here, a boosting DFG from AA‐stacked WS 2 coupled with a tunable dual‐resonant nanocavity, which is composed of Au nanodisk arrays and TiO 2 /Au film, is experimentally demonstrated. In this nanocavity, the Fabry‐Perot mode and localized surface plasmon resonance mode can be independently tuned to match well with the pump (515 nm) and the signal (750–900 nm) wavelength in DFG, respectively. Over two order‐of‐magnitude enhancement of DFG is successfully achieved ranging from 1200 to 1650 nm (e.g., 812 at 1445 nm). This work proposes a dual‐resonance hybrid nanocavity and shines the way to the DFG enhancement in transitional metal dichalcogenides, expanding the prospects in subwavelength coherent light sources and other optoelectronic devices.

The integration of perovskite nanolasers into on-chip photonic circuits demands high stability under strong electric fields. In this study, we developed high-quality all-inorganic CsPbBr 3 submicron cubes through an oleic acid-assisted crystallization method and demonstrated their excellent lasing performance even in high-electric-field environments. The oleic acid-controlled synthesis yields uniform cubes with high crystallinity and smooth surfaces, enabling lower threshold and single-mode lasing with a narrow linewidth (0.22 nm) and a high Q factor (2447). Notably, these cubes exhibit strong electric field resistance, maintaining stable lasing under biases up to 75 kV/cm, including in highly nonuniform fields between electrodes. This work underscores the importance of morphology control in achieving optically efficient and electrically robust nanolasers for practical on-chip photonic applications.

All-dielectric metasurfaces have emerged as promising platforms for studying strong coupling due to their low absorptive losses and diverse resonance modes. However, previous works have primarily focused on all-dielectric metasurfaces with electric/magnetic dipole (ED/MD) modes, while higher-order multipoles, such as electric quadrupole (EQ), have not been fully explored. Here, we report on an EQ-involved strong coupling based on the monolayer WS2 integrated all-dielectric metasurfaces. By precisely tuning the EQ modes to be spectrally overlapped with the A exciton resonance of WS2 (at 614 nm), angle-resolved transmission spectra show a clear anticrossing behavior with a 31.7 meV Rabi splitting. EQ modes feature spatially varying field gradients and strong in-plane electric fields at the interface, thus increasing the coupling efficiency with two-dimensional materials. This work presents a novel strategy for developing low-loss, compact polaritonic devices, contributing to a deeper understanding of the strong coupling mechanisms between higher-order multipoles and excitons.