Abstract Terahertz (THz) modulators are essential components for dynamic wavefront control in imaging, sensing, and wireless communication systems. However, practical THz modulators require simultaneous optimization of multiple functional metrics, including modulation depth, resonance quality, insertion loss, switching speed, and device integration, which are often difficult to balance within a single device architecture. Graphene is an attractive active material because of its electrical tunability, high carrier mobility, and compatibility with planar integration, but its intrinsically weak interaction with THz waves limits the modulation efficiency of pristine monolayer devices. Here, we demonstrate a monolithic graphene-quasi-bound-state-in-the-continuum (qBIC) resonator platform that addresses this multidimensional performance trade-off. By integrating patterned graphene within symmetry-broken double-rod metallic resonators, the dissipative loss of a high-Q qBIC resonance can be continuously controlled through electrostatic gating. The fabricated modulator achieves a modulation depth of 35%, a resonance bandwidth of 0.12 THz centered at 1.1 THz, an equivalent Q factor of 9.28, a 3 dB modulation speed of 14 kHz, and a low insertion loss of 2.08 dB. These results demonstrate balanced optimization of key modulation metrics in a compact, planar architecture, providing a practical route toward active THz photonic devices.

Abstract Van der Waals (vdW) crystals supporting in-plane anisotropic hyperbolic phonon polaritons (HPhPs) provide a promising route toward mid-infrared (MIR) and terahertz (THz) nanophotonics by enabling deep-subwavelength confinement, low-loss propagation, and directional control of light within the crystal plane. Although such anisotropic HPhPs have been reported in a few vdW crystals, most studies have relied on bulk crystals or mechanically exfoliated flakes, which limits scalability and compatibility with device fabrication. At present, α-MoO3 is essentially the only vdW material available as large-area nanoflakes for fabrication-ready anisotropic polaritonic structures. Here, we demonstrate the scalable physical vapor deposition (PVD) growth of free-standing, monocrystalline α-V2O5 nanoflakes with lateral dimensions up to hundreds of micrometers and thicknesses down to a few nanometers. Using infrared nano-imaging together with theoretical analysis, we uncover low-loss in-plane anisotropic HPhPs in α-V2O5 with strong confinement (λ0/λ ≈ 100), ultraslow group velocities (~3×10-4c), and lifetimes up to 9 ps. We further show that patterned microdisk cavities enable geometry-defined shaping of the polaritonic near field, allowing tailored in-plane electromagnetic confinement. Our results establish α-V2O5 as a large-area, monocrystalline, fabrication-ready vdW material for low-loss anisotropic polaritonics, and expand the materials basis for scalable infrared flat photonics and future on-chip polaritonic devices.

Terahertz (THz) polarization detection provides critical insights into material properties but faces a fundamental constraint upon miniaturization: subwavelength metallic electrodes induce strong field localization, convoluting the intrinsic device response with electrode-induced artifacts. Here, we overcome this limitation with a ring-shaped electrode architecture that suppresses field perturbations across 2.0-5.0 THz. The resonant frequency can be detuned from the target operation frequency by adjusting its inner and outer radii, while the smooth geometry minimizes the lightning-rod effect. Numerical simulations reveal an 8.48 & times;reduction in local field strength compared with conventional rod-shaped electrodes. Consistent with this, experimental measurements on graphene-based detectors exhibit a 6.95 & times;decrease in photocurrent for the ring-shaped electrode relative to the rod-shaped configurations. Moreover, the ring geometry reduces the linear polarization ratio of the photocurrent from >3 to <1.4, confirming its effectiveness in mitigating electrode-induced polarization anisotropy. Beyond single-frequency experimental validation, multiphysics simulations across 2.0-5.0 THz show that detectors with ring-shaped electrodes maintain consistently low photovoltage over the entire range, suggesting broadband suppression capability. Our design decouples the detection response from electrode-induced artifacts, enabling compact THz detectors that preserve intrinsic signal fidelity for high-quality polarization-resolved imaging and diagnostics.

Abstract Cold-cathode ultrafast electron source is a key component for probing ultrafast dynamics behavior in materials, as well as developing high-frequency and high-power electromagnetic radiation devices. Developing large-current, high-brightness and tunable ultrafast electron sources by leveraging the intrinsic properties of nanomaterials is significant. In this study, we report the in-situ assembly of a double-walled carbon nanotube (DWCNT) cold-cathode based on a tungsten (W) tip via nanotransfer manipulation within a SEM chamber, enabled by electron-beam-induced carbon deposition. The resulting ultrafast electron emission exhibits excellent performance under dual-regime modulation by multiphoton photoemission (MPP) and optical field emission (OFE). Under co-excitation by 800 nm femtosecond laser pulses and a static electric field, the DWCNT cold-cathode demonstrates significantly enhanced emission in both regimes with a maximum peak current of ~65 A and corresponding brightness of 4.98 × 1018 A m-2 sr-1 V-1, and its optical excitation threshold reduces by an order of magnitude compared to the conventional metallic W tip cathode at equivalent emission current levels. Comprehensive material characterizations combined with density functional theory (DFT) calculations reveal that the semiconducting nature of the DWCNT emitter, along with its favorable electronic density of states and correspondingly lower effective work function, provides distinct advantages over metallic CNT and W for ultrafast electron emission. Furthermore, quantitative models are developed for both MPP- and OFE-dominated regimes, which elucidate polarization-dependent electron emission behavior and its underlying physical mechanisms. This work presents a promising semiconducting DWCNT cold-cathode for high-performance ultrafast electron sources, and provides a path for investigating ultrafast electron emission dynamics from multiple perspectives.

Broadband photodetectors spanning from UV to terahertz wavelengths are crucial for applications in medical diagnostics, industrial inspection, and advanced imaging. However, scalable fabrication of suitable materials remains a major bottleneck. Here, the molecular beam epitaxy growth of centimeter‐scale metallic crystalline Au 0.58 Te 0.42 film is reported—marking an important advancement in the scalable synthesis of metal tellurides. Exploiting its intrinsic broadband free‐electron absorption, ultra‐broadband photonic detection from 325 nm to 119 µm, with strong THz coupling that circumvents the need for complex antenna structures is demonstrated. The detector achieves a room‐temperature responsivity up to 136 mA W −1 (1.34 × 10 −3 mA W −1 ) and a noise‐equivalent power of 14.7 nW Hz −0.5 (9.5 × 10 4 nW Hz −0.5 ) at 2.52 THz, operating effectively in biased (self‐powered) mode. A response time of 320 ms is also achieved. Moreover, broadband spectral imaging of multilayered targets is realized, underscoring the potential for high‐contrast, multi‐spectral diagnostics. This work highlights metal tellurides as a scalable platform for high‐performance broadband photonic detection and imaging, offering new opportunities for next‐generation THz optoelectronics.

Active modulation of ultrafast electron pulses is essential for tunable terahertz sources and high-resolution imaging. Here, we demonstrate a polarization-tuned ultrafast pulsed electron source using a vertical few-layer graphene (vFLGs) cold cathode. Excited by a linearly polarized femtosecond laser, the vFLGs yield high optical switching ratios of 277 at 0° and 235 at 90° under a 400 V vacuum gap voltage. The emission current exhibits a distinct periodic polarization dependence, evolving from a cosine trend (0°–30°) to a sine trend (30°–90°). This tunable conversion between the multiphoton photoemission (MPP) and photoassisted thermionic emission (PTE) mechanisms provides a vital foundation for the construction of carbon-based ultrafast coherent electron sources.

Two-dimensional (2D) material terahertz (THz) detectors offer a promising platform for compact, room-temperature detection, yet their performance is fundamentally constrained by weak absorption in atomically thin layers. Here, we demonstrate a graphene plasmon polariton atomic cavity (PPAC) THz detector in which intrinsic graphene plasmon absorption is enhanced through vertical cavity-assisted field redistribution. By incorporating a metallic back reflector beneath a silicon substrate of designed thickness, a Fabry-Pérot (FP) interference cavity is formed that positions the standing-wave antinode near the graphene plasmonic layer. Electromagnetic simulations reveal that the Fabry-Pérot cavity itself primarily redistributes the vertical electromagnetic field, thereby enhancing the local in-plane driving field responsible for intrinsic graphene plasmon excitation. Experimental measurements at the optimized cavity condition confirm a pronounced increase in plasmon-induced photothermoelectric response, consistent with the predicted absorption enhancement. As a result, the detector exhibits an approximately 30-fold increase in responsivity compared with the corresponding structure without the cavity, while maintaining a fast response time below 130 μs. The detector further enables discrimination of concealed polar and nonpolar liquids through continuous-wave THz imaging at 2.52 THz, achieving a discrimination speed 30-fold faster than that of conventional time-domain spectroscopy. This result highlights the potential of cavity-enhanced intrinsic plasmon absorption for compact, high-sensitivity, and high-speed THz photodetection.

Deep ultraviolet surface-enhanced Raman scattering (DUV-SERS) technology demonstrates its potential for biomolecule detection. Materials exhibiting localized surface plasmon resonance (LSPR) effects generate intense electromagnetic field enhancements, which are critical for improving DUV-SERS detection sensitivity. However, the chemical stability of SERS substrates in complex biological environments remains a substantial challenge in practical applications. In this work, we have developed an LSPR-enhanced DUV-SERS substrate material utilizing molybdenum (Mo) random nanoholes fabricated on monocrystalline films, providing an efficient and anticorrosion solution for ultrasensitive molecular detection in complex biological environments. Under 325 nm DUV laser excitation, the Mo random nanoholes substrate achieves a Raman enhancement factor (EF) of approximately 107 for adenine detection and a detection limit as low as 10-10 mol=L. Finite-difference time-domain (FDTD) simulations further confirm the Mo's significant localized field enhancement effect, providing theoretical support for the high EF value. Additionally, a deep-learning-based spectral analysis approach was implemented to achieve a classification accuracy of 92.89% for adenine as a representative of biological molecules. The sensitive and robust Mo nanoholes material offers a cost-effective, high-reliability platform for detection of biomolecules, promoting the future of and molecular devices. (c) 2026 Chinese Laser Press

Non-stoichiometric molybdenum trioxide (MoO3-x) thin films are promising electrochromic materials owing to their abundant oxygen vacancies that enhance ion storage and redox activity. However, translating these intrinsic properties into superior device performance critically depends on the film microstructure, which governs ion transport kinetics and interfacial electrochemistry. In this study, we achieved effective regulation of MoO3-x thin film microstructure by manipulating critical process parameters during thermal evaporation. A phase-field theoretical model was established, successfully elucidating the fundamental conditions governing the formation of typical crystalline and amorphous MoO3-x films. Our findings demonstrate that, compared to the crystalline structures, the isotropic three-dimensional ion migration channels provided by the continuous and dense amorphous MoO3-x films significantly reduce ion intercalation barriers and effectively buffer volume strain during lithium-ion insertion and extraction, thereby exhibiting superior electrochromic properties and cycling stability. This research not only presents amorphous MoO3-x thin films as a promising candidate for high-performance electrochromic electrode materials, but also establishes structure-property relationships that provide effective guidance for the structural design of related electrode materials.

Phase-change materials provide nonvolatile optical tunability for programmable metasurfaces, yet their practical implementation remains fundamentally limited by their intrinsically narrow operational bandwidths. Addressing this limitation, we demonstrate broadband polarization manipulation using In3SbTe2 (IST) phase-change metasurfaces fabricated via laser direct writing of crystalline-IST grating patterns embedded within an amorphous-IST matrix. The mixed-phase architecture enables strong and continuous transmission contrast between transverse-electric and transverse-magnetic polarizations spanning the infrared to terahertz spectral regimes. As a proof of concept, an infrared quarter-wave plate is realized through geometry-controlled amplitude matching and pi/2 phase retardation, achieving efficient linear-to-circular polarization conversion. Large-area (2-in.-diameter) metasurfaces are rapidly produced and successfully integrated into a terahertz imaging platform for polarization-resolved imaging. This work establishes laser-written IST metasurfaces as a scalable solution for broadband, manufacturable polarization photonics.

Hyperbolic phonon polaritons (HPhPs) in van der Waals (vdW) alpha-MoO3 crystals enable deep subwavelength confinement of infrared (IR) and terahertz (THz) waves, making them promising for nanophotonic and optoelectronic applications. HPhPs propagation and loss are strongly influenced by the dielectric properties of the supporting substrate, necessitating careful substrate selection. Here, we systematically investigate HPhPs in alpha-MoO3 on substrates with varying dielectric constants, including insulators, semiconductors, and suspended (air) configurations. Using an analytical model supported by numerical simulations and experimental validation via scattering-type scanning near-field optical microscopy (s-SNOM), we demonstrate that substrate permittivity significantly modulates HPhP behavior. Suspended alpha-MoO3 exhibits elongated polariton wavelengths, longer propagation lengths, and a figure of merit (FOM) improved by up to 93%, 50%, and 6%, respectively, compared to supported counterparts. These results offer critical insight into substrate-mediated polaritonic tuning and pave the way for design of high-performance, low-loss polaritonic devices.

Room-temperature, miniaturized, polarization-resolved terahertz (THz) detection of high speed is vital for high-resolution imaging in radar, remote sensing, and semiconductor inspection, and is essential for large-scale THz focal plane arrays. However, miniaturization below deep-subwavelength scales (< 1/50 wavelength) remain challenging due to weak light-matter interaction, which degrades responsivity and polarization sensitivity. Here, we present a graphene plasmon polariton atomic cavity (PPAC) monolithic detector that overcomes this limitation by maintaining and even enhancing performance at a deep-subwavelength channel length of just 2 micrometers (1/60 wavelength). The device integrates graphene rectangle PPAC arrays with dissimilar metal contacts, where graphene functions as both absorber and conductor, simplifying the architecture. Exploiting plasmon polariton resonances and the photothermoelectric (PTE) effect, the detector achieves polarization-sensitive, frequency-selective, and fast THz detection spanning 0.53 to 4.24 THz with a polarization ratio of 93, featuring a responsivity (RV) of 1007 V/W, a noise-equivalent power (NEP) of 16 pW/Hz^0.5, a specific detectivity (D*) of 2.9 x 10^7 Jones, and a response time of 230 ps. We further demonstrate monolithic integration for polarization imaging and non-destructive semiconductor chip inspection, advancing room-temperature, compact, and polarization-sensitive THz technologies.

Strong light-matter interactions are central to advancing next-generation photonic technologies. However, the theoretical tools currently employed often fall short of accurately modeling highly dissipative systems. Traditional Hermitian approximations, which treat plasmon-exciton coupling with real-valued strengths, fail to capture defining experimental signatures such as asymmetric spectral line shapes and complex triplet structures. We address this limitation by introducing a non-Hermitian coupling theory built upon the quasinormal mode formalism for coupled plasmon-exciton systems. This approach achieves a complete theoretical description by intrinsically and self-consistently incorporating the physical realities of material dispersion, absorption, and radiation loss. By constructing a multimode, non-Hermitian Hamiltonian without fitting parameters, we directly derive hybridized eigenstates from complex-valued coupling strengths. This framework provides a unified and quantitative interpretation of strong-coupling features, offering the essential physical insights necessary for engineering truly dissipative, high-performance nanophotonic devices.

The polarization states of terahertz (THz) waves carry key physical information such as the spatial orientation of target objects and dielectric anisotropy. Miniaturized, high-performance polarization-sensitive detector is crucial for revealing the microscopic anisotropy of materials and enabling non-destructive characterization. However, conventional THz polarization-sensitive detectors are often limited by narrow bandwidths, bulky sizes, and requirement of polarization optical components, which remains a formidable challenge in scalable THz focal-plane arrays and on-chip platform. Here, we propose a polarization-sensitive THz micro-detector based on a graphene plasmon polariton atomic cavity (PPAC), which achieves a responsivity of 4.12 mV/W and a polarization sensitivity of 0.9 at a frequency of 2.52 THz. Furthermore, a polarization-resolved imaging of metal−silicon structures reveals that the detector is capable of distinguishing conducting and non-conducting regions. This study lays a solid experimental foundation for the application of PPAC-based polarization-sensitive THz detectors in target recognition, non-destructive testing, and advanced imaging systems.

High-frequency radiation source devices based on vacuum electronics hold significant application value in high-speed wireless communication and high-resolution radar imaging. Herein, we propose a photoelectric synergistic excitation of cold cathode for high-frequency radiation source device scheme, which can generate electromagnetic waves in the GHz-to-THz frequency range without the need for complex electron beam modulation components or additional microwave feed sources. In this study, based on a carbon nanotube (CNT) cold-cathode electron gun, we designed and successfully implemented a radiation source device capable of producing 12.2-GHz electromagnetic wave output under the coexcitation of picosecond laser pulses and a static electric field, achieving a peak output power of 22.7 mu W. Based on the strategy, a radiation source device for 1 THz has been designed, which provides an option for the development of novel high-performance miniaturized terahertz radiation sources.

Hyperbolic phonon polaritons (HPhPs) in polar van der Waals crystals, such as α-MoO3 and α-V2O5, enable deep subwavelength light confinement and in-plane manipulation via hyperbolic dispersion, yet momentum mismatch hinders efficient excitation. We present a far-field method to excite in-plane HPhPs using plasmonic gold nanorods. Surface plasmon resonance (SPPR) in nanorods mediates strong interactions between free-space waves and HPhPs in α-MoO3. Excitation efficiency is highly sensitive to nanorod length and angular orientation relative to the [100] crystal axis of α-MoO3. Maximum HPhP near-field intensity is achieved when the nanorods are aligned to optimize coupling with the anisotropic polaritonic modes, with efficiency closely linked to the resonance conditions of the plasmonic antennas. Numerical designs are corroborated well with near-field optical characterizations. Therefore, our approach establishes a platform to study interactions between plasmonic antennas and anisotropic polaritons, advancing controlled manipulation of in-plane polaritonic modes in van der Waals materials.

The interaction between terahertz (THz) photons and phonons of materials is crucial for the development of THz photonics. In this work, typical two-dimensional (2D) van der Waals (vdW) transition metal chalcogenide (TMD) layers and heterostructures are used in THz time-domain spectroscopy (TDS) measurements, low-wavenumber Raman spectroscopy measurements, calculation of 2D materials’ phonon spectra, and theoretical analysis of thermal responses. The TDS results reveal strong absorption of THz photons in the frequency range of 2.5–10 THz. The low-wavenumber Raman spectra show the phonon vibration characteristics and are used to establish phonon energy bands. We also set up a computational simulation model for thermal responses. The temperature increases and distributions in the individual layers and their heterostructures are calculated, showing that THz photon absorption results in significant increases in temperature and differences in the heterostructures. These give rise to interesting photothermal effects, including the Seebeck effect, resulting in voltages across the heterostructures. These findings provide valuable guidance for the potential optoelectronic application of the 2D vdW heterostructures.

Vacuum electronic devices offer superior electron mobility and spatiotemporal electron manipulating precision, with recent challenges focusing on ultrafast electron pulses for high‐frequency, high‐energy, and high‐resolution applications. Plasmon‐mediated electron emission (PMEE) nanocathodes provide a promising solution by producing high‐quality ultrafast electron pulses while simplifying the electron beam manipulation. In this study, we developed a PMEE Au‐on‐Gr nanocathode using vertically aligned few‐layer graphene decorated with gold nanoparticles, enabling synchronized generation of picosecond pulsed electron beam and electromagnetic radiation. The nanocathode achieved 80 MHz electron pulses with a 500 ps pulsewidth, 0.91 A·cm −2 peak current density, 6.53% external quantum efficiency, and 8.81 × 10 9 A·m −2 ·sr −1 ·V −1 reduced brightness. Additionally, it exhibited a 7.1° divergence angle and 0.97 eV energy spread under low excitations. Synchronized radiation pulses at 2.3, 5.7, and 9.2 GHz corresponded to electron pulse features. The excellent performance stems from plasmonic field enhancement and efficient hot electron generation driven by localized surface plasmon resonance (LSPR) in the PMEE nanocathode. The dynamic effects of high‐energy hot electron injection at the Au‐Gr interface also play a critical role. This system enables compact, room‐temperature, low‐power vacuum electronic devices for ultra‐high spatiotemporal resolution and high‐frequency applications, driving progress in materials science and nanotechnology.