Huanjun Chen

Ultraviolet (UV) plasmon-enhanced field emission offers a route toward dynamically tunable cold-cathode electron sources operating under low-light-intensity excitation. However, cathode materials and structures that simultaneously exhibit outstanding UV plasmonic and high field-emission performance still require further development. In this study, a Mo nanocone array cold cathode is proposed and fabricated for photoelectric co-excited electron emission with UV plasmonic enhancement. The Mo nanocone array supports localized surface plasmon resonance (LSPR) in the UV region, and the high-energy electrons generated by the resonance, together with the localized near-field effect, synergistically enhance cold-cathode electron emission. Surface-enhanced Raman scattering (SERS) measurements demonstrate that the array exhibits the strongest enhancement under 325 nm excitation, supporting the wavelength-dependent optical response of the Mo nanocone array associated with its UV-LSPR characteristics. Under photoelectric co-excitation at a low light intensity on the order of W cm–2, the Mo nanocone array cold cathode achieves emission current and brightness increases of 314.2% and 300% relative to pure electric field excitation, respectively. Meanwhile, the effective work function decreases by 0.23 eV. The wavelength-dependent electron emission current agrees with the photocurrent contribution from plasmon decay-generated carriers, supporting that the enhanced electron emission originates from the designed UV plasmonic resonance. Furthermore, deep-UV plasmon-enhanced Mo nanocone arrays targeting shorter wavelengths (266 and 160 nm) are designed. This study provides a viable strategy toward developing UV plasmon-enhanced cold-cathode electron sources with tailorable emission characteristics, which presents potential for application in short-wavelength vacuum micro/nanoelectronic devices.

To address the requirements of vacuum electronic and optoelectronic device applications for next-generation low-power-consumption, high-brightness, low-energy-spread, and tunable electron sources, the pursuit of high-efficiency metal nanocold-cathodes with plasmon-enhanced properties in specific waveband emerges as one of potent solutions. In this study, by screening and comparing the cathode materials and structures, we propose a high-brightness, low-energy-spread Au needle nanocold-cathode point electron source adapted to low-intensity photoelectric coexcitation. The experimental results demonstrate sensitivity to excitation light and outstanding electron emission performance of this point electron source: under the coexcitation of an electrostatic field below ∼1.25 V μm–1 and a supercontinuum-spectrum laser with average intensity of ∼24 W cm–2, the source achieved an emission current of 130 nA, improving by one order of magnitude when compared to the field emission current (∼13 nA) under pure electrostatic field excitation. As the laser excitation intensity increased, its effective work function decreased from 5.1 to 2.7 eV, resulting in a significant reduction of the excitation threshold for electron emission. Benefiting from the plasmon-enhanced light-harvesting effect, the cathode exhibits high sensitivity and selectivity to the wavelength and polarization of the incident light. In the 532–675 nm plasmon resonance band, the emission current exceeds 225 nA at an average excitation intensity of only 7.05 W cm–2, ∼17 times the pure electrostatic field emission current. Meanwhile, the plasmon relaxation preferentially populates high-energy electrons and suppresses the energy spread broadening induced by the space charge effect, thereby achieving a high reduced brightness of 6.89 × 109 A m–2 sr–1 V–1 and a low-energy spread of approximately 0.5 eV. This low-power photoelectric coexcitation scheme greatly relaxes the requirements on the excitation source, providing a route for the miniaturization and integration of vacuum optoelectronic point electron sources.

Room-temperature, miniaturized, multiparameter terahertz (THz) detectors with ultrafast response, particularly in the high-frequency THz regime (> 0.3 THz), are essential for next-generation information perception, including radar imaging, remote sensing, semiconductor inspection, and scalable TH...

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.

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.

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.

Hyperbolic phonon polaritons (HPhPs) in van der Waals (vdW) α‐MoO ₃ 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 α‐MoO ₃ 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 α‐MoO ₃ 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.

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 anti-corrosion 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 10 ⁷ 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 development of portable biosensing and rapid molecular recognition devices.