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.

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.