Thermally Evaporated NbSe2 Thin Films Designed as Frequency-Selective Reflective Surfaces

In this work, niobium diselenide (NbSe₂) thin films were deposited by thermal evaporation under a base pressure of ~10⁻⁵ mbar and investigated as broadband frequency-selective reflective surfaces spanning the microwave, infrared, visible, and ultraviolet spectral regions for high-frequency applications. X-ray diffraction indicated weak crystallinity with reflections assignable to the hexagonal NbSe₂ phase. Surface morphology and topographical analyses revealed an average grain size of ~80 nm and an average surface roughness of ~12 nm. Optical characterization yielded an energy bandgap of 1.50 eV and an Urbach energy of 0.21 eV. For photonic terahertz applications, NbSe₂ exhibited pronounced dielectric tunability from the infrared to the ultraviolet, accompanied by an increase in optical conductivity and a corresponding rise in terahertz cutoff frequency across 1.38–4.50 eV. In addition, when employed as a dielectric coating on metallic substrates, an Al/NbSe₂/Ag one-port reflective stack produced multiple impedance-matched resonances within the 9.0 kHz–6.0 GHz range, with reflection-loss minima approaching ~45 dB (|S11| ≈ 0.0056). A practical operating bandwidth of ~2.9 GHz was achieved under the reflection-loss criterion, while the most strongly matched conditions yielded a voltage standing-wave ratio of approximately 1.01. These findings demonstrate sub-6-GHz impedance engineering enabled by a thin NbSe₂ dielectric layer, supporting the development of compact frequency-selective surfaces for 5G/6G stopband implementation and microwave packaging integration.