Nb${}_2$O${}_5$ nanosheets designed as terahertz electro-optical filters

Herein, niobium pentoxide (Nb${}_2$O${}_5$) nanosheets grown by the thermal evaporation technique under a vacuum pressure of 10${}^{-5}$ mbar are treated as terahertz band filters. The amorphous thin layers of Nb${}_2$O${}_5$ exhibited high light transmittance and showed optical absorption within an energy band gap of 3.68 eV. The band gap contained wide-extended band tails with Urbach energy of 2.27 eV. As terahertz optical band filters, Nb${}_2$O${}_5$ nanosheets displayed a cutoff frequency of ~ 1.0 THz in the spectral range of 1.15-3.37 eV. Larger exciting light signals in the ultraviolet range increased the terahertz cutoff frequency from 1.2 THz to 170 THz as the light energy increased from 3.38 eV to 4.20 eV, respectively. Additionally, the fitting of the optical conductivity using the Drude-Lorentz model has shown that the optical conduction in Nb${}_2$O${}_5$ is associated with six optical oscillators. Two of these oscillators are dominant in the infrared range of light, three are active in the visible range, and one is sensitive to ultraviolet light radiation. The drift mobility of the charge carriers in the films varied in the ranges of 1.76-3.67 cm${}^2$/Vs. Electrical verification of the optical results was actualized through the design of Yb/p-Nb${}_2$O${}_5$/Ag band filters. This electrical device showed a negative capacitance effect and exhibited a cutoff frequency of 1.0 THz when a driving signal of frequency 0.597 GHz was imposed between the terminals of the device. These features prove the suitability of the terahertz band filters of Nb${}_2$O${}_5$ nanosheets for use as optical and electrical filters with an extremely wide range of tunability.