Exploiting the metal-insulator transition of VO₂ thin films for terahertz wave modulation and switching

VO₂ is one of the very few natural materials that can be used to modulate terahertz (THz) radiations. A 100-nm thick VO₂, when in its metallic phase, has a charge density of more than ∼ 10¹⁵ cm^-2 which will strongly reflect and absorb the THz radiation; while in its insulator state, the charge density is lowered by several orders of magnitude to be THz transparent. Therefore, exploiting the metal-insulator transition of VO₂ is a potential approach to modulate or even switch THz radiation for THz optics. Here we report that VO₂ epitaxial thin films on sapphire substrate exhibits 85% amplitude modulation depth in a broad bandwidth, while this value can be improved to 95% when VO₂ film is coated on both sides of a substrate. We further demonstrate that with wafer bonding, 4-layered VO₂ thin films exhibit a transmittance as low as -20 dB to -30 dB at their metallic state, enough for switching applications. We also report our proof-of-concept demonstration of THz spatial light modulator that exhibits amplitude modulation as large as 96%, -30 dB pixel-to-pixel crosstalk, and a broad THz bandwidth.