VO2 Thin Film Enabled Free Space Modulation of Infrared Light Using Pulsed Electric Field

VO₂ thin films synthesized via direct oxidation on piezoelectric GaN/AlGaN/GaN/Si and SiO₂/Si substrates have been used to demonstrate free space modulation of near and mid-IR light using a pulsed electric field. Interdigitated metal finger patterns deposited on high-quality 140 nm VO₂ thin films on suspended III-nitride or SiO₂/Si membranes were used to apply the pulsed electric field and modulate laser beams of wavelengths varying from 1064 to 2600 nm passing through it as it switched between metal and semiconducting phases. Strong wavelength dependence of the intensity modulation is observed with the modulation magnitude varying from 23.4% at 1550 nm to 52.1% at 2600 nm for VO₂ film grown on the III-nitride membrane. The VO₂ film on SiO₂/Si resulted in an intensity modulation of almost half of that on III-nitrides due to higher IR absorption in the SiO₂ and Si layers. Infrared microscopic images of the membrane recorded across the phase transition of the VO₂ film indicate significant temperature change over only a small fraction of the interdigitated finger pattern, clearly indicating the localized nature of the phase transition enabled by both the electric field and thermal heating. The intensity modulation depth did not change significantly over a frequency range of 10 kHz, which is likely limited by the thermal mass of the structure, so improved design concentrating electric field and reducing thermal mass and conductivity is expected to further improve the frequency response.