Effect of substrate orientation on terahertz optical transmission through VO_2 thin films and application to functional antireflection coatings

We report studies of the terahertz (THz) transmission through vanadium dioxide (VO2) thin films grown on c-, m-, and r-plane sapphire substrates. Our results revealed THz amplitude modulation as large as 84% for VO2 films grown on r-plane sapphire substrates upon crossing the metal–insulator phase transition temperature. Complex optical conductivity and refractive indices were determined for all investigated samples in the metallic state. Results are consistent with electrical resistivity measurements and described based on the Drude model. The real and imaginary parts of the optical conductivity and refractive index are obtained, and associations with variations in the grain morphology and crystal quality are described. We show that VO2 thin films can be used as tunable broadband THz frequency antireflecting coatings.