Emission of terahertz pulses from vanadium dioxide films undergoing metal–insulator phase transition

Abstract

This paper describes research on the optics of functional materials, which can change their dielectric properties according to their function. Vanadium dioxide is a good example of such a material where the insulator-to-metal phase transition offers the possibility to control dielectric properties and to use them as a triggering element for photonic applications in the wide spectral range from optical to terahertz frequencies. We observed emission of terahertz (THz) radiation from VO2 films in insulating and conductive phase states under femtosecond pulse irradiation. We found that the efficiency of THz emission increases up to 30 times after the insulator-to-metal phase transition. This process occurs in thin films while it is fundamentally forbidden in the bulk material, and polarization analysis of the emitted radiation reveals the crucial importance of interface contributions. The properties of the THz radiation emitted by VO2 are determined by displacement photocurrents at the VO2–air and VO2–substrate interfaces induced by the incident laser light. In each phase state the contributions of the two boundaries are different. Properties of the effective dielectric susceptibility χ(2) tensor for the insulating phase were defined. In demonstrating the conversion of optical into THz radiation in VO2 films, we found that fundamental symmetry restrictions are not applicable to problems of nonlinear optics of thin films.