Determination of the complex optical constants of thin-film vanadium dioxide on polar dielectrics*

Vanadium dioxide (VO2) has been widely used in photonics applications because the insulator-to-metal phase transition (IMT) near 67 C provides an active and reversible character to photonics devices. The device design requires precise information on the complex optical constants of VO2 across the IMT. VO2 deposited on polar dielectrics is considered a promising photonic platform hosting surface phonon polaritons at mid-/far-infrared. However, obtaining the VO2 optical constant is limited by the accessibility of an ellipsometer at infrared wavelengths. Here, we propose a useful method to extract the complex optical constants of VO2 on polar dielectrics across the IMT which relies solely on the normal incident reflection spectrum. We prepared 100 nm thick VO2 films deposited on silicon carbide and on sapphire substrates and measured the reflectance spectra at the Reststrahlen band under normal incidence at temperatures varying from 30 C to 80 C. A model combining Kramers-Kronig (KK) relations and thin film equation was used to precisely determine the changes in the spectrum during the IMT. The determined temperature-dependent complex optical constants of the VO2 films change steeply across the IMT and slight wavelength dependence of the optical constant was observed.