Pure optical phase control with vanadium dioxide thin films

Under certain conditions, films made of vanadium dioxide exhibit wavelengths at which transmittance or reflectance do not change as the material undergoes insulator to metal phase transition, in spite of refractive index changes on the order of unity. Exploiting this effect, we demonstrate control of optical phase at 800 nm in transmission and at 1310 nm in reflection. With a 68 nm film, the optical phase is adjusted while leaving all other properties of light unchanged, including amplitude, polarization and frequency. The phase change per unit of propagated distance is Δk=107 rad/m, orders of magnitude higher than typically obtained with electro-optic effects. We discuss potential application to nano-sized phase devices or thin film lenses.