Measurement of the Refractive Index at Cryogenic Temperature of Absorptive Silver Thin Films Used As Reflectors in a Fabry-Perot Cavity.

Data on the refractive index of silver thin films are scarce in the literature, and largely dependent on both the deposition method and thickness. We measure the refractive index of silver films at cryogenic temperature with a technique that takes advantage of the absorption of the films and the corresponding peculiar properties of Fabry-Perot cavities: a frequency shift between the reflection and transmission peaks, together with a modified cavity bandwidth. We demonstrate a decrease in the real value of the refractive index, together with a decrease in its imaginary value at 4 K.