Temperature evolution of optical constants and their tuning in silver

• Temperature (T) dependence of refractive indices of silver thin films determined.
• Thermo-optic coefficients (TOC) of silver as a function of incident energy is computed.
• TOC show maximum dispersion and sign changes around the L-interband transition.
• TOC show that refractive index can be tuned by appropriate wavelengths and temperatures.
• Determined T dependence of refractive indices of silver at 632.8 nm.

In this study, high temperature spectroscopic ellipsometry is employed to deduce the temperature dependent refractive indices of silver. The thermo-optic coefficients (thermo-refractive: dn/dT, thermo extinction coefficient: dk/dT) computed from the temperature dependent refractive indices, showed maximum dispersion and sign changes around the L (direct transition from the d band to the Fermi level i.e., L3–L′2(EF)) interband transition. The positive and negative dn/dT shows that tuning of refractive indices is possible by appropriate choice of incident wavelengths (or energies) and temperatures. The dispersion in dn/dT indicates the possibility of silver thin films being used as thermal lens for either converging or diverging purposes. A negative dk/dT for incident energy radiation around the L transition, indicates, increased transmission and decreased attenuation for these energies. The above dispersion of the thermo-optic coefficients are attributed to a natural outcome of the temperature dependent smearing in the Fermi distribution near the L transition and shift of energy levels due to the temperature dependent deformation potential. This paper also reports the experimental temperature dependence of refractive indices of silver corresponding to 632.8 nm, which has hitherto been obtained from a theoretically based Drude model.