  Understanding the temperature dependence of the optical properties of thin
metal films is critical for designing practical devices for high temperature
applications in a variety of research areas, including plasmonics and
near-field radiative heat transfer. Even though the optical properties of bulk
metals at elevated temperatures have been studied, the temperature-dependent
data for thin metal films, with thicknesses ranging from few tens to few
hundreds of nanometers, is largely missing. In this work we report on the
optical constants of single- and polycrystalline gold thin films at elevated
temperatures in the wavelength range from 370 to 2000 nm. Our results show that
while the real part of the dielectric function changes marginally with
increasing temperature, the imaginary part changes drastically. For
200-nm-thick single- and polycrystalline gold films the imaginary part of the
dielectric function at 500 0C becomes nearly twice larger than that at room
temperature. In contrast, in thinner films (50-nm and 30-nm) the imaginary part
can show either increasing or decreasing behavior within the same temperature
range and eventually at 500 0C it becomes nearly 3-4 times larger than that at
room temperature. The increase in the imaginary part at elevated temperatures
significantly reduces the surface plasmon polariton propagation length and the
quality factor of the localized surface plasmon resonance for a spherical
particle. We provide experiment-fitted models to describe the
temperature-dependent gold dielectric function as a sum of one Drude and two
Critical Point oscillators. These causal analytical models could enable
accurate multiphysics modelling of gold-based nanophotonic and plasmonic
elements in both frequency and time domains.
