  Due to their exceptional plasmonic properties, noble metals such as gold and
silver have been the materials of choice for the demonstration of various
plasmonic and nanophotonic phenomena. However, noble metals' softness, lack of
tailorability and low melting point along with challenges in thin film
fabrication and device integration have prevented the realization of real-life
plasmonic devices.In the recent years, titanium nitride (TiN) has emerged as a
promising plasmonic material with good metallic and refractory (high
temperature stable) properties. The refractory nature of TiN could enable
practical plasmonic devices operating at elevated temperatures for energy
conversion and harsh-environment industries such as gas and oil. Here we report
on the temperature dependent dielectric functions of TiN thin films of varying
thicknesses in the technologically relevant visible and near-infrared
wavelength range from 330 nm to 2000 nm for temperatures up to 900 0C using
in-situ high temperature ellipsometry. Our findings show that the complex
dielectric function of TiN at elevated temperatures deviates from the optical
parameters at room temperature, indicating degradation in plasmonic properties
both in the real and imaginary parts of the dielectric constant. However, quite
strikingly, the relative changes of the optical properties of TiN are
significantly smaller compared to its noble metal counterparts. Using
simulations, we demonstrate that incorporating the temperature-induced
deviations into the numerical models leads to significant differences in the
optical responses of high temperature nanophotonic systems. These studies hold
the key for accurate modeling of high temperature TiN based optical elements
and nanophotonic systems for energy conversion, harsh-environment sensors and
heat-assisted applications.
