Harsha Reddy

Temperature-dependent optical properties of gold thin films

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.

Ultra-thin transition metal nitrides for tailorable plasmonic devices (Conference Presentation)

As a result of recent developments in nanofabrication techniques, the dimensions of metallic building blocks of plasmonic devices continue to shrink down to nanometer range thicknesses. The strong spatial confinement in atomically thin films is expected to lead to quantum and nonlocal effects, making ultra-thin films an ideal material platform to study light-matter interactions at the nanoscale. Most importantly, the optical and electronic properties of ultra-thin plasmonic films are expected to have a strong dependence on the film thickness, composition, strain, and local dielectric environment, as well as an increased sensitivity to external optical and electrical perturbations. Consequently, unlike their bulk counterparts which have properties that are challenging to tailor, the optical responses of atomically thin plasmonic materials can be engineered by precise control of their thickness, composition, and the electronic and structural properties of the substrate and superstrate. This unique tailorability establishes ultra-thin plasmonic films as an attractive material for the design of tailorable and dynamically switchable metasurfaces. While continuous ultra-thin films are very challenging to grow with noble metals, the epitaxial growth of TiN on lattice matched substrates such as MgO allows for the growth of smooth, continuous films down to 2 nm. In this study, we present both a theoretical and an experimental study of the dielectric function of ultrathin TiN films of varying thicknesses. The investigated ultrathin films remain highly metallic, with a carrier concentration on the order of 1022 /cm3 even in the thinnest film. Additionally, we demonstrate that the optical response can be engineered by controlling the thickness, strain, and oxidation. The observed plasmonic properties in combination with confinement effects introduce the potential of ultra-thin TiN films as a material platform for tailorable plasmonic metasurfaces.

Temperature evolution of optical properties in plasmonic metals (Conference Presentation)

Understanding the temperature evolution of optical properties in thin metals is critical for rational design of practical metal based nanophotonic components operating at high temperatures in a variety of research areas, including plasmonics and near-field radiative heat transfer. In this talk, we will present our recent experimental findings on the temperature induced deviations in the optical responses of single- and poly-crystalline metal films – gold, silver and titanium nitride thin films - at elevated temperatures upto 900 0C, in the wavelength range from 370 to 2000 nm. Our findings show that while the real part of the dielectric function changes marginally with temperature, the imaginary part varies drastically. Furthermore, the temperature dependencies were found to be strongly dependent on the film thickness and microstructure/crystallinity. We attribute the observed changes in the optical properties to predominantly three physical processes: 1) increasing electron-phonon interactions, 2) reducing free electron densities and, 3) changes in the electron effective mass. Using extensive numerical simulations we demonstrate the importance of incorporating the temperature induced deviations into numerical models for accurate multiphysics modeling of practical high temperature plasmonic components. We also provide experiment-fitted models to describe the temperature-dependent metal dielectric functions as a sum of Drude and critical point/Lorentz oscillators. These causal analytical models could enable accurate multiphysics modeling of nanophotonic and plasmonic components operating at high temperatures in both frequency and time domains.

Ultra-thin plasmonic metal nitrides: Tailoring optical properties to photonic applications

In this study, we present recent developments on growing epitaxial quality, ultra-thin titanium nitride films that exhibit very good metallic/plasmonic properties, comparable with their bulk counterparts. The potential of these films for extreme light confinement and electrical control is discussed.

Optical properties of ultrathin plasmonic TiN films

Epitaxial, ultrathin (<10 nm) plasmonic TiN films are characterized using spectroscopic ellipsometry and Hall measurements. Thin films with thicknesses down to 2 nm remain highly metallic with a carrier concentration on the order of 10²² cm⁻³.

New Materials for Plasmonics: Designs and Applications from Flat Optics to Quantum Nanophotonics

We will review the list of alternative plasmonic materials and provide a focused discussion on transition metal nitrides for refractory plasmonics. Nanostructures made of alternative plasmonic materials and their performance will be presented. Article not available.