All practical plasmonic metals suffer from inherent ohmic losses that naturally increase the temperature of resonantly excited plasmonic components. For instance, the operation temperature of plasmonic near-field transducers in heat assisted magnetic recording (HAMR) is estimated to be close to 300 - 500 0C. It is therefore imperative to understand the influence of temperature on the evolution of optical properties of thin metals films to perform systematic and rational design of practical high temperature nanophotonic components in a wide variety of research areas, including HAMR, photothermal therapy, thermophotovoltaics, and near field radiative heat transfer. In this talk, we will present the experimentally probed temperature induced deviations to the optical response of important plasmonic metals: gold, silver and titanium nitride up to 900 0C, and outline the dominant microscopic physical mechanisms governing the optical response. Using extensive numerical calculations, we demonstrate the importance of incorporating the temperature induced deviations into numerical models for accurate multiphysics modeling of practical high temperature nanophotonic applications - transducers for HAMR, broadband emitters for thermophotovoltaics and high temperature sensors.
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 optical and electronic properties of ultra-thin plasmonic films are expected to have a strong dependence on the film thickness, composition, and strain, as well as an increased sensitivity to external optical and electrical perturbation. This unique tailorability establishes ultra-thin plasmonic films as an attractive material for the design of tailorable and dynamically switchable metasurfaces. Due to their epitaxial growth on lattice matched substrates, TiN is an ideal material to investigate the tailorable properties of plasmonic films with thicknesses of just a few monolayers. MXenes, a class of two-dimensional (2D) nanomaterials formed of transition metal carbides and carbon nitrides, are yet another promising material platform for tailorable plasmonic metamaterials. MXenes have been widely explored in a variety of applications, such as electromagnetic shielding and SERS. However, investigations of MXenes in the context of nanophotonics and plasmonics have been limited leading to this current exploration of MXenes as building blocks for plasmonic and metamaterial devices. In this study, we investigate these two emerging classes of materials, MXenes and ultra-thin transition metal nitrides, as potential material platforms for tailorable plasmonic metamaterials. We report on the strain and oxidation dependent optical properties of ultrathin TiN. Applications of MXenes as a broadband plasmonic metamaterial absorber and a random laser device are also discussed.
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.
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.