Dynamic Nanophotonics in Epsilon‐Near‐Zero Conductive Oxide Films and Metasurfaces: A Quantitative, Nonlinear, Computational Model

Abstract

The promise of dynamic nanophotonic technologies relies on the confinement and spatiotemporal control of light at the nanoscale. Confinement via plasmonics, dielectric resonators, and waveguides can be complemented with materials whose optical properties can be controlled using nonlinear effects. Transparent conducting oxides (TCOs) exhibit strong optical nonlinearities in their near‐zero permittivity spectral region, on the femtosecond timescale. Harnessing full spatiotemporal control over the nonlinear response requires a deeper understanding of the process. To achieve this, a self‐consistent multiphysics time‐domain model for the nonlinear optical response of TCOs is developed and implemented into a 3D finite‐difference time‐domain code. Simulations are compared and tuned against recently published experimental results for intense laser irradiation of thin indium tin oxide (ITO) films, achieving good quantitative agreement; the time‐domain dynamics of the nonlinear response and the phenomenon of time‐refraction are also explored. Finally, by simulating intense laser irradiation of a plasmonic particle on an ITO film, the applicability of the approach to time‐varying metasurfaces is demonstrated. As expected, significant enhancement of the nonlinear response of an ITO‐based metasurface over bare ITO thin films is found. This work thus enables quantitative nanophotonics design with conductive oxides in their epsilon‐near‐zero region.

Inverse design of optical pulse shapes for time-varying photonics

“…In fact, 2D and 3D implementations of this method are entirely possible, depending upon the spatial extent of the objective function. However, 1D is also useful in itself because much current work in active nanophotonic has involved planar geometries like thin-films [17,29] and metasurfaces [12,18], which can be described using bulk (for thin-films) or effective [30,31] (for metasurfaces) refractive indices. This enables the problem to be scaled to 1D wherein our provided pulse-shaping code can be directly applied.…”

“…The unpumped ITO film has a plasma frequency everywhere of 𝜔 𝑝 = 2.97 × 10 15 rad/s. Upon irradiation with the pump pulse, the plasma frequency in the ITO decreases as the excited conduction electrons occupy higher energy levels [17,29,37]. In Fig.…”

“…Its nonlinear optical properties are well studied and can be modelled using a self-consistent multiphysics model that couples electrodynamics and thermodynamics introduced in Ref. [29].…”

“…This pump pulse creates a time-varying medium through a temperature dependent plasma frequency, that we simulate by implementing the model of Ref. [29] into a 1D-FDTD solver (with the code provided as supplemental material). We simulate a modulated Gaussian pump pulse with peak intensity 𝐼 𝑝𝑒𝑎𝑘 = 13.3 TW/cm 2 , pulse duration 𝜏 = 100 fs, and center wavelength 𝜆 0 = 1.23 𝜇m incident on a 320 nm thin film of ITO.…”

“…By allowing the plasma-frequency to be time-varying (see, for example, Ref. [29]), we are allowing for the material to have a time-varying dispersion.…”