Structure-dependent optical nonlinearity of indium tin oxide

Abstract

We use post-deposition vacuum annealing of epsilon-near-zero (ENZ) indium tin oxide (ITO) nanolayers in order to modify their structural properties and enhance the third-order optical nonlinear response around the ENZ wavelength. We find that room temperature magnetron sputtering deposition results in polycrystalline thin films with an intrinsic tensile strain and a ⟨110⟩ fiber axis preferentially oriented normal to the substrate. Moreover, we demonstrate that post-deposition vacuum annealing treatments produce a secondary anisotropic phase characterized by compressive strain that increases with the annealing temperature. Finally, we use the Z-scan optical technique to accurately measure the complex nonlinear susceptibility [Formula: see text] and the intensity-dependent refractive index change [Formula: see text] for samples with different structural properties despite featuring similar ENZ wavelengths. Our intensity-dependent analysis demonstrates that an enhancement of the optical nonlinearity can be achieved by tuning the structure of ENZ nanolayers with values as large as [Formula: see text]. This study unveils the importance of structural control and secondary phase formation in ITO nanolayers with ENZ optical dispersion properties for the engineering of integrated highly nonlinear devices and metamaterials that are compatible with the scalable silicon photonics platform.

Enhanced Nonlinearity of Epsilon‐Near‐Zero Indium Tin Oxide Nanolayers with Tamm Plasmon‐Polariton States

“…[27,28] However, recent work has established the need to extend the Drude-Lorentz (DL) dispersion model in order to accurately describe the complex permittivity of ITO across a broad spectral range. [17,18,29] In fact, while the DL model well accounts for a direct optical bandgap (E g ), it also overestimates the optical losses due to the "long-tailed" nature of the Lorentzian functions involved. [29] To avoid this problem, here we employ the Tauc-Drude-Lorentz (TDL) oscillator model that is consis-tent with Kramers-Kronig causality relations.…”

“…Recently, using magnetron deposition in partnership with X-ray diffraction (XRD), linear and nonlinear optical characterization of ITO thin films, we reported significant enhancement of the nonlinear response of ITO nanolayers through control of their material microstructure, specifically driven by a secondary phase with anisotropic texturing, which is largely influenced by the deposition and post-deposition vacuum annealing conditions. [18] In this work, building on the enhanced optical nonlinearity of ITO thin films with anisotropic crystallographic texturing, we propose and demonstrate Tamm plasmon-polariton nonlinear structures that further boost the nonlinear refractive index modulation in 30 nm-thick ITO nanolayers coupled to silicon dioxide/silicon nitride (SiO 2 /SiN) multilayer structures fabricated by magnetron sputtering (MSP). In particular, we develop and utilize variable-angle spectroscopic ellipsometry (VASE) models that accurately retrieve the optical dispersion properties of the deposited materials over a large spectral range and we perform Z-scan measurements as a function of the pump intensity in order to precisely quantify their Kerr optical nonlinear coefficients.…”

Broadband Nonlinear Optical Response of Indium–Zirconium Oxide in the Epsilon‐Near‐Zero Region

“…The graded optical properties along the thickness of the TCO films, also studied by others, [ 27,37,38 ] can have different origins (or a combination of effects) such as nonuniform composition or dopant distribution, varying density or microstructure, and different types of defects. For instance, graded permittivity due to the nonuniform distribution of free electrons in ITO and inhomogeneous microstructure in AZO has been reported using SE analyses in Refs.…”

“…Similar experimental results showing an increase in Δn for ITO films with preferential crystal orientation have been also reported. [27] The nonlinear optical properties of TCO-based ENZ materials have attracted much attention recently, in part because of the large and ultrafast intensity-dependent refraction phenomenon exhibited by these materials. Despite the large interest in this subject, little information has been given on the bandwidth of the nonlinear response and its dependence on intensity.…”

“…Similar experimental results showing an increase in Δ n for ITO films with preferential crystal orientation have been also reported. [ 27 ]…”