Electrodynamics of conductive oxides: Intensity-dependent anisotropy, reconstruction of the effective dielectric constant, and harmonic generation

Abstract

We study electromagnetic pulse propagation in an indium tin oxide nanolayer in the linear and nonlinear regimes. We use the constitutive relations to reconstruct the effective dielectric constant of the medium, and show that nonlocal effects induce additional absorption resonances and anisotropic dielectric response: longitudinal and transverse effective dielectric functions are modulated differently along the propagation direction, and display different epsilon-near-zero crossing points with a discrepancy that increases with increasing intensity. We predict that hot carriers induce a dynamic redshift of the plasma frequency and a corresponding translation of the effective nonlinear dispersion curves that can be used to predict and quantify nonlinear refractive index changes as a function of incident laser peak power density. Our results suggest that large, nonlinear refractive index changes can occur without the need for epsilon-near-zero modes to couple with plasmonic resonators. At sufficiently large laser-pulse intensities, we predict the onset of optical bistability, while the presence of additional pump absorption resonances that arise from longitudinal oscillations of the free electron gas give way to corresponding resonances in the second and third harmonic spectra. A realistic propagation model is key to unraveling the basic physical mechanisms that play a fundamental role in the dynamics.

All-optical switching of an epsilon-near-zero plasmon resonance in indium tin oxide

“…We find best agreement with the data for an intensity-dependent red-shifting of the plasma frequency, described by ω p,2 = − 0.38% cm 2 GW −1 . This is the general behaviour expected for heating of the electron plasma in ITO, an effect that arises due to the non-parabolicity of the conduction band in this material 18,23,37 . However, as discussed in Supplementary Note S3, comparison between experiment and modelling suggests that the electron heating is weaker in our ITO compared to that previously reported in ref.…”

All-Optical Switching of an Epsilon-Near-Zero Plasmon in ITO

“…We find best agreement with the data for an intensity dependent red-shifting of the plasma frequency, described by ω p,2 = −0.35 %/ GW cm 2 . This is the general behaviour expected for heating of the electron plasma in ITO, an effect which arises due to the non-parabolicity of the conduction band in this material 22,36,37 . However, as discussed in supplementary information section S3, comparison between experiment and modelling suggests that the electron heating is weaker in our ITO compared to that previously reported in 36 , as our extracted value for ω p,2 is similar despite an increased local intensity arising due to the Kretchman geometry.…”

Broadband terahertz wave generation from an epsilon-near-zero material

“…At the low pump fluence, the THz generation peak is blueshifted from the static λ ENZ of the bulk ITO film measured through spectroscopic ellipsometry. This may be attributed to the nonlocal effect occurring in the ultrathin ITO film, as the electron doping concentration at the surface of the ITO film, where the THz wave is generated, may be higher than that in the bulk ITO film 39 , 43 . However, as the pump fluence increases, due to the photoinduced heating of conduction band electrons and the consequent time-dependent λ ENZ of ITO, the THz generation peak redshifts.…”