Intraband and Interband Competition Drives Ultrafast Modulations of Indium Tin Oxide

TL;DR

This study reveals the complex ultrafast carrier dynamics in indium tin oxide near its epsilon-near-zero frequency, driven by high-fluence nonlinear processes, with implications for ultrafast photonic device engineering.

Contribution

It introduces a detailed analysis of high-fluence carrier dynamics in ITO using a novel optical gating technique and an extended two-temperature model.

Findings

Ultrafast modulations of reflectivity and transmissivity are observed.

Non-monotonic oscillatory behavior in complex Fresnel coefficients is demonstrated.

High-fluence dynamics involve Auger-type scattering affecting plasma frequency and damping.

Abstract

Transparent conducting oxides near their epsilon-near-zero frequency exhibit near-unity ultrafast modulations of the refractive index which have enabled the field of time-varying metamaterials, yet the underlying carrier dynamics at high driving fluences remain poorly understood. Here, we report ultrafast modulations in the reflectivity and transmissivity of indium tin oxide, and a non-monotonic oscillatory behavior. This is especially evident in the time evolution of the complex Fresnel coefficients retrieved directly from pump-probe spectrograms using a optical gating technique, GRUMPY FROG. The dynamics of the retrieved plasma frequency and damping coefficient are well captured by an extended two-temperature model incorporating a competing nonlinear interband process: at high fluences, Auger-type scattering of hot conduction electrons promotes valence band carriers, increasing the plasma frequency while accelerating hot-electron cooling and raising the damping coefficient. These results clarify the origin of anomalous high-fluence dynamics in indium tin oxide and identify a fluence-tuneable modulation dynamic with direct implications for ultrafast refractive index engineering in time-varying photonic devices and optical switching