Second harmonic generation at a time-varying interface

TL;DR

This paper investigates second harmonic generation at a time-varying interface, revealing significant enhancement effects due to nonlinear susceptibility modulation, with potential applications in optical computing and sensing.

Contribution

It demonstrates non-perturbative effects on harmonic generation at a time-varying interface, highlighting the role of nonlinear susceptibility modulation in enhancement.

Findings

Up to 93% modulation contrast enhancement at second harmonic wavelength

Significant contribution from temporal modulation of nonlinear susceptibility

Enhanced frequency shift, broadening, and modulation depth in time diffraction spectra

Abstract

Time-varying metamaterials rely on large and fast changes of the linear permittivity. Beyond the linear terms, however, the effect of a non-perturbative modulation of the medium on harmonic generation and the associated nonlinear susceptibilities remains largely unexplored. In this work, we study second harmonic generation at an optically pumped time-varying interface between air and a 310 nm Indium Tin Oxide film. We observe an enhancement of the modulation contrast at the second harmonic wavelength, up to 93% for a pump intensity of 100 GW/cm$^2$, leading to large frequency broadening and shift. We demonstrate that, in addition to the quadratic dependence on the fundamental field, a significant contribution to the enhancement comes from the temporal modulation of the second order nonlinear susceptibility, whose relative change is double that of the linear term. Moreover, the spectra resulting from single and double-slit time diffraction show significantly enhanced frequency shift, broadening and modulation depth, when compared to the infrared fundamental beam, and could be exploited for optical computing and sensing. Enhanced time-varying effects on the harmonic signal extends the application of materials to the visible range and calls for further theoretical exploration of non-perturbative nonlinear optics.