Saturable Time-Varying Mirror Based on an Epsilon-Near-Zero Material

Abstract

We report a switchable time-varying mirror, composed of an indium-tin-oxide--gold bilayer, displaying a tenfold modulation of reflectivity ($\mathrm{\ensuremath{\Delta}}R\ensuremath{\approx}0.6$), which saturates for a driving-pump intensity ${I}_{\mathrm{pump}}\ensuremath{\approx}100\phantom{\rule{0.2em}{0ex}}\mathrm{GW}/{\mathrm{cm}}^{2}$. Upon interacting with the saturated time-varying mirror, the frequency content of a reflected pulse is extended up to 31 THz, well beyond the pump spectral content (2.8 THz). We interpret the spectral broadening as a progressive shortening of the mirror rise time from 110 fs to below 30 fs with increasing pump power, which is confirmed by four-wave-mixing experiments and partially captured by a linear time-varying model of the mirror. A temporal response unbounded by the pump bandwidth enables applications for spectral manipulation from time-varying systems with impact for communication networks, optical switching, and computing.