  We report a switchable time-varying mirror, composed of an ITO-Au stack,
which can be efficiently modulated in time with over a ten-fold increase in
reflectivity, with a change of 0.6. Upon interacting with the time-varying
mirror, the frequency content of a reflected pulse is extended to 31 THz. This
originates from the shortening of the response time of the mirror beyond
saturation, as confirmed by a time-varying model and by further four-wave
mixing experiments. A temporal response unbounded by the pump bandwidth opens
new avenues for spectral manipulation from time-varying systems with impact for
communication networks, optical switching and computing.
  We report a switchable time-varying mirror, composed of an ITO-Au bilayer,
displaying a ten-fold modulation of reflectivity ($\Delta R \approx 0.6$),
which saturates for a driving pump intensity $I_{\mathrm{pump}}\approx
100$~GW/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 sub 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 opens new avenues for spectral
manipulation from time-varying systems with impact for communication networks,
optical switching and computing.
