  Ultrafast control of light-matter interactions constitutes a crucial feature
in view of new technological frontiers of information processing. However,
conventional optical elements are either static or feature switching speeds
that are extremely low with respect to the timescales at which it is possible
to control light. Here, we exploit high-quality-factor engineered
epsilon-near-zero (ENZ) modes of a metal-insulator-metal nanocavity to realize
an all-optical ultrafast modulation of the reflectance of light at a tailored
wavelength. Our approach is based on the presence of the two, spectrally
separated, ENZ absorption resonances of the cavity. Optical pumping of the
system at its high energy ENZ mode leads to a strong red-shift of the low
energy mode because of the transient increase of the local dielectric function,
which leads to a sub-3-ps control of the reflectance at a specific wavelength
with a relative modulation depth approaching 120%.
