  Materials with a spatially uniform but temporally varying optical response
have applications ranging from magnetic field-free optical isolators to
fundamental studies of quantum field theories. However, these effects typically
become relevant only for time-variations oscillating at optical frequencies,
thus presenting a significant hurdle that severely limits the realisation of
such conditions. Here we present a thin-film material with a permittivity that
pulsates (uniformly in space) at optical frequencies and realises a
time-reversing medium of the form originally proposed by Pendry [Science 322,
71 (2008)]. We use an optically pumped, 500 nm thick film of epsilon-near-zero
(ENZ) material based on Al-doped zinc oxide (AZO). An incident probe beam is
both negatively refracted and time-reversed through a reflected
phase-conjugated beam. As a result of the high nonlinearity and the refractive
index that is close to zero, the ENZ film leads to time reversed beams
(simultaneous negative refraction and phase conjugation) with near-unit
efficiency and greater-than-unit internal conversion efficiency. The ENZ
platform therefore presents the time-reversal features required e.g. for
efficient subwavelength imaging, all-optical isolators and fundamental quantum
field theory studies.
