  The wave nature of light is revealed by diffraction from physical structures.
We report a time-domain version of the classic Young's double-slit experiment:
a beam of light twice gated in time produces an interference in the frequency
spectrum. The 'time slits', narrow enough to produce diffraction at optical
frequencies, are generated from a thin film of Indium-Tin-Oxide illuminated
with high-power infrared pulses, inducing a fast reflectivity rise, followed by
a slower decay. Separation between the time slits determines the period of
oscillations in the frequency spectrum, while the decay of fringe visibility in
frequency reveals the shape of the time slits. Here we find a surprise: many
more oscillations are visible than expected from existing theory, implying a
rise time for the leading edge of around 1-10 fs, approaching an optical cycle
of 4.4 fs. This is over an order of magnitude faster than the width of the pump
and can be inferred from the decay of the frequency oscillations.
