Abstract 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 expe

Abstract 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. ... In conclusion, we report a direct observation of spectral oscillations, at optical frequencies, resulting from double-slit time diffraction, the temporal analogue of the Young’s slits experiment. The measurements show a clear inverse proportionality between the oscillation period and the time slit separation. These oscillations reveal a 1-10 fs temporal dynamics of the ITO/Au bilayer, much faster than previously thought and beyond the adiabatic and linear intensity dependence assumed so far in most theoretical models (10)(11),(17), calling for a new fundamental understanding of such ultrafast non-equilibrium responses. The observation of temporal Young’s double-slit diffraction paves the way for optical realizations of time- varying metamaterials, promising enhanced wave functionalities such as nonreciprocity (18), new forms of gain (19)(20), time reversal (21)(22) and optical Floquet topology (23)(24). Double-slit time diffraction could be extended to other wave domains, e.g. matter waves (6), optomechanics (25) and acoustics (26)(27), electronics (28), and spintronics (29), with applications for pulse shaping, signal processing and neuromorphic computation. Finally, the visibility of the oscillations can be used to measure the phase coherence of the wave interacting with it, similarly to matter-wave interferometers (6).