Fig. 2: Synthetic motion from a single, continuous reflectivity modulation.
a We study space-time diffraction from synthetically moving reflectivity modulations using a pump-probe experiment, featuring one of two pump beam lines (red) and a central probe line (green) with non-degenerate angles of incidence θi, determining synthetic velocities on the surface. The space-time diffraction of the probe beam is measured by the angular dependence (δθ) of the spectrum diffracted about the reflected probe. b Sketches of the space-time profile of an idealised moving reflectivity slit (blue) and of the probe beam (green), along with their synthetic velocities (dashed lines). c The experimental diffraction efficiency (red crosses) of the probe from a single moving modulation as a function of pump-probe delay δt, defined as the ratio of the scattered to the initial, unmodulated probe power. d, e The frequency-momentum hyperspectral plots of the diffracted probe power density for the case of a slit that travels slower (vr = 0.95 vp, top row) and faster (vr = 1.08 vp, bottom row), than the probe, taken at the delay of maximum efficiency (shaded grey region in c). The sketches illustrate the elongated nature of both pump and probe beams, their relative size, and their incident angles, leading to measurable signatures of synthetic motion. The correlation between frequency and momentum depends on the relative speed of probe and modulation, in excellent agreement with analytical theory and numerical simulations (white dashed lines and colormaps on the right).
