Fig. 6: Schematic diagram of an optical set-up for far-field intensity distribution measurements.

Two divergent beams (pump in blue and probe in red) illuminate from the left the phase mask situated in the plane z = 0, positioned after the lens (L). Each beam has its own radius of curvature which causes different beam radius at the phase mask plane. Due to the Talbot effect, transient grating excitation is formed on the sample (placed at a distance z₀ from the phase mask). The far-field diffraction pattern is recorded by a two-dimensional position-sensitive detector, which is situated on the distance Z from the phase mask. The coordinate system is the following: ξ represents the in-plane coordinate of the grating (z = 0), η denotes the coordinate at the sample plane (z = z₀), and x is the coordinate at the plane of the position-sensitive detector (z = Z).