Fig. 5.

Verification of harmonic production through saturation. (a) Profiles through images of a thin fluorescent layer illuminated by a Gaussian laser beam modulated by sinusoidal stripes with a period of 2.5 μm, with a peak energy density per pulse of 0.58 mJ/cm² (bottom curve) and 37 mJ/cm² (top curve). The bottom curve approximately follows the sinusoidal illumination pattern, because the peak energy density is below the saturation regime. In the top curve, the higher pulse energy causes fluorescence to saturate near the peaks of the pattern, leading to an asymmetric curve form with broad peaks and sharp valleys. (That the observed valleys do not reach zero is expected because of blurring by the observation optics and does not necessarily imply that the actual intensity minima are nonzero.) (b) Fourier transforms corresponding to the profiles shown in a showing five detectable harmonics in the high-energy pattern (arrows). Only the lowest harmonic is detectable in the low-energy pattern (arrowhead). The vertical axis is logarithmic (base 10); the curves in b have been separated vertically for clarity.