Visible Harmonic Emission as a Way of Measuring Profile Steepening

First observed visible harmonics of CO₂-laser---irradiated plane and microballoon targets are reported. For intensities >510^14 W/cm^2, the harmonic production efficiency is constant over several visible harmonics with a cutoff at a high harmonic number. Two-dimensional particle simulations performed where there is a highly steepened density profile show a flat spectrum for high harmonics with a cutoff at the harmonic where the upper density shelf is underdense. Harmonics thus offer a means of measuring the upper density shelf and its dynamics.