Evidence in the second-harmonic emission for self-focusing of a laser pulse in a plasma

Abstract
Short‐pulse (300 psec), high‐intensity (1014−1015 W/cm2) Nd‐laser light was propagated into variable scale length plasmas (Lnn/∇n=200–400 μm at 0.1 critical density) preformed by long‐pulse (4 nsec), low‐intensity (≂6×1012 W/cm2) irradiation of planar targets. For high short‐pulse intensities (≥5×1014 W/cm2), time‐integrated images show filament‐shaped regions of second‐harmonic (2ω0) emission from the low density (0.01≤ne/nc≤0.2) region of the ablation plasma. Two‐dimensional computer calculations of the hyrodynamics and laser beam propagation indicate that these filaments are consistent with ponderomotive self‐focusing of the short pulse. A theoretical model that explains the 2ω0 generation mechanism within low‐density filaments is also presented.