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

Short‐pulse (300 psec), high‐intensity (1014−1015 W/cm2) Nd‐laser light was propagated into variable scale length plasmas (Ln≡n/∇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.

[1]  P. Corkum,et al.  Self-focusing of 10. 6-. mu. m radiation in an underdense plasma , 1980 .

[2]  R. Craxton,et al.  Hydrodynamics of thermal self‐focusing in laser plasmas , 1984 .

[3]  S. Obenschain,et al.  Variation of laser absorption with plasma scale length in long-scale-length plasmas , 1984 .

[4]  R. Whitlock,et al.  Evidence from x-ray, 3/2. omega. /sub 0/, and 2. omega. /sub 0/ emission for laser filamentation in a plasma , 1981 .

[5]  Zong Qi Lin,et al.  Optical probe observations of non-uniformities in laser produced plasmas , 1981 .

[6]  O. Willi,et al.  Filamentation on laser irradiated spherical targets , 1981 .

[7]  B. Luther-Davies,et al.  Evidence of filamentation (self-focusing) of a laser beam propagating in a laser-produced aluminium plasma , 1979 .

[8]  C. Max.  Strong self‐focusing due to the ponderomotive force in plasmas , 1976 .

[9]  T. K. Chu,et al.  Measurements of electron density evolution and beam self-focusing in a laser-produced plasma , 1974 .

[10]  R. Short,et al.  Two-plasmon decay and three-halves harmonic generation in filaments in a laser-produced plasma , 1984 .

[11]  Nonlinear aspects of hydrodynamic instabilities in laser ablation , 1982 .

[12]  P. Kaw,et al.  Filamentation and trapping of electromagnetic radiation in plasmas , 1973 .

[13]  H. Pépin,et al.  Fine-scale structures in plasmas stimulated by a CO/sub 2/ laser , 1978 .

[14]  Francis F. Chen,et al.  Direct observation of laser beam filamentation in an underdense plasma , 1982 .

[15]  A. Langdon,et al.  Filamentation and subsequent decay of laser light in plasmas , 1975 .

[16]  M. J. Boyle,et al.  Evidence of localized heating in CO2‐laser‐produced plasmas , 1976 .

[17]  D. S. Bailey,et al.  Two‐dimensional ray‐trace calculations of thermal whole beam self‐focusing , 1985 .

[18]  A. Ng,et al.  Filamentation of CO/sub 2/-laser radiation in an underdense hydrogen plasma , 1979 .

[19]  A. Ghatak,et al.  V Self Focusing of Laser Beams in Plasmas and Semiconductors , 1976 .

[20]  Zhang Yanzhen,et al.  Second‐harmonic emission from laser‐plasma interactions , 1983 .

[21]  F. Tappert,et al.  Self‐trapped laser beams in plasma , 1977 .

[22]  Diana Anderson,et al.  Variational approach to nonlinear self‐focusing of Gaussian laser beams , 1977 .

[23]  R. Evans,et al.  Numerical simulation of self-focusing in laser produced plasmas , 1983 .