ABSORPTION OF ULTRASHORT, ULTRA-INTENSE LASER-LIGHT BY SOLIDS AND OVERDENSE PLASMAS

Absorption mechanisms for ultra-intense (I > 10(17) W/cm(2)) laser pul ses incident on solids and overdense plasma slabs are discussed, We fo cus on the ultrashort pulse regime, i.e., where the laser pulse length is only a few to perhaps thousands of femtoseconds. Starting from wel l-known results at low intensity and long pulse length, we begin with absorption mechanisms such as inverse Bremstrahlung and classical reso nance absorption and survey several additional absorption mechanisms s ignificant for ultrashort, ultra-intense laser light interacting with overdense plasmas, Estimates for the fraction of laser energy absorbed by various mechanisms are given, It is found that the fraction of ene rgy absorbed by the plasma, and the resulting electron temperatures, c an depend considerably on the scale length of the plasma at the critic al surface, It is also found that two-dimensional (2-D) effects greatl y increase the amount of absorption into hot electrons, over the amoun t predicted using one-dimensional (1-D) theory. The inclusion of kinet ic effects, collisionless absorption, and multidimensional effects are crucial to obtaining a complete picture of the interaction, We also r eview some of the experimental efforts to understand this complex proc ess of absorption.