Absorption of ultrashort, ultra-intense laser light by solids and overdense plasmas

Absorption mechanisms for ultra-intense (I>10/sup 17/ W/cm/sup 2/) laser pulses incident on solids and overdense plasma slabs are discussed. We focus on the ultrashort pulse regime, i.e., where the laser pulse length is only a few to perhaps thousands of femtoseconds. Starting from well-known results at low intensity and long pulse length, we begin with absorption mechanisms such as inverse Bremstrahlung and classical resonance absorption and survey several additional absorption mechanisms significant 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 energy absorbed by the plasma, and the resulting electron temperatures, can depend considerably on the scale length of the plasma at the critical surface. It is also found that two-dimensional (2-D) effects greatly increase the amount of absorption into hot electrons, over the amount predicted using one-dimensional (1-D) theory. The inclusion of kinetic effects, collisionless absorption, and multidimensional effects are crucial to obtaining a complete picture of the interaction. We also review some of the experimental efforts to understand this complex process of absorption.