| Absorption mechanism for ultra-intense (I > 1017W/cm2) laser pulses incident on solid targets and overdense plasma slabs are discussed. It is focused 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 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. We also review some of the experimental efforts to understand this complex process of absorption.An analytical fluid model for the generation of the strong quasistatic magnetic fields during the normal incidence by a short ultraintense Gaussian laser pulse with a finite spot size on an overdense plasma is also proposed. The steepening of an originally homogeneous overdense plasma and the formation of an electron cavitation as the electrons are pushed inward by the laser are included self-consistently. It is shown that the appearance of the cavitation plays an important role in the generation of the quasistatic magnetic fields: the strong plasma inhomogeneities caused by the formation of the electron cavitation lead to the generation of a strong axial quasistatic magnetic field Bz. In the overdense regime, the magnitudes of the quasistatic magnetic field increases with the laser intensity while decreases with the plasma density. It is also found that in a moderately overdense plasma, highly intense laser pulses can generate magnetic fields～100MG and greater due to the transverse linear mode conversion process.
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