Fast electron heating and thermalization using circular polarization

Simple laboratory generation of warm dense matter in a spatially homogeneous, thermalized state would be a valuable tool for studies of high-energy-density physics. We use the particle-in-cell code SMILEI to investigate the effect of Coulomb collisions and ionization on the interaction of ultra-short laser pulses with solid-density copper plasmas. In such moderately high-$Z$ materials, electron-ion collisions result in significant laser energy absorption through inverse Bremsstrahlung. We consider the case of circular polarization~(CP), which is known to lead to inefficient electron energization through collisionless mechanisms such as $j \times B$ heating. In collisional settings, however, not only is CP comparable with linear polarization~(LP) in terms of electron heating efficiency, but it also produces thermalized electrons on $<100\,\rm fs$ timescales, i.e., much faster than LP, which yields a more pronounced and longer-lived energetic electron tail. Such isochoric heating can potentially be used to study atomic physics models or equations of state under extreme conditions.