Strong optical field ionisation of solids

Abstract

Population transfer from the valence to conduction band in the presence of an intense laser field is explored theoretically in semiconductors and dielectrics. Experiments performed on dielectrics exposed to an intense laser field have divulged a population dynamics between valence and conduction band that differs from that observed in semiconductors. Our paper explores two aspects of ionisation in solids. (i) Contemporary ionisation theories do not take account of the coupling between the valence and conduction bands resulting in the absence the dynamic Stark shift. Our single-particle analysis identifies the absence of the dynamic Stark shift as a possible cause for the contrasting ionisation behaviours observed in dielectric and semiconductor materials. The dynamic Stark shift results in an increased bandgap as the laser intensity is increased. This suppresses ionisation to an extent where the main population dynamics results from virtual oscillations in the conduction band population. The dynamic Stark shift mainly affects larger bandgap materials which can be exposed to decidedly higher laser intensities. (ii) In the presence of laser dressed virtual population of the conduction band, elastic collisions potentially transmute virtual into real population resulting in ionisation. This process is explored in the context of the relaxation time approximation.