Two-dimensional Keldysh theory for non-resonant strong-field ionization of monolayer 2D materials

The Keldysh theory of photoionization for bulk dielectrics is generalized to atomically thin two-dimensional semiconductors. We derive closed-form formulas and their asymptotic forms for a two-band model with a Kane dispersion, without and with the corrected multiphoton-order-dependent phase in the transition amplitude. We also derive selection rules related to the parity of multiphoton orders near the band edge. These results exhibit characteristically different behaviors from those of the bulk, which are explained in terms of density of states scaling in the reduced dimensionality. We validate our formulas by comparing them to recent experiments and modeling of strong-field ionization in monolayers with good agreement. Our work is expected to find a wide range of applications in intense light–2D material interaction.