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

The Keldysh theory of photoionization for solids is generalized to atomically thin two-dimensional semiconductors. We derive a closed-form formula and its asymptotic forms for a two-band model with a Kane dispersion. These formulas exhibit characteristically different behaviors from their bulk counterparts which are attributed to the scaling of the 2D density of states. We validate our formulas by comparing them to recent strong-field ionization experiments in monolayer MoS2 with good agreement. Our work is expected to find a wide range of applications in intense light - 2D material interaction.