Keldysh theory for non-resonant strong-field photoionization of low-dimensional materials

Keldysh's celebrated 1964 theory of strong-field ionization in crystalline solids was the first model able to describe both non-resonant multiphoton and tunneling ionization simultaneously. Recently, we have extended this theory to monolayer 2D materials [Her et al., Optica 2025], which was applied to literature data with good agreement. In this presentation, we show additional experimental evidence to support our 2D Keldysh theory, based on wavelength dependence of femtosecond laser ablation of monolayer 2D materials, where the enhanced density of states at the band edge can significantly enhance ablation. In addition, we further extend Keldysh theory of photoionization to 1D. We derive a general closed-form formula for the ionization rate and its multiphoton and tunneling limits using a two-band model with Kane dispersion. We compare the prediction of the 1D ionization rate with those from 2D and 3D, showing that the multiphoton rate increases with reducing dimensionality, whereas the trend is opposite for tunneling ionization. We provide explanations for these behaviors based on the energy scaling of the density of states in each dimensionality. We expect that our 1D analytical formulas will be useful for describing strong-field ionization in 1D nanostructures such as nanotubes and nanorods.