Crossover among different nonlinear absorption channels in the correlated van der Waals insulator NiPS₃

In 1964, Keldysh pioneered studies on ionization of atoms and solids in a strong subgap electromagnetic field by introducing the dimensionless Keldysh parameter to categorize the adiabaticity of the excitation process. Within this framework, multiphoton and adiabatic quantum tunneling transitions represent two asymptotic limits of nonlinear absorption. However, the precise nature of charge excitations near the Keldysh crossover remains elusive and poorly captured by existing analytical theories. In this talk, I will present transient reflectivity studies of nonlinear charge dynamics in the correlated van der Waals insulator NiPS₃. Our experiments reveal distinct absorption channels and their crossover behavior, which are analyzed in terms of the Keldysh parameter. A model calculation based on the time-dependent Gaussian variational approach that reproduces experimental observations will also be discussed.