We provide first-principles modelling of the ultrafast nonlinear optical response of low electron-density Drude materials (specifically, ITO) near their unique near-IR “epsilon-near-zero” point. We show that their 100’s-of-percent nonlinearity originates from stronger electron heating and faster cooling compared to noble metals, and unique dynamics of the chemical potential. The resulting drastic permittivity changes causes significant detunings from resonance, a rapid drop of absorptivity with increased illumination intensity and a sub-linear increase of phonon temperature with pump intensity, reaching the melting point at the experimentally-observed damage threshold. This furthers shows that the ITO nonlinearity is not electronic (“saturable”), but rather thermal.
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