Origin, Strength, and Speed of the Nonlinear Optical Response of Transparent Conducting Oxides to Single‐Cycle Optical Pulses

We present a first‐principles study of the nonlinear optical response of transparent conducting oxides at the nanoscale due to excitation by intense, extremely short pulses based on a density matrix framework. We identify a strong thermal nonlinearity, which is complemented with stimulated emission and excited‐state absorption; it yields a cumulative permittivity change decorated by quantum coherent oscillations. Further, rigorous calculations under far‐from‐equilibrium conditions show that electron–electron thermalization occurs within a few femtoseconds, supporting interpretations of high‐harmonic generation measurements and in agreement with a generalization of Fermi liquid theory.