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 (Formula presented.) 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.