Nonlinear Susceptibility: Perturbative Expansion

This lecture covers the foundations of nonlinear optics with two-level systems, including seminal papers on semiclassical models and experiments. It delves into the dressed-state picture in the quantized field, phase-matched four-wave mixing, and the connection to Lorentz oscillator model. The computation of linear susceptibility, intrinsic and overall permutation symmetries, and identifying near-resonant terms are discussed. The lecture concludes with the application of the dressed atom concept to matter-light interactions, explaining phenomena like the Autler-Townes effect and triplet spectrum of resonance fluorescence.