Nonlinear optics: 3rd order response, coherence in optics

This lecture covers the definition of nonlinear optics, focusing on the response of matter to electromagnetic fields. It discusses the interaction of a single atom with an optical wave, the isotropic nature of the process, and the intensity-dependent refractive index. Additionally, it explores atomic polarisability, incident waves, resulting polarisations, and frequency components. The lecture also delves into the characteristic electric field strength inside the atom, nonlinear polarizability observability, and coherence in optics, emphasizing the fixed phase relationship between waves. Deeper insights are provided on temporal and spatial coherence, dephasing in dipoles, and constructive interferences. The content is supported by references to R. W. Boyd's work on nonlinear optical susceptibility.