Second-harmonic generation (SHG) and third-harmonic generation (THG) processes are both nonlinear processes, related to the interaction of intense light with matters. SHG process describes the generation of light wave that is twice the frequency (with half of the original wavelength) of the original one while THG process describes the generation of light wave that triples the frequency (with one third of the original wavelength) of the original one. The harmonic light wave generation is coupled from the excited nonlinear polarization P ^NL under intense laser excitation. The interaction of nonlinear polarization P ^NL and the excitation light is usually related through a nonlinear susceptibility χ, as previously described in Chaps. 9 and 10.