# Second Harmonic Generation and Absorption Spectra

Second Harmonic Generation and Absorption Spectra

Light of frequency shining on a nonlinear material can generate, in addition to the fundamental frequency , higher harmonics of frequencies , , , and so on. If the material is noncentrosymmetric, the dipoles produced by the valence electronic motion driven by the nonlinear polarization can produce even-order harmonic generation. The simplest even harmonic generation has double the fundamental frequency of and is known as second harmonic generation (SHG).

ω

ω

2ω

3ω

4ω

2ω

As in the linear optic case, the nonlinear susceptibility can be investigated classically using an anharmonic Lorentz model. From the model, one can calculate the resonance SHG intensity (from the absolute value of the susceptibility) as well as the SHG absorption spectrum (from the imaginary value of the susceptibility). This Demonstration shows that there are two resonance and absorption peaks at the resonance and half-resonance frequencies.