Introduction to Nonlinear Optics: SHG, OPO, SPDC and Phase Matching for Quantum Optics

In short Nonlinear optics starts where intensity matters. At high intensity a material’s polarisation is no longer proportional to the field, and the χ⁽²⁾ and χ⁽³⁾ terms generate new frequencies. Four processes do most of the work: second-harmonic generation (SHG), sum- and difference-frequency generation (SFG, DFG) and optical parametric oscillation (OPO). Phase matching decides efficiency. […]
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