https://doi.org/10.1351/goldbook.OT07438
Light source similar to a laser, but based on 'non-linear optical gain' from parametric amplification rather than on stimulated emission.
Notes:
- The device is a powerful solid-state source of broadly tunable coherent radiation. It consists of a crystal, usually BBO ($\ce{BaB2O4}$), located inside an optical resonator and pumped by a very intense laser beam (typically provided by a pulsed neodymium laser or a diode laser). The pump beam (wavelength \(\lambda_{\rm{p}}\) frequency \(\nu_{\rm{p}}\)) is partially converted into two coherent beams, the signal and the idler, with wavelengths (\(\lambda_{\rm{s}}\), \(\lambda_{\rm{l}}\)) and frequencies (\(\nu_{\rm{s}}\), \(\nu_{\rm{l}}\)) such that \(\nu_{\rm{s}} + \nu_{\rm{i}} = \nu_{\rm{p}}\). By simultaneous rotation of the crystal and adjustment of the optical resonator, the wavelength of the signal beam is continuously tunable, theoretically from \(\lambda_{\rm{p}}\) to \(2 \times \lambda_{\rm{p}}\) and practically over a slightly more reduced range.
- For example, for \(\lambda_{\rm{p}} = 355\ \rm{nm}\) (3rd harmonic of a Nd:YAG laser), \(\nu_{\rm{s}}\) can be tuned from \(400\ \rm{nm}\) (with \(\lambda_{\rm{i}} \approx 3.15\ \unicode[Times]{x3BC}\rm{m}\)) up to \(600\ \rm{nm}\) (with \(\lambda_{\rm{i}} \approx 870\ \rm{nm}\)).
- This 'splitting of one photon into two photons' is the reverse of the 'sum frequency mixing' used, for instance, to generate the 3rd harmonic of a laser emission by mixing in a convenient crystal the fundamental and the frequency doubled beams (a way to get the 3rd harmonic much more efficiently than by pure frequency tripling as described under harmonic frequency generation).