Frequencytripling

Frequency tripling, also called third-harmonic generation (THG), is a nonlinear optical process in which photons with angular frequency ω are converted into photons with frequency 3ω. In most transparent media THG arises from the third-order nonlinear susceptibility χ(3). In centrosymmetric materials χ(2) vanishes, so there is no competing second-harmonic contribution; THG then proceeds via the instantaneous Kerr-type response of the material. In some systems THG can also occur through cascaded second-order processes, such as a sequence of SHG and sum-frequency generation, which effectively yields a 3ω output despite a nonzero χ(2).

Efficiency depends on pump intensity, nonlinear coefficient, interaction length, and phase matching. Phase matching minimizes walk-off between the fundamental and third-harmonic waves; in bulk crystals this is often difficult at visible or near-infrared wavelengths. Techniques include birefringent phase matching and, in integrated devices, modal phase matching or dispersion engineering in waveguides and microresonators, which provide enhanced confinement and longer effective interaction.

Materials and configurations for THG include glasses such as fused silica, other optical glasses, and certain semiconductors and chalcogenide glasses with sizable χ(3). THG signals are typically generated with high-peak-power ultrafast lasers, and can be enhanced in engineered waveguides or resonators that improve confinement and phase relationships. The process is used for frequency conversion to shorter wavelengths, nonlinear optical spectroscopy, and imaging techniques such as third-harmonic generation microscopy, which can provide contrast at interfaces and material inhomogeneities.

Limitations include generally low conversion efficiency, the need for high peak powers, dispersion management requirements, and potential material damage or absorption at the harmonic wavelength.