Frequencydoubled

Frequencydoubled is the term used for the nonlinear optical process of second-harmonic generation (SHG). In SHG, two photons at a fundamental frequency f combine to yield a single photon at frequency 2f, effectively halving the wavelength of the light. The process relies on a material with a nonzero second-order susceptibility and requires phase matching so that the generated second-harmonic wave stays in phase with the driving polarization as light propagates through the crystal.

Achieving efficient SHG involves selecting appropriate nonlinear crystals and employing techniques such as angle tuning, temperature tuning, or quasi-phase matching (periodically poled structures) to satisfy phase-matching conditions. Common SHG materials include beta barium borate (BBO), lithium niobate (LiNbO3), lithium triborate (LBO), and potassium titanyl phosphate (KTP, including periodically poled variants). Crystal length, pump power, and optical quality, as well as thermal effects, influence conversion efficiency and damage thresholds.

A widely cited example is frequency doubling of 1064 nm light from Nd:YAG lasers to 532 nm green light, which is used in displays, spectroscopy, and biomedical imaging. SHG is also employed to generate blue and UV light from near-infrared sources and in ultrafast laser systems for pulse characterization and nonlinear microscopy.

See also nonlinear optics, second harmonic generation, quasi-phase matching, and frequency mixing.