I heard once that with a special crystal it's possible to "convert" somehow two green photons to a ultraviolet one and that some UV-lasers are based on this fact. Is this right, and even more important: Does anyone have a keyword I can use for some additional research? My (german-language-based) research only led to some similar effects.
There is an effect called harmonic generation where two photons of similar wavelength are combined together into a single higher wavelength one in a nonlinear optical crystal.
The effect is used in DPSS 532nm green lasers for converting IR light from an ND:YaG crystal into green light by using passing the IR light into an KTP crystal.
There also exists a process called photon upconversion whereby multiple photons of lesser wavelength are absorbed to produce a photon of shorter wavelength
As for key-words for research, nonlinear optics, harmonic generation, photon upconversion,