An optical frequency multiplier is a nonlinear optical device in which photons interacting with a nonlinear material are effectively "combined" to form new photons with greater energy, and thus higher frequency (and shorter wavelength).
Two types of devices are currently common: frequency doublers, often based on lithium niobate (LN), lithium tantalate (LT), potassium titanyl phosphate (KTP) or lithium triborate (LBO), and frequency triplers typically made of potassium dihydrogen phosphate (KDP).
Direct third-harmonic generation (THG, also called frequency tripling) also exists and can be used to detect an interface between materials of different excitability.
[1] Optical frequency multipliers are common in high-power lasers, notably those used for inertial confinement fusion (ICF) experiments.
In order to avoid this problem much shorter wavelengths needed to be used, and experiments on the OMEGA laser and Novette laser validated the use of frequency tripling KDP crystals to convert the laser light into the ultraviolet, a process that has been used on almost every laser-driven ICF experiment since then, including the National Ignition Facility.