the process by which two photons with frequencies ν2 and ν3 combine in a non-linear medium to produce a higher energy photon with frequency ν1 such that ν1=ν2+ν3, protsess, mille puhul kaks footonit sagedustega ν2 ja ν3 mittelineaarses keskkonnas kombineeruvad, moodustades kõrgema energiaga footoni sagedusega ν1=ν2+ν3