downconversion

Process by which a photon with frequency $ν 1$ interacts with a non-linear medium and splits into two simultaneously emitted photons with frequencies $ν 2$ and $ν 3$ so that the energy is conserved:

$ν 1 = ν 2 + ν 3$

Notes:

Also known as a parametric downconversion.
The efficiency of the conversion process depends on the parametric gain in the non-linear material. This in turn depends on the power of the incident radiation, the photon frequencies, their indices of refraction in the material, and the non-linear 'hyper-susceptibility' of the material.

Source:

PAC, 2007, 79, 293 (Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)) on page 326