upconversion

The process by which two photons with frequencies $ν 2$ and $ν 3$ combine in a non-linear medium to produce a higher energy photon with frequency $ν 1$ such that $ν 1 = ν 2 + ν 3$

Notes:

Also known as a parametric upconversion or sum frequency generation. Upconversion is the reverse process of 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 437