https://doi.org/10.1351/goldbook.08686
Nonlinear optical technique in which pulsed lasers at frequencies \(\omega_{1}\) and \(\omega_{2}\) are overlapped at a surface and light emitted at the sum of the two frequencies, \(\omega_{1} + \omega_{2}\), is detected. One laser is normally in the mid-infrared and the other in the visible or near-infrared.