Sum- and difference-frequency mixing
Created | Updated Oct 10, 2002
Introduction
Sum and difference frequency mixing are nonlinear optical processes similar in effect to harmonic generation. This entry tells a little about what they are and how they are used.
What are they?
Well, as mentioned before, they are nonlinear optical processes. Specifically they are X(2) processes, which means that they are three-wave effects1. The actual description is relatively straightforward. Whereas in second harmonic generation we have a single input, which is frequency doubled (which can be thought of as two waves at the original frequency being added together) in sum frequency mixing we have two different frequency waves, whose frequencies are added together to give a sum frequency, and in difference frequency generation we have two waves whose frequencies subtract to give a difference frequency. Simple as that, really2.
How are they used?
Well, in common with most of the nonlinear effects, they are used to generate frequencies that normal lasers cannot generate. However, in the specific case of difference frequency mixing, if two input frequencies are chosen such that they are very similar, the difference frequency can be exceptionally low, and can have a wavelength longer than that of light or even beyond the infra-red. This region of the spectrum is very difficult to reach by other means3 and so this process can be very useful.
1For explanations of χ and the various effects, see the nonlinear optics page.2Well, not that simple, but it'll do as a simple explanation3Not impossible, just extremely difficult
