Harmonic generation


Harmonic generation is a nonlinear optical process in which n photons with the same frequency interact with a nonlinear material, are "combined", and generate a new photon with n times the energy of the initial photons.

General process

In a medium having a substantial nonlinear susceptibility, harmonic generation is possible. Note that for even orders, the medium must have no center of symmetry.
Because the process requires that many photons are present at the same time and at the same place, the generation process has a low probability to occur, and this probability decreases with the order n. To generate efficiently, the symmetry of the medium must allow the signal to be amplified, and the light source must be intense and well-controlled spatially and temporally.

Sum-frequency generation (SFG)

A special case where the number of photons in interaction is n=2, but with two different photons at and.

Second-harmonic generation (SHG)

A special case where the number of photons in interaction is n=2. Also a special case of sum-frequency generation where both photons are at the same frequency.

Third-harmonic generation (THG)

A special case where the number of photons in interaction is n=3, if all the photons have the same frequency. If they have different frequency, the general term of four-wave mixing is preferred. This process involves the 3rd order nonlinear susceptibility.
Unlike SHG, it is a volumetric process and has been shown in liquids. However, it is enhanced at interfaces.

Materials used for THG

Nonlinear crystals such as BBO or LBO can convert THG, otherwise THG can be generated from membranes in microscopy.

Fourth-harmonic generation (FHG or 4HG)

A special case where the number of photons in interaction is n=4.
Reported around the year 2000, powerful lasers now enable efficient FHG. This process involves the 4th order nonlinear susceptibility.

Materials used for FHG

Some BBO are used for FHG.

Harmonic generation for n > 4

Harmonic generation for n = 5 or more is theoretically possible, but the interaction requires a very high number of photons to interact and has therefore a low probability to happen : the signal at higher harmonics will be very low, and requires very intense lasers to be generated. To generate high harmonics, a substantially different process can be used : High harmonic generation.