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N-th harmonic generation
Harmonic generation (HG, also called multiple harmonic generation) is a nonlinear optical process in which photons with the same frequency interact with a nonlinear material, are "combined", and generate a new photon with times the energy of the initial photons (equivalently, times the frequency and the wavelength divided by ).
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 (non-centrosymmetrical).
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 . To generate efficiently, the symmetry of the medium must allow the signal to be amplified (through phase matching, for instance), and the light source must be intense and well-controlled spatially (with a collimated laser) and temporally (more signal if the laser has short pulses).
Sum-frequency generation (SFG)
A special case in which the number of photons in the interaction is , but with two different photons at frequencies and .
Second-harmonic generation (SHG)
A special case in which the number of photons in the interaction is . Also a special case of sum-frequency generation in which both photons are at the same frequency .
Third-harmonic generation (THG)
A special case in which the number of photons in the interaction is , 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 (?-BaB2O4) or LBO can convert THG, otherwise THG can be generated from membranes in microscopy.
Fourth-harmonic generation (FHG or 4HG)
A special case in which the number of photons in interaction is .
Reported around the year 2000, powerful lasers now enable efficient FHG. This process involves the 4th order nonlinear susceptibility .
Harmonic generation for (5HG) 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 (like and so on), the substantially different process of high harmonic generation can be used.