Second harmonic generation

Second harmonic generation (also called frequency doubling or abbreviated SHG) is a nonlinear optical process, in which two photons with the same frequency interacting with a nonlinear material are effectively "combined" to generate new photons with twice the energy, and therefore twice the frequency and half the wavelength of the initial photons. Second harmonic generation, as an even-order nonlinear optical effect, is only allowed in media without inversion symmetry.

It is a special case of sum frequency generation. Half-harmonic generation (a special case of spontaneous parametric down-conversion) is its reverse process where one photon leads to a pair of photons with half the energy, and occurs in parallel of the SHG with a lower probability though. The second-order non-linear susceptibility of a medium is responsible for the creation of SHG, which can convert a small or a large part of the excitation wave depending on some interference conditions that can happen in the medium.

Generating the second harmonic, often called frequency doubling, is also a process in radio communication; it was developed early in the 20th century, and has been used with frequencies in the megahertz range. It is a special case of frequency multiplication.

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