Second harmonic generation (also called
frequency doubling or abbreviated
SHG) is a
nonlinear optical process, in which
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 mediums without inversion symmetry. It is a special case of
sum frequency generation.