Half-harmonic generation
Half-harmonic generation (also called wavelength doubling or frequency halving) is a nonlinear optical process in which photons "split" to generate pairs of new photons with half the energy, therefore half the frequency and twice the wavelength of the initial photons. The half-harmonic generation process is the inverse of second-harmonic generation and can occur in optical parametric oscillators at degeneracy, and is a phase- and frequency-locked down-conversion process.