Quasiphasematching

Quasiphasematching is a nonlinear optical technique used to enhance the efficiency of nonlinear optical processes, such as second-harmonic generation or frequency mixing. It relies on periodically modulating the nonlinear susceptibility of a material. In a typical nonlinear optical process, light waves interact within a material, leading to a change in their frequencies. For this interaction to be efficient, the interacting waves must remain in phase as they propagate through the material. However, dispersion in materials causes different frequencies to travel at different speeds, leading to a loss of phase matching and reduced efficiency.

Quasiphasematching overcomes this limitation by periodically reversing the sign of the nonlinear susceptibility along the propagation path. This reversal is typically achieved by periodically altering the material's properties, for example, by poling ferroelectric crystals like lithium niobate. By reversing the sign of the nonlinear polarization at specific intervals, the interacting waves are effectively rephased, allowing for continuous energy transfer and significantly enhanced nonlinear conversion efficiency. The optimal length of these periodic structures, known as the coherence length, is determined by the wavelengths of the interacting light and the material's dispersion. This technique is crucial for applications requiring efficient nonlinear frequency conversion, such as laser frequency doubling and optical parametric oscillators.