Random quasi-phase-matching in bulk polycrystalline isotropic nonlinear materials.

Three-wave mixing in nonlinear materials--the interaction of two light waves to produce a third--is a convenient way of generating new optical frequencies from common laser sources. However, the resulting optical conversion yield is generally poor, because the relative phases of the three interacting waves change continuously as they propagate through the material. This phenomenon, known as phase mismatch, is a consequence of optical dispersion (wave velocity is frequency dependent), and is responsible for the poor optical conversion potential of isotropic nonlinear materials. Here we show that exploiting the random motion of the relative phases in highly transparent polycrystalline materials can be an effective strategy for achieving efficient phase matching in isotropic materials. Distinctive features of this 'random quasi-phase-matching' approach are a linear dependence of the conversion yield with sample thickness (predicted in ref. 3), the absence of the need for either preferential materials orientation or specific polarization selection rules, and the existence of a wavelength-dependent resonant size for the polycrystalline grains.