Baudrier-Raybaut M, Haïdar R, Kupecek Ph, Lemasson Ph, Rosencher E
DMPH and DOTA/ONERA, Office National d'Etudes et de Recherches Aérospatiales, Chemin de la Hunière, 91761 Palaiseau, France.
Nature. 2004 Nov 18;432(7015):374-6. doi: 10.1038/nature03027.
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.
非线性材料中的三波混频——两束光波相互作用产生第三束光波——是一种利用普通激光源产生新光频率的便捷方法。然而，由此产生的光转换效率通常较低，因为三束相互作用的光波在通过材料传播时，它们的相对相位会不断变化。这种现象被称为相位失配，是光色散（波速与频率有关）的结果，也是各向同性非线性材料光转换潜力低的原因。在此，我们表明，利用高透明多晶材料中相对相位的随机运动可以成为在各向同性材料中实现高效相位匹配的有效策略。这种“随机准相位匹配”方法的独特之处在于，转换效率与样品厚度呈线性关系（参考文献3中有预测），无需优先的材料取向或特定的偏振选择规则，并且多晶粒存在与波长相关的共振尺寸。
