通过测量后向反射波与入射波的强度比，可以确定介质的三阶极化率。

Third-order susceptibility has been determined by the ratio of the input probing beam in tensity to the backward wave intensity in four-wave mixing.

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文中解析地导出此情况下等离子体介质的三阶非线性极化率，并讨论其位相匹配条件。

The third-order nonlinear susceptibilities of the plasma are derived analytically and the condition to satisfy the phase matching is discussed.

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