Evaluation of third order nearly resonant optical nonlinearity of dye-doped films

由于染料掺杂聚合物薄膜易于制备，并且在较低功率的连续激光下可以观察到非线性光学现象，因此它们被应用于光学信息处理。我们用几种测量染料掺杂薄膜的三阶非线性系数的方法：(1)透过率随平面光束和高斯光束的抽运功率的变化(z扫描法)，(2)近简并两波混频的自衍射(移动光栅法)和(3)用标准位相共轭干涉法测量简并四波混频的三阶非线性系数。测量泵浦透过率的方法简单，适用于三阶非线性系数虚部的估计。我们认为最合适的方法是移动光栅法，它适用于三阶极化率的实部和虚部。较高的S/N测量有望改善微弱信号的检测。位相共轭干涉术的实用性正在研究中。对于吸光度较大的染料掺杂薄膜，z扫描法在评价光学非线性方面的简单性较差。

As dye-doped polymer films can be easily prepared and nonlinear optical phenomena are observed at relatively low powers of cw lasers, they are applied for optical information processings. It is desirable to develop the simple evaluation of the nearly resonant optical nonlinearity of dye-doped films.We evaluated the third order nonlinear susceptibility of the dye-doped films with several measuring methods of (1) the transmittance as a function of the pump powers of the plane beam and the Gaussian beam (z-scanning method), (2) self diffraction by nearly degenerate two-wave mixing (moving grating method) and (3) phase conjugate interferometry with a standard phase conjugator by degenerate four-wave mixing.The method of measuring the pump-transmittance is a simple one and applicable for the evaluation of the imaginary part of the third order susceptibility. We believe that the most suitable method is the moving grating one which is applied for real and imaginary parts of the third order susceptibility. The higher S/N measurement is expected to improve the weak signal detection. The usefulness of the phase conjugate interferometry is under study. For the dye-doped films with large absorbance, the z-scanning method is weak in the simplicity of the evaluation of the optical nonlinearity.