以衍射理论为基础考虑双光子吸收的Z-扫描理论

以菲涅尔-基尔霍夫衍射理论为基础,建立了非线性介质对高斯光束的衍射模型,对高斯光束通过非线性介质后的传输行为进行了详细的理论推导和数值计算,从一种新的角度解释了Z-扫描现象.理论推导出了考虑双光子吸收的闭孔和开孔Z-扫描曲线的统一公式,是计算非线性折射率和双光子吸收系数的一种新方法.数值模拟计算表明,对于考虑双光子吸收的闭孔Z-扫描曲线,其结果与经典的Z-扫描理论完全一致.而对于已给定的开孔Z-扫描曲线,用该理论计算出的双光子吸收系数是经典开孔Z-扫描理论计算值的1/3,其它结论与传统Z-扫描理论完全吻合.该理论的近似条件只要求薄样品和小非线性吸收,比传统理论具有更好的准确性.

A Z-Scan Theory Based on Diffraction Theory with Consideration of Two-photon Absorption

Based on Fresnel-Kirchhoff diffraction theory, a diffraction model of a nonlinear optical medium to a Gaussian beam is built. The propagation behavior of the Gaussian beam passing through the nonlinear optical medium is theoretically deduced and numerically simulated, which can explain the Z-scan phenomenon from a new approach. From this theory, a unified equation is derived, which can describe both the closed-aperture Z-scan curve and the open-aperture Z-scan curve with consideration of the twophoton absorption. It provides a new way to calculate the nonlinear refraction index and the two-photon absorption coefficients. Numeric simulation shows that for the closed-aperture Z-scan considering the twophoton absorption, the simulation results are well consistent with the conventional theory. For a given open-aperture Z-scan curve, the two-photon absorption coefficient computed by the new Z-scan theory is 1/3 of the value fitted by the conventional Z-scan theory, whereas other conclusions are identical to the conventional Z-scan theory. This theory has better accuracy than the conventional Z-scan theory because the approximate conditions are only thin sample and small nonlinear absorption.