The closed aperture continuous wave Z-scan of L-tryptophan at 661 nm was performed to investigate the variation of on-axis nonlinear phase shift $({\Delta {\Phi _0}})$ with the change of optical field strength. $\Delta {\Phi _0}$ was found to vary nonlinearly with irradiance (${I_0})$ in the range from 150 to ${{290}}\;{\rm{\;{\rm MW}/}}{{\rm{m}}^2}$. This nonlinear variation is explained by considering the effect of linear and nonlinear absorption of radiation on the thermo-optical refractive index. Using the quadratic fitting of $\Delta {\Phi _0}$ with ${I_0}$, we have found the thermo-optic coefficient of refractive index $dn/dT$, thermal coefficient of nonlinear refractive index $n_2^T$, and the nonlinear absorption coefficient $\beta$ in the observed power regime.

中文翻译：

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L-色氨酸的闭孔 CW Z 扫描，用于确定热状态中的光学非线性

进行L-色氨酸在661 nm处的闭孔径连续波Z扫描，以研究轴上非线性相移$({\Delta {\Phi_0}})$随光场强度变化的变化。发现 $\Delta {\Phi _0}$在 150 至${{290}}\;{\rm{\;{\rm MW}/}范围内随辐照度 ( ${I_0})$呈非线性变化}{{\rm{m}}^2}$。这种非线性变化是通过考虑辐射的线性和非线性吸收对热光折射率的影响来解释的。使用$\Delta {\Phi _0}$与${I_0}$的二次拟合，我们已经找到了折射率的热光系数$dn/dT$，非线性折射率的热系数$n_2^T$，以及在观察到的功率范围内的非线性吸收系数$\beta$。