Closed aperture CW Z-scan of L-tryptophan for determination of optical nonlinearity in the thermal regime

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 ( Δ Φ 0 ) with the change of optical field strength. Δ Φ 0 was found to vary nonlinearly with irradiance ( I 0 ) in the range from 150 to 290 M W / 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 Δ Φ 0 with I 0 , we have found the thermo-optic coefficient of refractive index d n / d T , thermal coefficient of nonlinear refractive index n 2 T , and the nonlinear absorption coefficient β in the observed power regime.