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

Abstract

The closed aperture continuous wave Z-scan of L-tryptophan at 661 nm was performed to investigate the variation of on-axis nonlinear phaseshift .180/ with the change of optical fieldstrength. 180 was found to vary nonlinearly with irradiance (I0) in the range from 150 to 290 MW/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 180 with I0, we have found the thermo-optic coefficient of refractive index dn=dT, thermal coefficient of nonlinear refractive index nT2 , and the nonlinear absorption coefficient β in the observed power regime.