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.

Proteins are large biomolecules in the form of polypeptide chains consisting of amino acid (AA) residues. Ultraviolet–visible absorption spectroscopy and continuous wave (CW) z-scan of bovine serum albumin (BSA) and some of its constituent AAs were examined to deduce the relationship between the optical properties of this protein molecule and its constituents. From the analysis of their optical spectra, the absorption at 278 nm by BSA is found to be the outcome of the cumulative effects of the absorptions by constituent aromatic AA residues, cysteine disulfide bonds, and methionine. Similarly, the closed aperture CW z-scan of BSA and those of the constituent AAs at 74–106 mW incident optical power at 655 nm indicate that thermally generated third-order optical effects arise in BSA and its aromatic AA residues due to multiphoton absorptions. The nonlinear optical (NLO) responses of BSA and those of the AA residues are compared in terms of their molar phase shift per unit power, which indicate a possible relationship between the NLO property of BSA and its AA residues.