Nonlinear Optical Interactions in Bacteriorhodopsin Using Z-Scan

Nonlinear refractive index coefficient n₂ of bacteriorhodopsin suspensions in water is measured by the Z-scan technique with a low power continuous wave laser at 647.1 manometer wavelength. Our results indicate that both the magnitude and the sign of n₂ depend strongly on the light intensity. Negative values for n₂ are obtained for on axis laser irradiance at the focus above 3 W/cm². The observed self-defocusing phenomena can be attributed to the index change due to the light induced transition between the photochromic states. The results elucidate the origin of n₂ and offer a plausible explanation for the differences in the reported n₂ measurements.Presented at the International Commission for Optics Topical Meeting, Kyoto, 1994.