Nonlinear refractive index coefficient n
2 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 n
2 depend strongly on the light intensity. Negative values for n
2 are obtained for on axis laser irradiance at the focus above 3 W/cm
2. 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 n
2 and offer a plausible explanation for the differences in the reported n
2 measurements.
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