Francisco J. ARANDA,¹ Devulapalli V.G.L.N. RAO,¹ Chi L. WONG,¹ Ping ZHOU,¹ Zhong CHEN,² Joseph A. AKKARA,² David L. KAPLAN² and Joseph F. ROACH²

¹Department of Physics, University of Massachusetts at Boston, Boston, MA 02125, USA, ²U.S. Army Natick Research, Development and Engineering Center, Natick, MA 01760, USA

(Accepted February 22, 1995)

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.

Key words : bacteriorhodopsin, Z-scan, nonlinear, optics