[OPTICAL REVIEW Vol. 2, No. 3 (1995) 204-206]

Nonlinear Optical Interactions in Bacteriorhodopsin Using Z-Scan

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