NONLINEAR-OPTICAL STUDIES OF BACTERIORHODOPSIN

We report interesting results on nonlinear optics at low powers in bac teriorhodopsin films with applications in all-optical switching and mo dulation. Chemically stabilized films of bacteriorhodopsin in a polyme r matrix for which the lifetime of the excited M state is 3 to 4 order s of magnitude longer than that of water solutions of wild-type bR wer e used in these experiments. Due to the sensitivity of the films, very small powers of order microwatts are required for optical phase conju gation. The influence of the fast photochemical M to B transition indu ced by blue light on the saturation intensity, phase conjugate intensi ty and switching time was established. We also report our measurements of the intensity dependence of the self-focusing and self-defocusing properties of wildtype bR in water solution using the Z-scan technique with low power cw lasers at two wavelengths on either side of the abs orption band. Our measurements indicate that the sign of the nonlinear ity depends on the wavelength and the magnitude depends on the fluence of the incident laser beam. The observed self-focusing and defocusing is not due to the intrinsic electronic nonlinearity. The observations can be explained in terms of the Kramers-Kronig dispersion relation t hat relates the real and imaginary parts of the complex index of refra ction.