Wavelength Dependence of the Photorefractive and Photodiffractive Properties of Holographic Thin Films Based on Bacterio
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Wavelength Dependence of the Photorefractive and Photodiffractive Properties of Holographic Thin Films Based on Bacteriorhodopsin Robert R. Birge, K. Can Izgi, Jeffrey A. Stuart and Jack R. Tallent Department of Chemistry and Center for Molecular Electronics Syracuse University Syracuse, New York 13244-4100
ABSTRACT The photorefractive and photodiffractive properties of a 2 x 10-3 M, 30gim thin film of bacteriorhodopsin at - 40°C are analyzed by using optical absorption spectroscopy, the KramersKronig transformation and coupled wave theory. Conversion of M to bR generates a dispersion in the refractive index that has a broad negative band from 450 to 540 nm [An500nm - -0.0016] and a broad positive band from 590 to 700 nm [An605nm - 0.0016]. The large change in refractive index for moderate solute concentration is due to the formation of the protonated Schiff base chromophore in bR which generates a large red shift in the absorption spectrum as well as a large increase in oscillator strength. The integrated diffraction efficiency from 300 - 800nm is dominated by refractive index contributions ('iphase) which are maximum in regions of minimal bR and M absorption. The maximum in the refractive (phase) component occurs at 451 nm (flphase 9.7%) whereas the maximum in the absorption component occurs at 575 nm (rlabs - 2.2%). The maximum efficiency of diffraction is observed at -440 nm (ntotal - 10.7%). Adequate diffractive performance for most applications is predicted for write wavelengths in the regions 380 - 420 & 500 - 650 nm and for read wavelengths from 380 to 740 nm. INTRODUCTION Bacteriorhodopsin (MW - 26,000) is the light harvesting protein in the purple membrane of Halobacterium halobium [1]. This halophilic archaibacterium grows in salt marshes where the concentration of NaC1 can exceed 4 M, roughly six times higher than seawater (-0.6 M NaCL). The purple membrane, which contains the protein bacteriorhodopsin in a lipid matrix (3:1 protein:lipid), is grown by the bacterium when the concentration of oxygen becomes too low to sustain the generation of ATP via oxidative phosphorylation. Upon the absorption of light, bacteriorhodopsin converts from a dark adapted state to a light-adapted state. Subsequent absorption of light by the latter generates a photocycle which pumps protons across the membrane, with a net transport from the inside (cytoplasmic) to the outside (extraceihular) of the membranes. The resulting pH gradient (ApH - 1) generates a proton-motive force which is used by the bacterium to synthesize ATP from inorganic phosphate and ADP. There are advantages inherent in the use of biological molecules, either in their native form, or modified via chemical or mutagenic methods, as active components in optically coupled devices. These advantages derive in large part from the natural selection process, and the fact that nature has solved, through trial and error, problems similar to those encountered in harnessing organic molecules to carry out logic, switching, and data storage functions. The light harves
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