Bacteriorhodopsin as a Photochromic Retinal Protein for Optical Memories.

Technology which makes use of biological materials has advanced dramatically in the last few decades. Production of specific biochemicals by selected microbial strains, the use of enzymes for stereospecific biosynthesis of materials and gene technological production of biologically important macromolecules are a few examples of these developments.

Bacteriorhodopsin: a biological material for information processing

Films of oriented bacteriorhodopsin have been formed in polyvinyl alcohol with excellent optical quality. Images with high contrast have been impressed and erased on these films. Second-harmonic microscopy has been used to read the image on a bacteriorhodopsin–polyvinyl alcohol film without erasure. The potential of these films for molecular information storage and computation is discussed.

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Bacteriorhodopsin oriented in polyvinyl alcohol films as an erasable optical storage medium

The sign of the optoelectrical response of bacteriorhodopsin is highlighted as a means to emulate excitation and inhibition in neural computation. A classic example of a neural computation that is based on such excitation and inhibition is chosen to highlight the unique applicability of bacteriorhodopsin in highly parallel computational schemes. The classic example chosen is that of the ganglion receptive field, which is a fundamental element in retinal edge detection. Dried bacteriorhodopsin films are constructed that effectively act as receptive fields because of the sign of their photoresponse. The results on these simple bacteriorhodopsin receptive fields are extended to schemes that incorporate with greater elegance this unique ability of bacteriorhodopsin to exhibit excitation and inhibition. Experiments are presented that test some of these advanced ideas in bacteriorhodopsin parallel computation.

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Implementing receptive fields with excitatory and inhibitory optoelectrical responses of bacteriorhodopsin films.

The optical limiting properties of a thin film (~150 μm) of chemically enhanced bacteriorhodopsin under cw laser illumination is investigated. The effective nonlinearity n2 of the film is measured with the z-scan method. Anomalous absorption at different wavelengths as a function of illumination intensity is observed.

Optical limiting by chemically enhanced bacteriorhodopsin films

Bacteriorhodopsin exhibits strong nonlinear and saturation behavior with very low light intensities (~10 mW/cm(2)). A model and an experimental study of the saturation are described. Four-wave mixing experiments are used for obtaining phase conjugation and information on the holographic resolution of the material. Two kinds of samples were prepared and used in the experiments: films of bacteriorhodopsin in a solid polyvinylalcohol (polymer) matrix and in water.

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Isaiah Nebenzahl

Papers

Saturable absorption, wave mixing, and phase conjugation with bacteriorhodopsin.