Q. Wang Song

Optical limiting by chemically enhanced bacteriorhodopsin films.

The optical limiting properties of a thin film (~150 microm) of chemically enhanced bacteriorhodopsin under cw laser illumination is investigated. The effective nonlinearity n(2) of the film is measured with the z-scan method. Anomalous absorption at different wavelengths as a function of illumination intensity is observed.

Chemically enhanced bacteriorhodopsin thin-film spatial light modulator

An optically addressed spatial light modulator based on a thin film of chemically enhanced bacteriorhodopsin is demonstrated. Incoherent-to-coherent light conversion is achieved by exploitation of both the large shift in absorption maxima accompanying the bR → M phototransformation and the extended M-intermediate lifetime resulting from the chemical enhancement of the protein at high pH. The device exhibits a linear dynamic range of 120:1 at 514 nm and a resolution of ~100 line pairs/mm.

Self-defocusing, self-focusing, and speckle in LiNbO 3 and LiNbO 3 :Fe crystals

The characteristics of light-induced scattering in LiNbO(3) and LiNbO(3):Fe crystals are studied. The experimental and theoretical results show that the light-induced scattering for an extraordinary illumination beam consists of self-defocusing, self-focusing, and speckle. They are caused by the light-induced lensing and fluctuation of the refractive index in the crystals.

All-optical logic gates using bacteriorhodopsin films

We demonstrate all-optical logic gates using bacteriorhodop- sin (bR) film. By studying the transmission of bR film, we advance an all-light-modulated transmission mechanism of bR film: complementary suppression-modulated transmission (CSMT). When a yellow beam (568 nm) and a blue beam (412 nm) illuminate bR film, the two transmitted beams suppress mutually. Based on this mechanism, we design an all- optical operating device in which we implement all 16 kinds of double- variable binary logic operations. The intensity of an incident yellow or blue beam acts as the input to the logic gate and the transmission bears the output of the gate. We can turn this all-optical device into different states using different wavelengths and different intensity illuminations.

Determination of the refractive index of a bacteriorhodopsin film

The wavelength-dependent refractive index of a bacteriorhodopsin thin film is measured by the use of a modified critical-angle technique. The effect of the host bovine skin gelatin on the refractive index is analyzed. The measured data on the thin film can be useful for system applications. The methods and procedures are generally applicable to any optically absorbing thin films.

Modified critical angle method for measuring the refractive index of bio-optical materials and its application to bacteriorhodopsin

The critical angle technique is modified for the accurate measurement of the refractive index of bio-optical materials. Based on the analysis of reflection from the boundary of the material as a function of incident angle and polarization direction, the critical illumination angle is obtained by numerical differentiation of the reflection curve. As an example, the dispersion curve of bacteriorhodopsin is given. The measurement error and the effect of the host bovine skin gelatin on the results are analyzed.

Real-time holographic imaging with a bacteriorhodopsin film.

We investigate the potential of high-resolution and large space–bandwidth product holographic lensless imaging by using bacteriorhodopsin film as a real-time material under He–Ne laser illumination. The imaging reflectivity is found to be linearly proportional to the reading light intensity when the hologram is under short laser pulse stimulation. The time response of the recording process is characterized by two exponentially increasing functions that are related to the intensity modulation of the illumination. An image resolution of 80 lines/mm and a space–bandwidth product of ~2 × 106 are obtained.

Photochemistry in Dried Polymer Films Incorporating the Deionized Blue Membrane Form of Bacteriorhodopsin

The preparation and photochemical properties of dried deionized blue membrane (dIbR600; lambdamax approximately 600 nm, epsilon approximately 54, 760 cm-1 M-1, f approximately 1.1) in polyvinyl alcohol films are studied. Reversible photoconversion from dIbR600 to the pink membrane (dIbR485; lambdamax approximately 485 nm) is shown to occur in these films under conditions of strong 647-nm laser irradiation. The pink membrane analog, dIbR485, has a molar extinction coefficient of approximately 39,000 cm-1 M-1 (f approximately 1.2). The ratio of pink --> blue and blue --> pink quantum efficiencies is 33 +/- 5. We observe an additional blue-shifted species (dIbR455, lambdamax approximately 455 nm) with a very low oscillator strength (f approximately 0.6, epsilon approximately 26,000 cm-1 M-1). This species is the product of fast thermal decay of dIbR485. Molecular modeling indicates that charge/charge and charge/dipole interactions introduced by the protonation of ASP85 are responsible for lowering the excited-state all-trans --> 9-cis barrier to approximately 6 kcal mol-1 while increasing the corresponding all-trans --> 13-cis barrier to approximately 4 kcal mol-1. Photochemical formation of both 9-cis and 13-cis photoproducts are now competitive, as is observed experimentally. We suggest that dIbR455 may be a 9-cis, 10-s-distorted species that partially divides the chromophore into two localized conjugated segments with a concomitant blue shift and decreased oscillator strength of the lambdamax absorption band.

Bacteriorhodopsin based photonic logic gate and its applications to grey level image subtraction

A novel photonic logic gate model is presented based on a thin film containing the light-controlled absorption of the biological photochrome bacteriorhodopsin (BR). Under illumination by two beams of wavelength around 568 and 412 nm, respectively, there exists a threshold intensity around which the transmission of the two beams are modulated in a complementary fashion. The local transmission within the film depends on the ratio of the forward and backward photoreaction rates which can be controlled by either light illumination or material modification. An example demonstrating the logic operation of the device is given for a possible practical application of an incoherent two-dimensional multiple grey level image subtraction.

The intensity-dependent refractive index of chemically enhanced bacteriorhodopsin

Abstract The intensity-dependent refractive index of a chemically enhanced bacteriorhodopsin film is shown to be composed of two components. The first arises from the shift in absorption band accompanying the bR → M phototransformation and follows the predictions made by the Kramers-Kronig transformation. An additional modulation is observed at high laser intensities and is the likely result of a thermal lensing effect induced by laser heating. This study provides for the first time new insight and quantitative descriptions of the regimes of the refractive index modulation in bacteriorhodopsin and is helpful in understanding the potential use of the BR refractive index modulation in optical applications.