Masato Nakashima

Enhancement of photoinduced anisotropy and all-optical switching in Bacteriorhodopsin films

Large enhancement of photoanisotropic effects is demonstrated in thin films of the biomaterial Bacteriorhodopsin by using two exciting beams of orthogonal polarization. The mechanism of the enhancement originates from optimization of direction-selected photoisomerization of the biomaterial controlled by the polarized exciting beams. The technique is applied for achieving an all-optical switch with the additional feature of output sign control.

All-optical logic gates with bacteriorhodopsin films☆

We demonstrate an all-optical switch using molecular states in a bacteriorhodopsin (bR) thin film. All-optical logic gates are implemented with wild-type and chemically stabilized films of bacteriorhodopsin using a two-color backward degenerate four-wave mixing geometry. The films are observed to be stable over a period of four years. The sensitivity of each bR film is markedly different due to the large differences in the saturation intensity which can be controlled by different means. Red light is used to form a grating due to the B to M transition and blue light is used to form a grating due to the fast photochemical transition from M to B. Each of the two wavelengths in the experimental system acts as an input to the all-optical gate and the phase conjugate signal beam bears the output of the gate.

Optical Fourier processing using photoinduced dichroism in a bacteriorhodopsin film.

The photoinduced dichroism in bacteriorhodopsin films has been investigated in view of its application for optical Fourier processing. A simple optical system for Fourier processing is demonstrated that does not require precise alignment and vibration isolation. The basic principle of operation relies on the intensity dependence of photoinduced dichroism in a bacteriorhodopsin film. Experimental results with Fourier processing are shown for edge enhancement.

Optical power limiting with photoinduced anisotropy of azobenzene films

We study the power-limiting properties of photoanisotropic azobenzene films with low-power laser. The trans-cis photoisomerization and molecular reorientation of azobenzene molecules induced by polarized laser beams result in intensity-dependent anisotropic effects. Consequently, the transmittance of the input beam that passes through the film between two crossed polarizers becomes enhanced at low intensities and clamped at high intensities. The limiting threshold is adjustable by changing the intensity of excitation beam.

Processing of medical images using real-time optical Fourier processing

Optical image processing techniques are inherently fast in view of parallel processing. A self-adaptive optical Fourier processing system using photoinduced dichroism in a bacteriorhodopsin film was experimentally demonstrated for medical image processing. Application of this powerful analog all-optical interactive technique for cancer diagnostics is illustrated with two mammograms and a Pap smear. Microcalcification clusters buried in surrounding tissue showed up clearly in the processed image. By playing with one knob, which rotates the analyzer in the optical system, either the microcalcification clusters or the surrounding dense tissue can be selectively displayed. Bacteriorhodopsin films are stable up to 140 degrees C and environmentally friendly. As no interference is involved in the experiments, vibration isolation and even a coherent light source are not required. It may be possible to develop a low-cost rugged battery operated portable signal-enhancing magnifier.

Optical implementation of the wavelet transform by using a bacteriorhodopsin film as an optically addressed spatial light modulator

An optical system utilizing the photoinduced dichroism in a bacteriorhodopsin film has been demonstrated for the optical implementation of wavelet transforms. The dichroism, induced by the image of a wavelet filter on a bacteriorhodopsin film leads to polarization rotation of the Fourier components of an image. The polarization-rotated Fourier components of an input scene are analyzed with a polarizer to give the wavelet transform components. The dichroism is induced with beams whose profiles are determined by wavelet filters in order to perform the optical wavelet transform.

Transient optical modulation with a disperse-red-1–doped polymer film

Transient optical modulation properties of a disperse-red-1-doped polymer film are investigated by use of four-wave mixing with two-color light. Interesting behavior is observed for the transient modulation on variation of the two-color light intensities because of competition between the two mechanisms of one-photon (reorientation) and biphoton (isomerization) gratings. Positive and negative modulation as well as double-frequency modulation relative to blue-light input is observed in the experiment. The results may have applications in optical information processing.

Optical power limiting in ZnO crystals

We report optical power limiting in undoped single crystals of the wide band-gap semiconductor Zinc Oxide (ZnO). The ZnO crystals were grown at the Sensors Directorate of the Air Force Research laboratory using the hydrothermal method. The crystals are of high optical quality and are transparent in the visible part of the spectrum. The contributions of the real and imaginary parts of the third-order nonlinear optical susceptibility were studied using the Z-scan technique. The Z-scan experiments were carried out using picosecond pulses at 532 nm wavelength with the electric field polarization oriented orthogonal to the crystal c- axis. The nonlinear refractive index of the crystals is found to be negative. The ZnO samples exhibit two photon absorption and high damage thresholds. Our results indicate that the ZnO crystals may be good candidates for applications in optical power limiting and pulse stabilization.

Spatial light modulation with an azobenzene-doped polymer by use of biphotonic holography

Spatial light modulation by use of a biphotonic holographic grating has been demonstrated in an azobenzene-doped polymer film. The mechanism of incoherent-to-coherent optical conversion originates from the photoinduced trans <--> cis isomerization of azobenzene molecules by two color lights. It is also found that when the intensity of the blue beam is varied the output replica of the input information changes sign.

Optical power limiting in some azulenic compounds

A series of azulenic compounds are synthesized and their third-order nonlinear optical properties are determined. The compounds are studied in solution using degenerate four wave mixing and z-scan at 532 nm with 30 ps pulses. Extrapolated values for the third-order nonlinear optical susceptibility (chi) (3) of the compounds are in the range 10-9 to 10-8 esu. Z-scan results indicate reverse saturable absorption. Optical power limiting with a f/5 system is demonstrated for all of the compounds studied.