Ralph Thoma

Bacteriorhodopsin films as spatial light modulators for nonlinear-optical filtering

The application of dry purple membrane films for nonlinear-optical filtering is described. The biological photochrome bacteriorhodopsin (BR) is contained in the purple membrane (PM) from Halobacterium halobium. BR has two dominant photoactive states, B and M, which have well-separated absorption bands with maxima at 570 nm (B) and 412 nm (M). Since the local transmission of a PM film depends on the ratio between the forward (B ? M) and the backward (M ? B) photoreactions, PM films can be used as light-controlled absorptive spatial light modulators. A model describing the nonlinear transmission of PM films containing the wild-type form of BR or one of its mutated variants, e.g., BR(D96N), and examples of their application in spatial filtering, e.g., edge enhancement, are presented.

Biological photochrome bacteriorhodopsin and its genetic variant Asp96 ? Asn as media for optical pattern recognition.

The biological photochrome bacteriorhodopsin (BR) is contained within the purple membrane (PM) of Halobacterium halobium. Artificial derivatives with improved optical properties can be generated by genetic methods and isolated from mutated halobacterial strains. The use of PM films that contain wild-type BR and BR variants as real-time recording media for various holographic applications has been reported previously, and the advantages of BR variants have been demonstrated. The high reversibility (>> 10(5) record/erase cycles), the fast time scale of its photoconversions (femtoseconds to milliseconds), and the large photochromic shift ( approximately 160 nm) occurring during its photocycle make it a promising material for real-time applications. A dual-axis joint-Fourier-transform (DA-JFT) correlator is used to demonstrate the applicability of PM films in holographic pattern recognition. One major advantage of PM films in this application is their high spatial resolution of more than 5000 lines/mm. Severe restrictions on the overall performance of the DA-JFT correlator system are caused by scattered light and result in a low signal-to-noise ratio. Since PM patches typically have a diameter in the range of the visible wavelengthsthat are used for hologram recording, light scattering is an intrinsic problem of PM films. The polarization recording properties of PM films are employed to overcome this problem. More than 20-fold improvement of the signal-to-noise ratio in the DA-JFT correlator output is obtained.

Real-time holographic correlation of two video signals by using bacteriorhodopsin films

A dual-axis joint-Fourier-transform correlator is described with two liquid-crystal television screens as input devices and a bacteriorhodopsin film as the active holographic material in the Fourier plane. The experimental data presented demonstrate that this system is capable of processing two independent video signals in real time with a signal-to-noise ratio of 45 dB.

ADAPTIVE BACTERIORHODOPSIN-BASED HOLOGRAPHIC CORRELATOR FOR SPEED MEASUREMENT OF RANDOMLY MOVING THREE-DIMENSIONAL OBJECTS

An adaptive correlator system is presented that uses a bacteriorhodopsin film as the active holographic material. Because M-type holography with bacteriorhodopsin films is used, the TV frame rate is met at fairly low intensities. No limitations arising from the heterodyne operation of the dual-axis joint-Fourier-transform correlator were observed. The systems capability to monitor autonomously the speed of an object moving randomly in the whole viewing field of a video camera is demonstrated.

Bacteriorhodopsin variants for optical information processing: A new approach in material science

By genetechnological modification of the biological photochrome bacteriorhodopsin a variety of new materials has been generated with photophysical properties adapted to the particular needs of various optical applications, like optically addressed spatial light modulators, real‐time interferometry, holographic pattern recognition and optical data storage. These materials were obtained by slight modifications of the amino acid sequence of wildtype bacteriorhodopsin. This new approach in material science illustrates the potential of gentechnological modifications of natural macromolecules in order to derive from them technically usable materials which advantageously employ their evolutionarily optimized properties.

All-optical nonlinear holographic correlation using bacteriorhodopsin films

Films made of the halobacterial photochrome bacteriorhodopsin (BR) can be used in a number of holographic real-time applications. Their application as active material in a dual-axis joint-Fourier-transform (DAJFT) real-time correlator was shown recently. The BR films have a strong nonlinear intensity dependence on the light-induced absorption and refractive-index changes. Therefore the holographic diffraction efficiency also shows a nonlinear dependence on the writing intensity. We investigate the effect of this nonlinearity on the result of the correlation process in a bacteriorhodopsin-based DAJFT correlator. Numerical models supporting the experimental observations are presented. It was found that the BR film combines the holographic function for most objects with that of a spatial bandpass filter, whose center frequency is tuned by the writing intensity. This results in smaller peak widths and a suppression of the sidelobes. BR films allow the application of this nonlinear behavior in real time to the all-optical correlation process.

Real-time holographic pattern recognition with bacteriorhodopsin films

The biological photochrome bacteriorhodopsin (BR) has attractive photophysical properties which allow its use as the photoactive component for dynamic recording media for optical applications. Purple membrane (PM) patches, which contain BR in a two-dimensional crystalline lattice, are isolated from Halobacterium halobium. Polymeric films with embedded PM are well suited reversible media for holographic recording. In addition, artificial derivatives of BR with improved optical properties can be generated by genetic methods and isolated from the mutated halobacterial strains. The high reversibility (> 106 record/erase cycles), the fast timescale of its photoconversions (fs - ms), and the high resolution (> 5000 lines/mm) make these films suitable media for real-time holographic applications. A dual-axis joint-Fourier-transform correlator is described with two liquid crystal television screens as input devices and a BR-film as active holographic material in the Fourier plane. The experimental data presented demonstrate that this system is capable of processing two independent video signals in real-time with a signal-to-noise ratio of 45 dB. The polarization recording properties of BR-films offer an efficient method to separate the correlation signal from scattered light.