Thorsten Juchem

Bacteriorhodopsin as a high-resolution, high-capacity buffer for digital holographic measurements

Recent trends in optical metrology suggest that, in order for holographic measurement to become a widespread tool, it must be based on methods that do not require physical development of the hologram. While digital holography has been successfully demonstrated in recent years, unfortunately the limited information capacity of present electronic sensors, such as CCD arrays, is still many orders of magnitude away from directly competing with the high-resolution silver halide plates used in traditional holography. As a result, present digital holographic methods with current electronic sensors cannot record object sizes larger than several hundred microns at high resolution. In this paper, the authors report on the use of bacteriorhodopsin (BR) for digital holography to overcome these limitations. In particular, BR is a real-time recording medium with an information capacity (5000 line-pairs/mm) that even exceeds high resolution photographic film. As such, a centimetre-square area of BR film has the same information capacity of several hundred state-of-the-art CCD cameras. For digital holography, BR temporarily holds the hologram record so that its information content can be digitized for numeric reconstruction. In addition, this paper examines the use of BR for optical reconstruction without chemical development. When correctly managed, it is found that BR is highly effective, in terms of both quality and process time, for three-dimensional holographic measurements. Consequently, several key holographic applications, based on BR, are proposed in this paper.

Multifunctional optical security features based on bacteriorhodopsin

Bacteriorhodopsin (BR), a photochromic retinal protein, has been developed into a new materials platform for applications in anti-counterfeiting. The combination of three different properties of the material on its molecular level, a light-inducible color change, photochemical data storage and traceability of the protein due to molecular marker sequences make this protein a promising material for security applications. The crystalline structure of the biopigment combines these properties with high stability. As BR is a biological material specialized knowledge for modification, cost- effective production and suitable processing of the material is required. Photochromic BR-based inks have been developed for screen printing, pad printing and ink jet printing. These prints show a high photochromic sensitivity towards variation of illumination. For this reason it is not possible to reproduce the dynamic color by photocopying. In addition to such visual inspection the printed symbols offer the possibility for digital write-once-read-many (WORM) data storage. Photochemical recording is accomplished by a two-photon process. Recording densities in a range from 106 bit/cm2 to 108 bit/cm2 have been achieved. Data structures are stored in a polarization sensitive mode which allows an easy and efficient data encryption.

Biomolecular optical data storage and data encryption

The use of bacteriorhodopsin (BR) as an active layer in write-once-read-many optical storage is presented. This novel feature of BR materials may be used on a wide variety of substrates, among them transparent substrates but also paper and plastics. The physical basis of the recording process is polarization-sensitive two-photon absorption. As an example for this new BR application, an identification card equipped with an optical recording strip is presented, which has a capacity of about 1 MB of data. The recording density currently used is 125 kB/cm/sup 2/, which is far from the optical limits but allows operation with cheap terminals using plastic optics. In the examples given, data are stored in blocks of 10 kB each. A special optical encryption procedure allows the stored data to be protected from unauthorized reading. The molecular basis of this property is again the polarization-sensitive recording mechanism. The unique combination of optical storage, photochromism, and traceability of the BR material is combined on the single-molecule level. BR introduces a new quality of storage capability for applications with increased security and anticounterfeiting requirements.

Improvement of the diffraction efficiency and kinetics of holographic gratings in photochromic media by auxiliary light.

Holographic gratings recorded in photochromic media often do not obtain the maximally achievable diffraction efficiency because of diminishing the fringe contrast caused, e.g., by a photochemically active readout beam or unequal intensities of object and reference waves. For nonreversible materials this problem causes a decrease in diffraction efficiency that is proportional to the signal-to-noise ratio (SNR). However, in nonlinear materials such as photochromic media, for which saturation effects need to be considered, an out-of-proportion decrease in the SNR results. It is shown that an overshooting peak during hologram growth, which then decays to a lower permanent level of diffraction, is an indicator for such a situation. Even a weak readout beam may cause such effects, which significantly affect the hologram kinetics. The observed overshooting diffraction efficiency may even be misinterpreted to be dependent on material properties. Experimental and theoretical proof that with low levels of auxiliary light this type of problem can be eliminated completely is presented. Throughout this research bacteriorhodopsin films were used, but the results are valid for photochromic media in general.

Bacteriorhodopsin modules for data processing with incoherent light

Incoherent optical processing with films made from the biological photochrome bacteriorhodopsin (BR) is accomplished by use of the photoinducible anisotropy of polymeric BR films. BR has two spectrally well separated states, B and M, both of which show a high degree of optical anisotropy. Whereas with green and blue light the BR molecule can be switched between the two states, the accompanying changes in the refractive index may be nondestructively read out by wavelengths in the red, e.g., at 676nm. As the sensitivity of CCD arrays is quite high for such wavelengths, even low-power light sources are sufficient for detection. We describe a BR-based XOR computing module for incoherent optical data processing.

Reflection-type polarization holograms in bacteriorhodopsin films for low-light recording.

Reflection-type polarization holograms with phase-conjugated readout are very useful for low-light recording with bacteriorhodopsin (BR) films. The dependence of reflection-type holograms with parallel and orthogonal recording beams on their intensity ratio (1:1-1:20) was investigated. It was found that for orthogonally polarized beams the phase-conjugated signal depends significantly less on the beam intensity ratio than predicted by coupled-wave theory. This finding is of particular relevance for recording of very low object-beam intensities with BR films, e.g., in interferometry, where signals with a high signal-to-noise ratio, owing to the different polarizations of the scattered light and the signal, and with low dependence of the diffraction efficiency on the ratio between the reference and object-beam intensities are obtained. With this asymmetric recording process, holograms were recorded successfully in BR films with a good signal-to-noise ratio at exposures (from the object side) as low as 50muJ/cm(2) . These exposures are in the range of those typically used for silver halide films.

Computer-generated holograms recorded in bacteriorhodopsin

Computer-generated holograms (CGHs) of phase modulation type have been designed and fabricated in the biological material Bacteriorhodopsin (BR). BR is a photochromic retinal protein which may be used in optical data storage and security applications. Using the permanent light-inducible refractive index change of BR, we demonstrate that both analog and digital optical data can be stored in this material in a write-once-read-many (WORM) mode. The calculation and the optimization of the phase function of the CGHs have been accomplished with iterative Fourier transform algorithm methods (IFTA) such as error reduction algorithms. In the fabrication procedure the optimized phase functions of the CGHs have been recorded in BR which was coated onto a glass substrate. A direct laser writing process employing the 532 nm line of a cw-Nd:YAG laser was used for recording the CGH as a modulation of the absorption coefficient as well as of the refractive index. The design and fabrication method of the CGHs with a pixel pitch of 20 μm and a total size of 10 mm x 10 mm are presented.

Large-diameter bacteriorhodopsin films for applications in nondestructive testing

Bacteriorhodopsin (BR) films with dimension of 100 mm X 100 mm have been developed for applications in non- destructive testing. Films made from the photochromic retinal protein BR have been successfully employed in a variety of optical applications. Among them holographic interferometry for non-destructive testing which is one of the applications where the aperture of the BR-films used for recording plays an important role. The BR-films available earlier had an aperture of about 2/3 inch. Due to this limit size it was necessary to focus the light scattered from the object under investigation to the BR-film by means of lenses which in turn limit the optical resolution of the whole system. Now BR-films with a more than 50-fold larger are have been developed which allow lensless recording of the holograms. The optical quality of the films, in particular the spatial homogeneity of their optical density and their light sensitivity, is constant over the full aperture within a few percent. Applications of this new type of BR-films in non-destructive testing of ceramic parts are presented.

Progress in read-write fast-access volume holographic data storage

We review recent progress made towards commercializable read-write, fast-access holographic data storage. This includes a recent demonstration of high areal density holographic storage, systems architectures for extending this high density to high capacity using phase-conjugate readout, and recent experimental progress along these lines. Other topics include using signal processing to relieve alignment and distortion constraints, optical elements for improving beam uniformity, and most importantly, requirements and prospects for improved photorefractive materials for two-color, gated nonvolatile holographic storage.

Wavelength-shifted reconstruction with bacteriorhodopsin for holographic particle image velocimetry

When large amounts of data are stored in Bacteriorhodopsin (bR), for example with Holographic Particle Image Velocimetry (HPIV), the volatile nature of the medium can be a serious problem. The loss of information has two causes; thermal erasure and photo-induced erasure. The thermal erasure can be reduced by cooling the film. We found that by cooling a film with an optical density of 1.5 (OD570) from 21.2°C to 1.7°C, the thermal erasure time leading to 50% loss of diffraction efficiency was increased from 31 to 185 seconds. The rate of photo-induced erasure does not only depend on the intensity of the reconstruction wave, but also on its wavelength. The influence of the shifted wavelength and the reconstruction intensity on the rate of photo-induced erasure were analysed experimentally and were found to agree with the theory. Reconstructing a hologram with 690 nm can potentially result in a 35 times larger integrated signal to be read from the hologram as when reconstructing with 532 nm.