Title: Techniques for depth-resolved imaging through turbid media including coherence-gated imaging
Author(s): C Dunsby and P M W French
Publication date: 21 July 2003 Volume: 36 Start page: R207
Publication: Journal of Physics D: Applied Physics

Abstract: This article aims to review the panoply of techniques for realising optical imaging through turbid media such as biological tissue. It begins by briefly discussing optical scattering and outlines the various approaches that have been developed to image through scattering media including spatial filtering, time-gated imaging and coherence-based techniques. The discussion includes scanning and wide-field techniques and concentrates on techniques to discriminate in favour of unscattered ballistic light although imaging with scattered light is briefly reviewed. Wide-field coherence-gated imaging techniques are discussed in some detail with particular emphasis placed on techniques to achieve real-time high-resolution three-dimensional imaging including through turbid media, providing rapid whole-field acquisition and high depth and transverse spatial resolution images.

Title: Diarylethene Materials for Rewritable Volume Holographic Data Storage
Author(s): Liu Guo-Dong, He Qing-Sheng, Ding De-Hua, Wu Min-Xian, Jin Guo-Fan, Pu Shou-Zhi, Zhang Fu-Shi, Liu Xue-Dong and Yuan Peng
Publication date: July 2003 Volume: 20 Start page: 1051
Publication: Chinese Physics Letters

Abstract: The photochromic diarylethene, 1,2-bis(2-methyl-5-(4-formylphenyl)-thien-3-yl)perfluorocyclopentene (1a) is studied and its applicable potential in rewritable volume holographic data storage is verified. Holographic recording films of 10-µm thickness have been fabricated. The refractive index modulation (Δn = 1.15×10^-3) between the open- and close-ring forms is detected to be large enough so that the films are suitable for the production of volume holographic storage. The experiments of angle multiplexing and rewriting holograms show that the materials are fit for volume holographic data storage.

Title: Insensitivity of Speckle Multiplexing to Multi-longitudinal Modes of Laser in Volume Holographic Storage
Author(s): Wang Jin-Nan, He Shu-Rong, He Qing-Sheng, Huang Dong and Jin Guo-Fan
Publication date: July 2003 Volume: 20 Start page: 1047
Publication: Chinese Physics Letters

Abstract: If a diode pumped solid state laser is used in a holographic storage system, its multi longitudinal modes may damage the angular selectivity of the hologram and introduce more cross talk in the system. By theoretical analysis, we found that with adopting the speckle multiplexing scheme, holographic systems are no longer sensitive to the multi longitudinal modes of the laser source, and consequently the damage described above could be well suppressed. Moreover, the following high density storage experimental results also express strong advocacy of this conclusion. This result may greatly prompt the miniaturization of a holographic storage system.

Title: Quantum-corrected entropy for (1 + 1)-dimensional gravity revisited
Author(s): A J M Medved
Publication date: 7 June 2003 Volume: 20 Start page: 2147
Publication: Classical and Quantum Gravity

Abstract: In this paper, we examine a generic theory of (1 + 1)-dimensional gravity with coupling to a scalar field. Special attention is paid to a class of models that have a power-law form of dilaton potential and can admit black-hole solutions. The study focuses on the formulation of a Lorentzian partition function. Extending a four-dimensional treatment by Makela and Repo, we incorporate the principles of Hamiltonian thermodynamics (as well as black-hole spectroscopy) and find that the partition function can be expressed in a calculable form. We then go on to extract the black-hole entropy, including the leading-order quantum correction. As anticipated, this correction can be expressed as the logarithm of the classical entropy. Interestingly, the prefactor for this logarithmic correction disagrees, in both magnitude and sign, with the findings from a prior study (on the same model). We comment on this discrepancy and provide a possible rationalization.

Title: Holographic Properties of BR-D96N Film and Its Application in Hologram Aberration Correction
Author(s): Zheng Yuan, Yao Bao-Li, Wang Ying-Li, Menke Neimule, Lei Ming, Chen Guo-Fu and Norbert Hampp
Publication date: May 2003 Volume: 20 Start page: 671
Publication: Chinese Physics Letters

Abstract: A biophotochromic material, i.e., genetic mutant bacteriorhodopsin (BR-D96N), was experimentally studied on its holographic recording properties. The saturation absorption curve and the diffraction efficiency curve were measured respectively. As holographic storage application, reflection type polarization holograms were recorded on the BR-D96N film. The 173° configuration between the object and reference beams proves that the spatial resolution of the film is over 6000 lines/mm. By using phase conjugate wave of reference beam as reconstruction beam, the distorted object image introduced by the optical components and the defects in the recording medium can be well corrected.

Title: Fabrication and application of holographic Bragg gratings in lithium niobate channel waveguides
Author(s): J Hukriede, D Runde and D Kip
Publication date: 7 February 2003 Volume: 36 Start page: R1
Publication: Journal of Physics D: Applied Physics

Abstract: Permanent refractive-index gratings in waveguide devices are of high potential for optical communication systems that make use of the high spectral selectivity of holographic filters, e.g. dense wavelength division multiplexing (DWDM), optical sensors, or narrow-bandwidth mirrors for integrated waveguide lasers. In this contribution we review our results on holographically recorded refractive-index gratings in Cu-doped LiNbO₃ channel waveguides. Elementary holograms are recorded with green light and read in reflection geometry in the insensitive infrared wavelength region around 1.5 μm. To enable long-term stability of the Bragg gratings a thermal fixing technique is applied. In this way strong and almost permanent refractive-index gratings are obtained and their application as narrow-bandwidth filters for DWDM applications is demonstrated. In comparison with Bragg gratings in silica fibres, the electro-optic effect in LiNbO₃ allows for a direct wavelength tuning and a fast, reliable electrical switching of these gratings.

Title: High-Pressure X-Ray Diffraction and Raman Scattering of LiTaO₃
Author(s): Zhang Wei-Wei, Cui Qi-Liang, Pan Yue-Wu, Dong Shu-Shan, Liu Jing and Zou Guang-Tian
Publication date: November 2002 Volume: 19 Start page: 1666
Publication: Chinese Physics Letters

Abstract: We study the energy-dispersive x-ray diffraction and Raman scattering of LiTaO₃ at high pressure. The result remains stable up to 36 GPa. The average isothermal bulk modulus and its pressure derivative are obtained to be k₀ = (225±6) GPa and k₀' = 1.3±0.5 at zero pressure by the Birch-Murnaghan equation of state and the ``universal'' equation of state. The linear incompressibility of LiTaO₃ between the a- and c-directions differs by a factor of four, which shows that the compression is anisotropic.

Title: Diffusional enhancement of holograms: phenanthrenequinone in polycarbonate
Author(s): Andrei Veniaminov and Eckhard Bartsch
Publication date: July 2002 Volume: 4 Start page: 387
Publication: Journal of Optics A: Pure and Applied Optics

Abstract: As a further development of the manufacture of holographic gratings by the diffusion of phenanthrenequinone in polymer glass, the traditionally employed poly(methyl methacrylate) is replaced by poly(bisphenol-A-carbonate). The post-exposure growth and decay of the gratings in the two materials are compared at different spatial periods and temperatures. The medium with polycarbonate demonstrates a fortunate combination of significantly improved stability of the gratings and their faster development. Our results indicate that the polycarbonate materials promise to be suitable media for holographic optical elements and data storage.

Title: Optically fixed photorefractive correlator
Author(s): Liu You-Wen, Liu Li-Ren, Zhou Chang-He and Xu Liang-Ying
Publication date: July 2002 Volume: 11 Start page: 710
Publication: Chinese Physics

Abstract: An optically fixed photorefractive correlator is presented, where two-centre non-volatile holographic recording is employed to write and fix the matched filter in doubly doped LiNbO₃ crystals. This correlator shows good correlation characteristics and insensitivity to the writing beam during readout. It can be used in cases requiring stability and not requiring modification for a long period and it is refreshed optically when new information needs to be registered.

Title: Holographic particle image velocimetry
Author(s): K D Hinsch
Publication date: July 2002 Volume: 13 Start page: R61
Publication: Measurement Science and Technology

Abstract: Holography is truly the key to three dimensions in particle image velocimetry, i.e. the measurement of all spatial components of the velocity vector - and this over a deep measuring field. Sophisticated instruments have been designed that successfully tackle practical problems such as the low scattering efficiency of particles, the inferior depth resolution or the aberrations and distortions in the reconstruction. Furthermore, efficient strategies are introduced to interrogate the holographic storage and process the huge amount of data towards a final flow field representation. Recently, phase-sensitive metrology, familiar in many fields of experimental mechanics, has been examined for use in particle velocimetry. Suitable methods are holographic and speckle interferometry or the optical processing of data for three-dimensional correlation. While in these techniques the power of optics is unrivalled, the practical advantage of video and digital techniques over photographic recording is obvious. The electronic version of speckle interferometry (ESPI/DSPI) is a well-established method used in laser metrology and has received further exploitation for applications in flow analysis recently. Finally, the state-of-the-art of digital particle holography is reviewed to allow estimates of its future in experimental flow analysis.

Title: Voltage-dependent orientational photorefractivity in a planar C₆₀-doped nematic film
Author(s): Wei Lee and Yuan-Lin Wang
Publication date: 7 May 2002 Volume: 35 Start page: 850
Publication: Journal of Physics D: Applied Physics

Abstract: Photorefractive diffraction gratings were studied in cells of homogeneously aligned nematic liquid crystal doped with buckminsterfullerene (C₆₀). These Raman-Nath gratings were induced by the interference modulation of two coherent optical beams in conjunction with an applied dc field across the sample thickness. The diffraction efficiency depends on the applied dc field. Regardless of the writing-beam power, the maximal efficiency is found to occur at ~0.8 kV cm^-1. This critical field value also determines three applied-dc-field regimes that lead to three distinct behaviours of the corresponding time-evolved diffraction-intensity curves.

Title: The fabrication of permanent holographic gratings in bulk polymer medium by a femtosecond laser
Author(s): Jianfeng Zhai, Yuquan Shen, Jinhai Si, Jianrong Qiu and Kazuyuki Hirao
Publication date: 21 December 2001 Volume: 34 Start page: 3466
Publication: Journal of Physics D: Applied Physics

Abstract: Refractive-index-modulated holographic gratings have been recorded for the first time in the interior of polymer bulk, by a femtosecond laser at 800 nm. The polymer bulk was prepared by doping an azo dye, HPNA in polymethylmethacrylate. This sample preparation process is quite simple. Multi-photon absorption and, as a result, photoinduced changes in isomer population were postulated as a brief discussion on the mechanism of the grating formation.

Title: Diffusion-based model of holographic grating formation in photopolymers: generalized non-local material responses
Author(s): J T Sheridan, M Downey and F T O'Neill
Publication date: November 2001 Volume: 3 Start page: 477
Publication: Journal of Optics A: Pure and Applied Optics

Abstract: It has recently been shown by Sheridan and Lawrence (Sheridan J T and Lawrence J R 2000 J. Opt. SA A 17 1108-14) that, introducing a Gaussian non-local material response function into the one-dimensional diffusion equation governing holographic grating formation in photopolymers, both high-frequency and low-frequency cut-offs in the spatial frequency response of the materials can be deduced.

In this paper generalized non-local responses are examined. These include a Chernov material response, and both the Chernov/Debye and Gaussian responses generalized to include a power-law response. The effects of these responses are examined for various material parameters, including the non-local variance, &bsigma;, the power-law degree, k, and the ratio of the rate of diffusion to the rate of polymerization, R. The resulting variations in monomer and polymer concentrations, the cross-sectional grating profiles and the spatial frequency responses predicted are presented and discussed. It is shown that in general, assuming an isotropic even, area-normalized response function, qualitatively acceptable growth curve and spatial frequency response behaviour are observed in all cases.

Title: Scattering-Suppression in Photochromic LiNbO₃:Fe:Mn Non-volatile Holographic Recording
Author(s): Liu De-An, Liu Li-Ren, Liu You-Wen, Zhou Chang-He and Xu Liang-Ying
Publication date: August 2001 Volume: 18 Start page: 1064
Publication: Chinese Physics Letters

Abstract: We propose and experimentally investigate a new scheme capable of suppressing light-induced scattering by periodical incoherent erasure during every non-volatile holographic recording cycle in photochromic LiNbO₃:Fe:Mn crystals. The results demonstrate that the scattering noise is suppressed effectively, and the final diffraction efficiency of the fixed grating is significantly enhanced, rather than decreased, by about 30% compared with the conventional recording procedure. The period of the recording and incoherent erasure cycle is theoretically calculated and experimentally optimized.

Title: A superconducting associative memory that is defect tolerant
Author(s): P Chandra and L B Ioffe
Publication date: 23 July 2001 Volume: 13 Start page: L697
Publication: Journal of Physics: Condensed Matter

Abstract: We present a content-addressable memory whose central component is a superconducting crossbar array with 2N elements connected by N² junctions. Because multiple pathways exist between any two elements, this storage device is tolerant to physical defects in the interconnections. Furthermore, each pattern of N bits is stored non-locally in the N² junctions, so information access and retrieval are tolerant to input errors. This superconducting memory should exhibit picosecond single-bit acquisition times with negligible energy dissipation during switching and multiple non-destructive read-outs of the stored data.

Title: Geometry Control of Photo-induced Microstructures in an Azobenzene Polymer Film
Author(s): Xu Ze-Da, Li Zhen, Ninulescu Valerica, Liang Li-Zhen, Peng Zhuo-Lun, Cai Zhi-Gang, Zhou Jian-Ying, Zhang Ling-Zhi and Liang Zhao-Xi
Publication date: March 2001 Volume: 18 Start page: 379
Publication: Chinese Physics Letters

Abstract: The mechanisms of photo-induced microstructures in an azobenzene polymer film are presented. They are based on the spatial periodic modulation of optical intensity and the photoisomerization of azobenzene molecules with the movement of main chains. Experiment and theory jointly point out the possibility of photo-inducing desired spatial microstructures in an azobenzene organic polymer via adequate optical lattices and adequately polarized `writing' beams.

Title: 3D bit-oriented optical storage in photopolymers
Author(s): Susanna Orlic, Steffen Ulm and Hans Joachim Eichler
Publication date: January 2001 Volume: 3 Start page: 72
Publication: Journal of Optics A: Pure and Applied Optics

Abstract: The bit-oriented data storage of conventional optical disks may be expanded into the third dimension by using microscopic reflection gratings instead of pits. Microgratings are holographically induced in a photopolymer layer. The Bragg selectivity of holographic volume gratings makes the application of multiplexing methods possible. High storage density may be achieved by combining wavelength multiplexing and multilayer storage. Wavelength multiplexing is realized by recording several gratings overlapping with write beams of different wavelengths. All gratings are recorded simultaneously in the same volume element. By translating a photopolymer sample perpendicularly to the laser beam axis during exposure, stripe-shaped microgratings are induced dynamically with constant linear velocity. The length of a grating is defined by the exposure time. Stripe-shaped gratings are required to realize an areal structure of recorded data similar to the pit-land structure of conventional disks that allows us to overtake the progress made in proceeding CD/DVD technology. Experimental results have been obtained for writing and reading of microholograms in different photopolymer materials including DuPont holographic recording films and CROP photopolymers from Polaroid.

Title: Long dark decay in highly sensitive Ce doped photorefractive KNbO₃ crystals
Author(s): Roland Ryf, Germano Montemezzani and Peter Günter
Publication date: January 2001 Volume: 3 Start page: 16
Publication: Journal of Optics A: Pure and Applied Optics

Abstract: We show that cerium doped KNbO₃ crystals exhibit high photorefractive sensitivities of S₁ = 25×10^-11 m³ J^-1 and S₂ = 78×10^-9 m² J^-1 at 488 nm. The grating build-up time at a grating spacing of 1.5 µm and an intensity of 1 W cm^-2 is 40 ms and the exponential gain is Γ = 6 cm^-1. The decay time of the photorefractive grating in the dark was measured and found to be 31 h. The sample was completely characterized by additional photoconductivity measurements. The ratio between the photoconductivity at 1 W cm^-2 and the conductivity in the dark is as high as 10⁷.

Title: Investigation of titanium- and copper-indiffused channel waveguides in lithium niobate and their application as holographic filters for infrared light
Author(s): J Hukriede, D Kip and E Krätzig
Publication date: September 2000 Volume: 2 Start page: 481
Publication: Journal of Optics A: Pure and Applied Optics

Abstract: Monomode channel waveguides for infrared light around 1550 nm are fabricated in lithium niobate by successive indiffusion of titanium stripes and thin evaporated layers of copper. The fabricated waveguides are optically characterized and investigated with an atomic force microscope. To evaluate the photorefractive properties holograms are recorded with green light of an argon ion laser and read in the various channels in reflection geometry with infrared light. The saturation values of the light-induced refractive-index change and the photoconductivity in the channels are measured. The results are consistent with a one-centre model for the charge transport. To demonstrate the multiplexing capabilities of the photorefractive waveguides three reflection holograms with 90% diffraction efficiency and a channel spacing of only 0.8 nm are superimposed in one sample.

Title: Optical fixing using shallow traps - application to La₃Ga₅SiO₁₄ doped with praseodymium
Author(s): Thomas Nikolajsen and Per Michael Johansen
Publication date: July 2000 Volume: 2 Start page: 255
Publication: Journal of Optics A: Pure and Applied Optics

Abstract: The purpose of this paper is twofold. First, we demonstrate optical fixing through two-step gated recording in a crystal of La₃Ga₅SiO₁₄ doped with praseodymium. Second, based on these experiments we suggest an excitation-recombination model for the photorefractive effect in the crystal and comment on the nature of the intermediate level involved in the gated recording process.

Title: Relaxation behaviour of electrically induced polar orientation and of optically induced non-polar orientation in an azo-chromophore side group polymer
Author(s): A Stracke, A Bayer, S Zimmermann, J H Wendorff, W Wirges, S Bauer-Gogonea, S Bauer and R Gerhard-Multhaupt
Publication date: 7 December 1999 Volume: 32 Start page: 2996
Publication: Journal of Physics D: Applied Physics

Abstract: We have investigated the relaxation of the polar order (P₁cos) in a poled styrene-maleic anhydride copolymer with side groups of modified dispersed red chromophores - using pyroelectric studies - as well as the relaxation of the orientational order (P₂cos) of the optical axes of the side groups previously induced by polarized light - using holographic techniques. Isomerization cycles of the chromophores cause in the latter case the formation of an optical grating. The relaxation processes were monitored within the glassy state. In addition we investigated molecular relaxation processes, in particular the glass relaxation, using dielectric techniques. The observation was that the relaxations of the pyroelectric response and of the holographic grating couple to the glass relaxation yet do not display identical kinetics within the solid glassy state, showing that the optical grating is less stable. The difference arises obviously from the excess free volume induced by the isomerization cycle in the optical storage experiment. The formation of this excess free volume is apparent from the formation of a surface relief pattern in the storage experiment. A straightforward conclusion is that the stability of the grating can be considerably enhanced if chromophores are used characterized by a significantly lower equilibrium concentration of cis-species during irradiation.

Title: Light-induced absorption in a Bi₁₂TiO₂₀ crystal
Author(s): Oleg V Kobozev, Stanislav M Shandarov, Alexei A Kamshilin and Victor V Prokofiev
Publication date: July 1999 Volume: 1 Start page: 442
Publication: Journal of Optics A: Pure and Applied Optics

Abstract: Evolution of the light-induced absorption in a Bi₁₂TiO₂₀ crystal is investigated at a wavelength of 633 nm. It is found that the absorption coefficient evolution consists of fast and slow components with different characteristic response time. The light absorption coefficient decreases from 0.9 to 0.16 cm^-1 after keeping the sample in darkness for six months. We propose a new charge-transport model that includes the deep donors in three valence states and the shallow traps. It is shown that the theory developed provides an explanation for the observed features of the increase and dark decay of the light-induced absorption. The material parameters of the Bi₁₂TiO₂₀ crystal are estimated from the best fit of the theoretical model with the experimental data.

Title: Efficient non-volatile holographic recording in doubly doped lithium niobate
Author(s): Karsten Buse, Ali Adibi and Demetri Psaltis
Publication date: March 1999 Volume: 1 Start page: 237
Publication: Journal of Optics A: Pure and Applied Optics

Abstract: We present an efficient, non-volatile holographic storage scheme in doubly doped lithium niobate, with red recording light and simultaneous illumination by homogeneous ultraviolet light.

Title: Polarization holographic gratings with surface relief in amorphous azobenzene containing methacrylic copolymers
Author(s): I Naydenova, Tz Petrova, N Tomova, V Dragostinova, L Nikolova and T Todorov
Publication date: July 1998 Volume: 7 Start page: 723
Publication: Pure and Applied Optics: Journal of the European Optical Society Part A

Abstract: Two types of high-efficiency amorphous methacrylic azobenzene polymer for reversible holographic storage are synthesized. It is shown that their efficiency is due to both photoinduced linear birefringence and surface relief. The contributions of these two effects on the diffraction efficiency of the holographic gratings are separated using the methods of polarization holography and it is found that they are strongly influenced by the differences in the chemical structure of the polymers. It is shown that higher relief corresponds to stronger coupling between the azobenzene side chains and the main chains of the polymer molecules.

Title: Double-exposure interferometry utilizing Denisyuk holograms in photorefractive crystals
Author(s): S Riehemann
Publication date: May 1998 Volume: 7 Start page: L33
Publication: Pure and Applied Optics: Journal of the European Optical Society Part A

Abstract: For practical applications, Denisyuk type holograms offer important advantages. So this recording geometry is utilized to develop a compact holographic double-exposure interferometer based on a photorefractive SBN:Ce crystal as storage material. A preliminary report on the realized interferometer is given, present limitations are determined and proposals for further improvements are outlined.

Title: Optical Kerr-like response of dye-doped nematics
Author(s): E Santamato, G Abbate, M Colicchio, P Maddalena, L Marrucci, E Massera and D Paparo
Publication date: March 1998 Volume: 7 Start page: 191
Publication: Pure and Applied Optics: Journal of the European Optical Society Part A

Abstract: When small amounts of suitable dyes are added to a pure nematic liquid crystal, its nonlinear optical response may be enhanced by over two orders of magnitude and even be reversed in sign. This feature may be interesting for applications. We measured the enhancement of the nonlinear optical Kerr coefficient of several dye-nematic mixtures, in order to gain some insight on the molecular mechanism that is supposed to be the basis of the phenomenon.

Title: All-optical bypass-exchange switch based on two types of photorefractive hologram
Author(s): Xiaona Yan, Liren Liu and Feng Wang
Publication date: February 1998 Volume: 29 Start page: 1
Publication: Journal of Optics

Abstract: We demonstrate a scheme of using two types of recorded photorefractive hologram - a photovoltaic hologram and a standard photorefractive hologram - to implement an all-optical bypass-exchange switch. A programmable polarization analyser array is used to select the desired operation. Issues about the elimination of noise are discussed with the help of previous calculations by Yeh and Gu et al. As the system can be implemented in one single-block photorefractive crystal, it has the properties of compactness and miniaturization. Moreover, as the input and output signals propagate in a direction normal to the switching array the switch is extremely suitable for a multistage interconnected network.

Title: Charge distribution and optical properties of and F centres in crystals
Author(s): E A Kotomin, R I Eglitis and A I Popov
Publication date: 2 June 1997 Volume: 9 Start page: L315
Publication: Journal of Physics: Condensed Matter

Abstract: Results of quantum chemical calculations for the and F centres in cubic and orthorhombic phases of a perovskite ferroelectric are presented and analysed in the light of existing experimental literature. It is shown that one (two) electrons of the and F centres, respectively, are considerably delocalized, even in the ground state of defects, over the two Nb atoms nearest to the O vacancy, and other close atoms. They resemble more electron defects in partly covalent crystals (the so-called centre) than F-type centres in ionic MgO crystals. We predict two or three absorption bands (depending on the crystalline phase) for each of the defects. The calculated absorption energies for the centre are by 0.3 - 0.4 eV lower than those for the F centre.

Title: The bulk photovoltaic effect of photorefractive :Fe crystals at high light intensities
Author(s): M Simon, St Wevering, K Buse and E Krätzig
Publication date: 7 January 1997 Volume: 30 Start page: 144
Publication: Journal of Physics D: Applied Physics

Abstract: Direct measurements of the bulk photovoltaic current density in photorefractive and crystals were performed at high light intensities. Illumination of the crystals with light pulses of a Q-switched frequency-doubled Nd:YAG laser (pulse duration 20 ns; light wavelength 532 nm) yielded a photovoltaic current through the crystals which charged a capacitor. Afterwards an electrometer measured the voltage of the charged capacitor. From this voltage and from capacity and exposure time the current density was deduced. For intensities above a contribution to which increased quadratically with light intensity I appeared for crystals. Co-doping with Mg and Zn lowered the quadratic component. Our results are in good qualitative and quantitative agreement with previous holographic measurements and support the claim that the intrinsic defect acts as a secondary photorefractive centre in at high light intensities.

Title: Dye-doped liquid crystal composite for real-time holography
Author(s): S Bartkiewicz, A Januszko, A Miniewicz and J Parka
Publication date: November 1996 Volume: 5 Start page: 799
Publication: Pure and Applied Optics: Journal of the European Optical Society Part A

Abstract: In this paper we report the results of dynamic phase grating formation in an anthraquinone derivative dichroic dye-doped nematic liquid crystal in which an external DC electric field was used to change the planar alignment of liquid crystal molecules into a near homeotropic one. This field was also needed to produce an efficient grating via photoconductivity-induced molecular reorientation in a degenerate two-wave mixing experiment with He - Ne (632.8 nm) laser light serving as a low power excitation source. The characteristic of the measured effect shows that the mechanism of grating formation is evidently a non-photorefractive one. A diffraction efficiency of up to 20% has been measured in the system. Fast hologram recording and erasing times (1 ms) make this system attractive for image processing applications.

Title: Photorefractive materials: specifications relevant to applications
Author(s): Ph Delaye, J M C-Jonathan, G Pauliat and G Roosen
Publication date: September 1996 Volume: 5 Start page: 541
Publication: Pure and Applied Optics: Journal of the European Optical Society Part A

Abstract: After a rapid presentation of generally quoted figures of merit of photorefractive materials, we show that these figures of merit do not allow us to select a `best material'. We illustrate this important point by accurately defining the materials' properties relevant to several applications taken in various domains such as optical storage, real time interferometry and pattern recognition.

Title: Photorefractive polymers for digital holographic optical storage
Author(s): D M Burland, R G Devoe, C Geletneky, Y Jia, V Y Lee, P M Lundquist, C R Moylan, C Poga, R J Twieg and R Wortmann
Publication date: September 1996 Volume: 5 Start page: 513
Publication: Pure and Applied Optics: Journal of the European Optical Society Part A

Abstract: The orientational enhancement mechanism responsible, in part, for the photorefractive effect observed in polymeric materials in which an anisotropic chromophore is free to orient in the presence of an external poling electric field is described. This mechanism does not require that the chromophore have a substantial first hyperpolarizability. This fact has been demonstrated by the demonstration of a substantial photorefractive effect in a chromophore with a tiny hyperpolarizability but a large linear polarizability anisotropy. The use of photorefractive polymers in holographic data recording is assessed using a sophisticated holographic data storage tester. These experiments have demonstrated the critical dependence of the recording bit error rate on background scattering and the severe requirements that this dependence places on sample quality.

Title: X-ray photoelectron diffraction for pure and Nb-doped : site determination for the Nb atoms
Author(s): R Niemann, H Hartmann, B Schneider, H Hesse and M Neumann
Publication date: 5 August 1996 Volume: 8 Start page: 5837
Publication: Journal of Physics: Condensed Matter

Abstract: We present a set of polar-angle-dependent x-ray photoelectron spectra (XPS) obtained from in situ cleaved single-crystalline pure . The variation of the intensity for different angles can be explained by the diffraction of the emitted electrons by the surrounding atoms of the emitter and thus may be used as a `fingerprint' for the location of emitting impurity atoms. Measurements were carried out on niobium-doped . The atomic position of the niobium atoms was deduced from the comparison of the angle-dependent spectra, giving a clear hint that tantalum is replaced by niobium.

Title: Pulsed laser deposition of /MgO bilayered films on Si wafer in waveguide form
Author(s): X L Guo, Z G Liu, X Y Chen, S N Zhu, S B Xiong, W S Hu and C Y Lin
Publication date: 14 June 1996 Volume: 29 Start page: 1632
Publication: Journal of Physics D: Applied Physics

Abstract: Strontium barium niobate (SBN) thin films were grown on Si (111) substrates coated with MgO buffer by the pulsed laser deposition (PLD) technique. The thickness of SBN and MgO films were of the order of 1200 nm and 840 nm respectively. X-ray energy dispersive spectrometry (XREDS) showed that SBN films have stoichiometric composition identical to the target material, and no Si diffusion into the SBN film was found. X-ray diffraction (XRD) scans indicated that MgO films were highly (111) textured, but the SBN films were polycrystalline without preferential orientation. The surface of the SBN film was smooth, dense and crack-free and no large droplets were observed when studied under a scanning electron microscope (SEM). A favourable optical waveguiding property of the bilayered films was demonstrated by a prism coupler method.

Title: The lattice position of Fe in Fe-doped LiNbO₃
Author(s): T Gog, P Schotters, J Falta, G Materlik and M Grodzicki
Publication date: 28 August 1995 Volume: 7 Start page: 6971
Publication: Journal of Physics: Condensed Matter

Abstract: The lattice location of Fe atoms in Fe-doped LiNbO₃ is determined experimentally employing X-ray standing waves. At a bulk concentration of 7.48*10¹⁸ cm^-3 Fe is found to occupy sites close to the ferroelectric Li position; shifted by 0.18 AA in the direction of the -c axis. Electronic structure calculations confirm this result insofar as computed spectroscopic data are more consistent with the respective experimental values for Fe located in the vicinity of the Li site than for Fe occupying the Nb lattice position.

Title: Applications of photorefractive crystals
Author(s): S I Stepanov
Publication date: January 1994 Volume: 57 Start page: 39
Publication: Reports on Progress in Physics

Abstract: The main suggestions in practical applications of dynamic photorefractive holographic recording for interferometry, processing of 2D pictures, operations with laser beams, and holographic memory systems are reviewed. Specific photorefractive techniques underlying these applications, and in particular, methods of continuous and nondestructive hologram reconstruction, nonlinearities in recording mechanisms, and basic limitations due to geometrical factors, Bragg diffraction, and optical noise are considered in detail. The paper is preceded by two introductory sections devoted to fundamentals of dynamic holography and phase conjugation in volume media, and to specific features of recording and reconstruction of the photorefractive holograms.

Title: An interferometric dilatometer for the determination of thermo-optic coefficients of NLO materials
Author(s): J Mangin, P Strimer and L Lahlou-Kassi
Publication date: August 1993 Volume: 4 Start page: 826
Publication: Measurement Science and Technology

Abstract: An interferometric dilatometer has been designed for determination of thermo-optic coefficients of millimetric nonlinear optical (NLO) materials. The apparatus is a combination of a modified Mach-Zehnder arrangement for dilation measurements and a Fizeau technique for determination of changes in optical length. Its configuration allows measurements under known conditions of applied electric field. Analysis of the linear thermal expansion coefficient alpha _b of a triglycine sulphate (TGS) single crystal around its second-order phase transition is used to evaluate the sensitivity of the interferometer, and measurements performed on a LiNbO₃ single crystal exhibit step-like variations of changes in optical length from 20 degrees C to 122 degrees C. These experiments show that an accuracy better than 1*10^-6 K^-1 is achievable for both dilation and thermo-optic coefficients with this dilatometer, provided that they are not highly temperature-dependent and are under the condition of suitably controlled applied electric field.

Title: Lithium niobate integrated optics
Author(s): M Lawrence
Publication date: March 1993 Volume: 56 Start page: 363
Publication: Reports on Progress in Physics

Abstract: Guided-wave devices demonstrated in lithium niobate have been seen for many years as highly functional components for applications in fields such as optical communications and sensor systems. The potential for their use has been recognized by the substantial research and development effort the technology has witnessed, and the subject matter of this article is born from these endeavours. The material reviewed includes a survey of devices built, including their design, functionality, and performance, as well as parallel activities that have been undertaken in materials processing, theory, and the development of lithium niobate itself, all of which are closely linked, and all of which have played an important part in the overall development of this major branch of contemporary optics research.

Title: Scattering and beam fanning in a BaTiO₃ crystal
Author(s): G Li, T Yang, Y Y Teng, F C Lin and M A Fiddy
Publication date: October 1992 Volume: 2 Start page: 303
Publication: Waves in Random Media

Abstract: Single-domain barium titanate crystals exhibit unusual scattering and fanning phenomena. Some models which contribute to this effect are described and an analysis presented and related to the experimental observations. These phenomena depend on the illumination geometry; in particular, for the case of illumination of the +c face, the fanning strongly depends on angle of incidence, illuminating beam profile, wavelength, polarization and temperature but only weakly depends on input laser power. In the case of -c face and a face illumination, a dependence of the (conical diffraction) ring angle on wavelength and input power was observed and modelled.

Title: Holographic collimator of diameter 200 mm in silver halide sensitized gelatin
Author(s): A Belendez, I Pascual and A Fimia
Publication date: September 1990 Volume: 21 Start page: 211
Publication: Journal of Optics

Abstract: The authors present the theoretical analysis and experimental results of a holographic collimator made in silver halide sensitized gelatin. Diffraction efficiency up to 70% over the whole plate and wave aberration as low as 20 lambda in the maximum diffraction efficiency geometry of reconstruction are its main properties as a holographic optical element.

Title: Spatially varying two beam coupling in photorefractive media in the presence of biasing incoherent illumination
Author(s): A Roy and K Singh
Publication date: July 1990 Volume: 21 Start page: 171
Publication: Journal of Optics

Abstract: The effect of biasing incoherent illumination has been investigated on the energy transfer in the case of co-directional and contra-directional two-wave mixing (TWM) in an absorbing photorefractive (PR) medium. Input beams are considered collinear Gaussian TEM₀₀ in nature. Nonlinear coupled wave equations are solved analytically and numerically, respectively in the case of transmission and reflection geometries. Numerical solution is obtained by using the shooting method.

Title: Imaging in white light with a thick-phase transmission holographic doublet
Author(s): A Belendez, I Pascual and A Fimia
Publication date: November 1989 Volume: 20 Start page: 263
Publication: Journal of Optics

Abstract: An afocal two-element holographic imaging system is presented. This system can be used to work in white light. The holographic optical elements (holographic lenses) are made as thick phase holograms on silver halide sensitized gelatin (SHSG) and they present a maximum diffraction efficiency of 75%. Geometrical conditions at reconstruction with coherent light and with white light are studied and a resolution test chart is imaged through the system, which shows a best resolution of 57 lines/mm for coherent illumination and 15 lines/mm for white light. The two-element holographic system presents a maximum diffraction efficiency of 56%.

Title: Anamorphic correlator for character recognition. Use of a multiple filter matched to signals of different scale
Author(s): M J Buades, A Moya and C Ferreira
Publication date: March 1989 Volume: 20 Start page: 79
Publication: Journal of Optics

Abstract: An anamorphic correlator working with spherical beam illumination is presented. It allows the recognition, through matched filtering, of alphanumerics with different sizes from those in the corresponding signals in the target used to build a multiple matched filter. Characters in an input with the same size are detected in different planes because the characters in the target have been chosen with different sizes. Thus, ambiguities in detection, due to possible overlappings, are avoided. The correlation is only obtained when the input is moved to the position where the Fourier transform has the suitable scale to match the filter. However, the positions of the lenses of the second transformer depend on the sizes involved in the recording and filtering processes. Linear movements are achieved with the setup proposed. Under these conditions, the output plane location also varies linearly with respect to the ratio between the size of the signals in the target and that in the input. Experimental results are shown.

Title: The influence of holography on measurement technology
Author(s): J W C Gates
Publication date: December 1986 Volume: 19 Start page: 998
Publication: Journal of Physics E: Scientific Instruments

Abstract: The introduction of holographic techniques into optical measurements is reviewed with some detail of selected examples. Adoption of these techniques seems to have been slow, and many well understood advantages have not yet been adequately exploited. Prospects of more rapid development seem hopeful, however, especially if more convenient sources of coherent light soon become available. Attention is drawn to the importance of holography contributing to better presentation of the results of measurement in visible displays, and the vital effect such a development could have on the acceptance and wider exploitation of holography in measurement.

Title: The fundamental absorption edge in Bi₁₂SiO₂₀
Author(s): T Toyoda, S Maruyama, H Nakanishi, S Endo and T Irie
Publication date: 14 May 1986 Volume: 19 Start page: 909
Publication: Journal of Physics D: Applied Physics

Abstract: The optical absorption spectra of facet-core-free and facet-core parts in Bi₁₂SiO₂₀ have been measured precisely at various temperatures between 30 and 300K and analysed in the region of the absorption coefficient from 50 to 1600 cm^-1. The data are interpreted as indicating there are indirect allowed transitions with energy gaps of 3.380 eV (facet-core-free) and 3.353 eV (with facet core) at 0K. The temperature dependence of the energy gaps in facet-core-free and facet-core parts are different from each other within experimental accuracy.

Title: The fitting parameters of exponential optical absorption in Bi₁₂SiO₂₀
Author(s): T Toyoda, H Nakanishi, S Endo and T Irie
Publication date: 30 April 1986 Volume: 19 Start page: L259
Publication: Journal of Physics C: Solid State Physics

Abstract: In Bi₁₂SiO₂₀, the fitting parameters of the exponential optical absorption were investigated from the experimental results reported by Toyoda and co-workers (1986) and analysed by Kurik. The exciton-phonon interaction in the facet-core part of Bi₁₂SiO₂₀ is larger than that in the facet-core-free part. Also the energy of the phonons involved in the formation of the absorption tail in the facet-core part is larger than that in the facet-core-free part.

Title: Exponential absorption edge in Bi₁₂SiO₂₀
Author(s): T Toyoda, S Maruyama, H Nakanishi, S Endo and T Irie
Publication date: 14 July 1985 Volume: 18 Start page: L87
Publication: Journal of Physics D: Applied Physics

Abstract: The optical absorption spectra of facet core free and facet core parts in Bi₁₂SiO₂₀ have been measured precisely at various temperatures between 30K and 300K, and analysed in the region below the fundamental absorption edges. In both cases, it can be seen that the absorption coefficients plotted semilogarithmically vary linearly with photon energy h(cross) omega (Urbach's rule). The values and the temperature dependence of Urbach parameter in facet core free are larger than those in facet core in Bi₁₂SiO₂₀.

Title: Applications of photochromic polymer films
Author(s): A.E.J. Wilson
Publication date: September 1984 Volume: 15 Start page: 232
Publication: Physics in Technology

Abstract: Organic photochromic compounds await major commercial exploitation. The main characteristics of these materials are outlined and their applications are discussed. These include: optical data recording, optical data processing, optical waveguide components, inks for security applications, displays, variable density filters, actinometry and various leisure products

Title: The electric sub-band structure of electron accumulation layers in InSe from Shubnikov-de Haas oscillations and inter-sub-band resonance
Author(s): E Kress-Rogers, G F Hopper, R J Nicholas, W Hayes, J C Portal and A Chevy
Publication date: 30 July 1983 Volume: 16 Start page: 4285
Publication: Journal of Physics C: Solid State Physics

Abstract: In bulk InSe crystals grown by the Bridgman method, electron accumulation layers are formed in the vicinity of stacking faults with charged impurities adsorbed to the defect plane. Shubnikov-de Haas experiments show that the 2D electron gas in InSe observed at low temperatures has a density of 10¹² cm^-2 and that several dielectric sub-bands are occupied. The g-factor is enhanced by electron-electron interactions. The optical absorption was studied in the frequency range 3-20 meV in perpendicular and tilted magnetic fields up to 8T. The cyclotron resonance at E_res=h(cross) omega _c, the intersubband resonance at E_res=E₀₁ and the combined resonance at E_res=E₀₁+h(cross) omega _c, are all observed for both parallel and tilted polarisation, where E₀₁ is equal to the sub-band separation as deducted from the Shubnikov-de Haas periodicities. With increasing magnetic fields the exciton-and depolarisation-shifted resonance E₀₁ approaches the true sub-band splitting E₀₁, and is the same time reduced in amplitude. Absorption due to bulk 1s-2p transitions in isolated shallow donors is also observed. The 2p level shows a three-dimensional Zeeman splitting (h(cross) omega _c)_3D with a weak anomalous mass anisotropy as observed in cyclotron resonance measurements of bulk carriers at temperatures of 30-100K. The effective electronic Rydberg is in agreement with results of exciton spectra.

Title: Holography-techniques and applications
Author(s): I Weingartner
Publication date: January 1983 Volume: 16 Start page: 16
Publication: Journal of Physics E: Scientific Instruments

Abstract: A principal survey of holographic techniques is presented and the following applications are considered: holographic interferometry, optical filtering, computer holography, multiplex holography, holographic storage, and holographic optical elements.

Title: Phonon echoes in CdS and related piezoelectric semiconductors
Author(s): J O Fossum and R M Holt
Publication date: 20 November 1982 Volume: 15 Start page: 6613
Publication: Journal of Physics C: Solid State Physics

Abstract: Wavevector-reversed phonon echoes in single crystals are generated by the interaction between a forward-propagating ultrasonic wave and an electric AC field. In piezoelectric semiconductors, the interaction consists of an electroacoustic (dielectric) and an electronic contribution. The latter is shown to be proportional to the conductivity in highly resistive crystals. This is demonstrated experimentally in CdS.

Title: Integrated optical devices and applications
Author(s): P K Cheo
Publication date: January 1979 Volume: 12 Start page: 2
Publication: Journal of Physics E: Scientific Instruments

Abstract: This paper reviews some of the integrated optical devices being developed for use either as a part of or as a complete laser system for optical radar, space communication, pollution detection and data processing. An introduction to optical waveguides and fabrication techniques is included.

Title: Applications of holography
Author(s): B J Thompson
Publication date: May 1978 Volume: 41 Start page: 633
Publication: Reports on Progress in Physics

Abstract: The current status of holography and its applications are discussed with some historical background to place the current activity in perspective. Basic types of holograms are described together with their properties. Applications in optical image formation include photography, microscopy, image storage and image replication. Acoustic, X-ray, electron beam and microwave holography are briefly commented upon. The second major area of application is contour generation and interferometry. Finally, the current status of holographic optical elements is assessed.

Title: Holographic storage of multiple images using spatial division in Fourier plane
Author(s): C P Grover and M May
Publication date: September 1973 Volume: 4 Start page: 281
Publication: Nouvelle Revue d'Optique

Abstract: A new holographic method for multiple storage of images has been suggested in which the spectra of low frequency signals are made to spread widely in the Fourier transform plane, by modulating them with a high frequency random diffuser. The Fourier transform plane is divided into a number of non-overlapping areas and with each of them is associated a signal. Due to symmetry of revolution about the optical axis, the spatial division of the frequency plane is made by using annular filters of different diameters. The directly transmitted beam corresponding to zero spatial frequency provides the reference beam. The method can be used to for multicolor wavefront reconstructions. The storage capacity of the method depends on the spread of the spectrum in the Fourier transform plane. This can, however, be increased by incorparating an additional periodic modulation in the Fourier transform plane.

Title: Optical holographic storage in KCl and KBr crystals at room temperature
Author(s): B Stádník and Z Tronner
Publication date: November 1972 Volume: 3 Start page: 347
Publication: Nouvelle Revue d'Optique Appliquée

Abstract: Les résultats d'étude de l'efficience de diffraction d'un enregistrement holographique produit à la température ordinaire dans les cristaux de KCl et KBr colorés par des méthodes différentes sont présentés.