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Three-dimensional optical data storage is demonstrated in an initially homogenous volume by first recording a reflection grating in a holographic photopolymer. This causes the entire volume to be weakly reflecting to a confocal read/write head. Superposition of two or three such gratings with slightly different k-vectors creates a track and layer structure that specialized servo detection optics can use to lock the focus to these deeply-buried tracks. Writing is accomplished by locally modifying the reflectivity of the preexisting hologram. This modification can take the form of ablation, inelastic deformation via heating at the focus, or erasure via linear or two-photon continued polymerization in the previously unexposed fringes of the hologram. Storage by each method is demonstrated with up to eight data layers separated by as little as 12 microns.

There are some "Grand Challenges" of data visualization which scientists in every application domain confront at one time or another. For example, how to access large volumes of data, how to isolate the "important" pieces of information, how to map scientific data to primitives suitable for graphical algorithms, how to achieve the speed of useful interaction, why won't off-the-shelf software do what I need it to do, how to make hardcopy, etc., are just some of the common problems. This panel session addresses issues like these, from the point of view of the scientist who uses visualization as a research tool. Each panelist was invited to submit a short "position paper", which is reproduced here.

A new optical switch design is reported consisting of two sets of parallel channel waveguides intersecting at right angles. Arrays based on Ti:LiNbO3 technology, with waveguide intersections iron doped to provide photorefractive sensitivity, have been fabricated. Interfering guided beams are used to write index gratings at the waveguide intersections, which in turn diffract a portion of an incident signal beam into the crossing waveguide. Arrays ranging from 15 x 15 up to 50 x 50 waveguides are constructed in an active area of 5 mm square. Diffraction efficiency and writing time measurements are reported along with a brief theoretical analysis. A proposal for a practical architecture offering nondestructive readout is described.

Optical implementations of interconnects for digital processors are being inves- tigated for their high bandwidth, large interconnect density and high level of immu- nity from interference. Implementations can be categorized into several classes, in- cluding fiber optics, holographic approaches and integrated optics. Photorefractives appear to be attractive media for interconnects requiring dynamic reconfiguration capabilities. The attractiveness stems from the ability to record large multiple inter- connect patterns on a time scale of milliseconds using modest laser powers, typically of a few mW/cm2. Few of the interconnect architectures that have been explored so far take advantage of the dynamic holographic recording properties of photore- fractives, and key issues related to holographic recording of interconnect patterns in these media still need to be considered. For example, fixing approaches are re- quired to record and store multiple holograms with minimum crosstalk, and update strategies are necessary for modifying or removing interconnect patterns without affecting the others. In this paper we dwell on the pertinent physical characteristics of photorefractives, and discuss implications for optical interconnects. For example, we present a procedure for prolonged readout, an architecture for Bragg matched diffraction with light of a different wavelength than is used for recording, a novel approach to a large recording medium consisting of an SBN fiber bundle, and a very high density integrated optical interconnect of 100x100 switches per square centimeter.

Back-seeded stimulated photorefractive scattering phase conjugation is demonstrated in SBN fibers for the first time. A 7 mm long by 0.8 mm diameter 0.01% Ce:SBN fiber oriented with its c-axis along the length of the fiber is used for the measurement. The seed is provided by placing a retroreflecting screen in the path of the beam that is transmitted by the fiber. Reflectivities of 28% and 49%, accounting for reflection losses, are achieved by using light with wavelengths of 515 nm and 638 nm, respectively. Previously seeded stimulated phase conjugation (SSPC) has been demonstrated in bulk photorefractive BaTiO3,1,2 by using retroreflector arrays. Good fidelity of the back-reflected signal is achieved with reflectivities approaching 60%. The geometry for optimum results requires that an index-matching fluid surround the crystal; pixel-based image processing-appears to be difficult to achieve in bulk crystals. In the new approach a synthetic material is constructed by bundling SBN fibers. This allows parallel, pixel-based image processing to be accomplished3 with low-power (milliwatt) laser beams. By employing a polarization beam splitter in conjunction with a wave plate, the reflected light can be coupled out without loss in amplitude. We report on the angular sensitivity, wavelength dependence, and amplitude threshold needed for SSFC to occur in these fibers.

Future generations of computing machines may require an optical swiching arrangement which provides a high speed, large size reconfigurable interconnect. The fundamental component of such a switch is based on a dynamic medium which can route optical beams in accordance with some external programming stimulus. In particular, photorefractive crystals have been invetigated for their potential to solve the dynamic interconnect problem1,2. These crystals allow for real-time recording of holograms which diffract signal beams to the desired optical detectors. Although the benefits of an ideal holographic interconnect scheme are tantalizing, the realization of such an approach is no simple matter. Establishing a communication network between points on a plane requires a very specific set of multiplexed holograms. The fact that a large number of holograms (most likely more than 10) is required before this approach becomes competitive with current electronic techniques places an inherent constraint on the ultimate performance of the crystal, since the diffraction efficiency of each hologram decreases with increased multiplexing.

A high resolution surface plasmon resonance sensor is proposed and demonstrated by measuring the plasmon resonance enhanced Goos-Hanchen effect at attenuated total internal reflection. The giant Goos-Hanchen shift results from the singular phase retardation at the resonance which leads to the superior sensitivity of the sensor.

We present a C-shaped nanoaperture-enhanced Ge photodetector that shows 2-5 times the photocurrent enhancement over that from a square aperture of the same area at 1310 nm wavelength. We demonstrate the polarization dependence of the C-aperture photodetector over a wide wavelength range. Our experimental observation agrees well with finite-difference time-domain simulation results.

We present results of nanosecond laser ablation of silicon over a broad range of laser intensities, investigated by studying the time evolution of the ablation plume ejection, which, as measured by transmission of a probe beam, consists of a fast ejection phase followed by a delayed slow ejection phase. Experimental results indicate that different physical processes dominate at different ablation stages. The laser intensity threshold for the appearance of the slow ejection is ∼30GW∕cm2. The plume kinetics parameters extracted from our experiments agree well with estimations based on blast wave and homogeneous nucleation theories, providing a physical explanation for the observed ablation rate dependence on the laser light intensity.

We describe laser ablation of Si under water by 5 ns, 355 nm and 100 fs, 800 nm pulses. Compared to that in air, an approximately twofold improvement in the ablation rate is found in water for femtosecond and nanosecond pulses. For higher laser irradiances, the plasma that forms at the water-air interface hampers further improvement of the ablation rate. We investigated the enhanced ablation process in water and found that the cavity-confinement geometry that increases the laser energy coupling to the target and allows more energy to be transferred to the cavity sidewalls plays an important role in the escalated material removal process. In addition, we show that the water layer that effectively reduces the oxidation and redeposition of the ablated debris is also responsible for improvements in the ablation process.

Micrometer-sized reflection holograms can be written into a rapidly rotating homogeneous photopolymer disk at the focus of a high-numerical-aperture beam and its retroreflection to implement high-capacity multilayer digital data storage. This retroreflection is generated by an optical system with positive unity magnification to ensure passive alignment of the counterpropagating beam. Analysis reveals that the storage capacity and transfer rate of this bit-based holographic storage system compare favorably with traditional page-based systems but at a fraction of the system complexity and cost. The analysis is experimentally validated at 532 nm by writing and reading 12 layers of microholograms in a 125-microm photopolymer disk continuously rotating at 3600 rpm. The experimental results predict a capacity limit of 140 Gbytes in a millimeter-thick disk or over 1 Tbyte with the wavelength and numerical aperture of Blu-Ray.

Rear surface spallation of single-crystal silicon under 5‐ns laser pulse ablation at intensities of 0.6–60GW∕cm2 is studied through postablation examination of the ablated samples. The spallation threshold energy and the spallation depth’s dependences on the energy and target thickness are measured. From the linear relation between the spallation threshold energy and the target thickness, an estimation of the material spall strength around 1.4GPa is obtained, in reasonable agreement with the spall strength estimation of 0.8–1.2GPa at a strain rate of 107s−1 using Grady’s model for brittle materials. The experiment reveals the internal fracturing process over an extended zone in silicon, which is controlled by the competition between the shock pressure load and the laser ablation rate. The qualities of the laser microstructuring and micromachining results are greatly improved by using an acoustic impedance matching approach.

Families of fractals are investigated as near-field aperture shapes. They are shown to have multiple transmission resonances associated with their multiple length scales. The higher iterations exhibit enhanced transmission, and spatial resolution exceeding the first order. Near-field enhancements of greater than 400 times the incident intensity and resolutions of better than ?/20 have been shown with apertures modeled after third iteration prefractals. Enhancements as large as 1011 have been shown, when compared with conventional square apertures that produce the same spot size. The effects of the complex permittivity values of the metal film are also addressed.

The adoption of resolution enhancement techniques (RET) for subwavelength lithography relies on performing accurate simulation of mask effects. Although topography effects have been successfully used in RET flows, the impact of electromagnetic effects such as surface plasmons tend to be ignored. It is known, however, from the performance of “C-shaped” apertures that extraordinary transmission and opacity can be achieved using these electromagnetic effects. We have examined simulations for representative 45nm features using both conventional and novel finite difference time domain simulators. When the mask material is assumed to be conducting chromium, we found that electromagnetic effects appear to be significant and highly dependent on polarization. This may place additional constraints on the specification of mask material composition for these integrated circuit generations.

We describe the design and implementation of a high-data-rate high-capacity digital holographic storage disk system. Various system design trade-offs that affect density and data-rate performance are described and analyzed. In the demonstration system that we describe, high-density holographic recording is achieved by use of high-resolution short-focal-length optics and correlation shift multiplexing in photopolymer disk media. Holographic channel decoding at a 1-Gbit/s data rate is performed by custom-built electronic hardware. A benchmark sustained optical data-transfer rate of 10 Gbits/s has been successfully demonstrated.

Small subwavelength apertures provide high spatial resolution that is not limited by the diffraction limit. However, application of these apertures to practical problems has been hindered by the critical problem of extremely low power transmission efficiency. Recently, we reported a specially designed subwavelength aperture that has a letter C shape (X. Shi, R. L. Thornton, and L. Hesselink, Opt. Lett. 28, 1320 (2003)]. A well-designed C aperture can provide both a high spatial resolution of ~λ/10 and a high power throughput greater than 1. We present the underlying design ideas of the C aperture and report interesting general properties of optical transmissions through a single two-dimensional subwavelength aperture, based on numerical finite-difference time-domain simulations and fundamental observations. These results are expected to provide helpful information for both C-aperture applications and general studies of subwavelength metallic structures.

We describe a three-dimensional theoretical model of holographic shift multiplexing in thin volumetric media in paraxial approximation. In the case of shift multiplexing with three-dimensional beams, additional cross talk is found and its origin is attributed to Bragg-matching synchronism which occurs when shift in both transverse dimensions (x and y) is considered. The origin of this cross talk is attributed to three-dimensional Bragg degeneracy. The experimental results obtained for the case of two spherical wave front beams are in excellent agreement with the theoretical predictions. We propose theoretically that the cross talk in spherical multiplexing can be minimized if reference and signal waves have the same radius of curvature.

Photorefractive grating recording with two orthogonally polarized eigenwaves (ordinary and extraordinary) and vectorial coherent oscillation in a backward four-wave mixing scheme in iron-doped periodically poled lithium niobate are reported.

Near-stoichiometric LiNbO3:Ce,Mn exhibited a reversible photochromic effect at room temperature. Irradiation with ultraviolet (UV) light enhanced a visible absorption band, and the photorefractive sensitivity to 532 nm-green recording light was greatly improved after UV irradiation. Hologram multiplexing was performed with a continuous-wave 532 nm-green laser by the angle-multiplexing method without and with UV pre-exposure. An M/# of ∼5 was obtained in a crystal of 2.55 mm-thickness in the absence of UV pre-exposure. In multiplexing after UV pre-exposure, however, the variable sensitivity and erasure-time constant made it very difficult to achieve a multiple hologram storage with nearly equivalent diffraction efficiency.

Photorefractive properties of a 0.05-wt. % Ce2O3-doped Pb0.5Ba0.5Nb2O6 crystal are investigated at visible wavelengths. Compared with the performance of the undoped crystal, sensitivities as high as 3 cm/J represent an increase of almost two orders of magnitude, and a photorefractive gain of 25 cm-1 for extraordinary polarization exhibits a fourfold increase. The saturated diffraction efficiency varies with the intensity ratio of the incident beams owing to strong beam coupling. Photoconductivity at 514.5 nm and dark conductivity are also measured. The intensity dependence of these light-induced parameters suggests a nonsingle-photon behavior. We use a two-center model that involves both a shallow level and a deep level to explain the measured results.

Tetragonal lead barium niobate (Pb(1-x)Ba(x)Nb(2)O(6);1-x

We demonstrate a novel apodization technique for holographic data storage using two-photon recording in stoichiometric lithium niobate. The gating light-intensity profile is used to achieve grating apodization inside the bulk of the crystal during recording in the transmission geometry. Experimental Bragg-selectivity curves and theoretical fits indicate a >20-dB drop in multiplexing cross talk.

We demonstrate a one-color one-beam pumping method for a three-dimensional two-step excitation fluorescent display with Er(3+) -doped ZBLAN glass. A localized green fluorescent spot is obtained by use of a focused pump-light beam at 979 nm. The quantum efficiency of the two-step excitation fluorescence is investigated in a time-dependent analysis and an experiment with a pulsed pump light.

We investigate the resonant two-photon (two-step) processes of photorefractive grating formation and identify the regimes in which nonvolatile holography is possible. We develop a charge-transport model to describe this behavior for a photorefractive crystal with a single active impurity species under continuous-wave illumination. For the cases that allow for nondestructive reconstruction of gratings, we evaluate the maximum refractive-index perturbation and the response rate with respect to illumination intensities and impurity characteristics. We evaluate the importance of the impurity intermediate-state occupancy. Holographic data storage system issues are also discussed. The present results are consistent with previously reported photorefractive behavior and predict additional properties that characterize these resonant processes. The analysis may be used further to study other related two-photon phenomena of interest in holographic storage systems.

Polymer films doped with 2-(5, 5-dimethyl-3-styryl-cyclohex-2-enylidene)-malononitrile chromophore and various sensitizers have been prepared and characterized. The material exhibits high sensitivity in the red region (S=0.26 cm(2)/J at lambda=633 nm), a large dynamic range (Dn=-0.05) , and large transient two-beam coupling gain (G=110 cm(-1)) . Self-diffraction effects, such as energy and phase coupling, and photoinduced scattering are studied.

Unwanted erasure during readout of holographic data can be reduced or eliminated by use of a different wavelength for reading than that which was used for writing. To prevent distortion and Bragg mismatch that would be unacceptable for digital data storage, one can format data to account for the wavelength difference. Techniques to format data and the results of this formatting are presented. Varying the formatting parameters is investigated to optimize diffraction efficiency.

We present a novel digital holographic data storage system based on wavelength multiplexing in 90 degrees geometry, using an automated tunable diode laser and a lithium niobate crystal. The automatic storage and retrieval of a 60-kbyte data file, as well as the limitations and future implementations of the system, are discussed.

We derive the relationships for coded wave fronts in volume holography for reference and signal waves coded with two-dimensional wave fronts. We develop estimates for cross talk that is due to grating degeneracy, nonorthogonality, and momentum-mismatch reconstruction. We compare a number of multiplexing techniques on the basis of cross talk and other noise sources in a system.

We describe a digital holographic data storage system that uses in situ thermal fixing to achieve nonvolatile readout. The system was used to store and fix 530 holograms representing 1.7 MB of digital data. The system demonstrates that fixing by heating after recording gives adequate performance for multiplex holography in the perpendicular recording geometry. The postrecording heating procedure is preferred over high-temperature recording in the perpendicular geometry to achieve Bragg matching for the entire signal angular bandwidth.

We study detuning effects that modify the Bragg condition in photorefractive crystals and analyze the influence of these effects on the retrieval of information that is stored in such crystals. We examine and integrate several mechanisms that induce Bragg detuning, including the effects of temperature changes, applied electric fields, and polarization changes, taking into account the anisotropic nature of photorefractive crystals. Transfer functions based on these detuning effects are defined and computed to examine and quantify the signal quality that is obtained during the readout of image-bearing holograms. The different recording configurations and geometries that are typically used in photorefractive applications are considered and compared. Finally, we study more specifically the consequences of temperature-induced detuning in the context of a holographic data storage system that uses thermal fixing. We show theoretically and experimentally that distortions (in this case limited field of view) occur on readout, and we propose solutions to improve the quality of the retrieved images.

A general analytical model is developed for analysis of the diffraction properties of layered structures of photorefractive gratings. With this model a simple closed-form expression is derived in both the transmission and the reflection geometry for the diffraction efficiency that describes the Bragg selectivity properties of these structures. We then propose and analyze a novel optical device, a dynamic multiple-wavelength filter, that combines the diffraction properties of such layered structures with the photorefractive effect and the electrically controlled diffraction effect.

Various channel codes, including binary, gray-scale, threshold, and differential techniques, are compared for digital holographic data storage. The tradeoffs among bit error rate, storage capacity, and system complexity are discussed.

An encryption method and apparatus for holographic data storage are disclosed. In a system using orthogonal phase-code multiplexing, data is encrypted by modulating the reference beam using an encryption key K represented by a unitary operator. In practice, the encryption key K corresponds to a diffuser or other phase-modulating element placed in the reference beam path, or to shuffling the correspondence between the codes of an orthogonal phase function and the corresponding pixels of a phase spatial light modulator. Because of the lack of Bragg selectivity in the vertical direction, the phase functions used for phase-code multiplexing are preferably one dimensional. Such phase functions can be one-dimensional Walsh functions. The encryption method preserves the orthogonality of reference beams, and thus does not lead to a degradation in crosstalk performance.

We have observed and explained three types of hexagon pattern formation in photorefractive crystal KNbO 3 :Fe. These are: (1) dynamic (laser induced), (2) semipermanent (holographically stored), (3) permanent (induced by a static domain grid) over a wide wavelength range.

The selection of reference beams for cross-talk-limited wavelength- and angular-multiplexed volume-holographic data storage is discussed. It is shown that sparse selection of the reference waves can lead to a significant improvement in the cross-talk-limited signal-to-noise ratio in a wavelength-multiplexed system. In a high-capacity angular-multiplexed system sparse selection offers essentially no advantage.

We present a comprehensive analysis of the evolution of a grating envelope, in both transmission and reflection geometry, during the erasure process in various readout configurations in bulk photorefractive media. We derive a single partial differential equation that describes grating evolution. This equation shows that under certain conditions the grating envelope propagates and shares several features with either bright or dark solitons. Details of the grating envelopes directly relate to the dynamics of the observed diffraction efficiency. We derive analytical expressions for diffraction during nondestructive readout. Finally, we discuss the effects of nonzero absorption, finite dark conductivity, fringe bending, fanning, and nonlinear responses on the propagating envelope.

We explore theoretically and experimentally how applying an electric field to a photorefractive crystal affects the recording and the readout of a hologram. We present a unified theory that describes several electric-field-related mechanisms relevant to photorefractive crystals, including amplitude coupling, phase coupling, linear and nonlinear electro-optic effects, and the piezoelectric effect. We analyze the influence of these different effects on the Bragg selectivity and the diffraction efficiency and derive general analytical expressions describing the Bragg detuning effects. Finally, we compare the theory with experiments that we have performed in a strontium barium niobate crystal by recording and retrieving holograms in the presence of an applied electric field.

We introduce the concept of traveling grating envelopes to study the erasure dynamics in bulk photorefractive media. This new technique provides considerable physical insight into the observed photorefractive transients. We propose and demonstrate experimentally, in a Ce-doped SBN:60 crystal, two novel ways of extending the grating lifetime by trapping the grating envelope inside the medium. The incident beam ratio and the readout geometry have the largest effect on how long the grating can be read out.

An analytical model is developed to describe transient grating formation in surface-charge-dominated Pockels readout optical modulator devices for long exposures. It is shown that, under a short-transport-length approximation, solutions to the one-dimensional transport equations reveal the existence of thin carrier accumulation and depletion regions in the inner photoconductive layer near the dielectric–photoconductive interfaces. Rate equations for the accumulation of the net surface charge at these interfaces are derived and are solved self-consistently in a small-modulation approximation. The temporal behavior of the internal field and the grating-spacing dependence of the time response and of the grating amplitude are examined. It is shown that field fringing significantly affects the time response and the device resolution at small grating spacings. The general dependence of device performance on geometry is also discussed.

We investigate diffraction from a thermally fixed grating in a photorefractive medium with an ion concentration that is large compared with the acceptor density. A numerical solution to the standard band transport model with mobile ions and the coupled-wave equations are used to study steady-state solutions. For the first time to our knowledge, transient responses are investigated during recording at elevated temperatures and during readout at room temperature. We consider the effect of large applied fields, which reduces the electronic screening and reveals the fixed ionic grating, on diffraction efficiency. The combined effects of self-diffraction, beam coupling, electrical revelation, and in some materials increased sensitivity to the applied field can result in a large diffraction efficiency even from a weak fixed grating. Results showing how transient solutions are affected by absorption in the photorefractive medium and by the magnitude of the incident intensity are also provided.

We compare the characteristics of cross talk for angular multiplexing and several phase-encoded multiplexing strategies for volume holography. We discuss the implications of these characteristics for holographic data storage and compare noise arising from cross talk with noise arising from scattering that is due to inhomogeneities within a holographic medium.

A two-center photorefractive model permitting all center-to-band transitions and electron–hole charge transport unifies several models describing subsets of these transitions. Using the quasi-steady-state approximation for low continuous-wave illumination, the model derives the first-order Fourier harmonic of the steady-state space-charge distribution and fields and the corresponding response rates. The model describes electron–hole competition, complementary gratings, double-exponential time-dependent behavior, and nonlinear dependence of response rates on intensity. The model can be used to make new predictions of photorefractive behavior of crystals in which two species of impurity center are present; an example is presented for BaTiO3.

An analytical model is developed to study layered structures of photorefractive thin films, a promising application of stratified volume holographic optical elements. The theory gives a closed-form solution for the first-order diffraction efficiency of such elements. A new holographic multiplexing scheme that uses the photorefractive phase shift induced by an applied electric field is also analyzed with this theory.

We explore theoretically and experimentally how application of an electric field to a photorefractive crystal affects the recording and the readout of holograms. We consider, for the first time to our knowledge, the effects of fringe bending caused by nonlinear two-wave mixing on the change in Bragg condition and diffraction efficiency as a function of the applied electric field. Practical performance limitations for holographic data storage and image amplification are discussed.

We have built an adaptive system capable of detecting submicrometer defects in periodic structures by using real-time holography in photorefractive crystals. We summarize the design and optimization of the defect-enhancement system. We present representative results of 0.2- to 0.5-µm diameter defect detection on two different periodic substrates: chrome-on-glass masks and patterned silicon wafers. On patterned silicon we detect 94% of the 0.5-µm diameter defects with three false positives, while inspecting an area greater than 1 mm(2) in 20 s or less. The throughput on the glass masks is somewhat less. To the best of our knowledge, these defects have an area 100 times smaller than those previously detected with any real-time holographic technique.

Growth-induced striations in photorefractive media constitute phase gratings that scatter transmitted light and therefore degrade the quality of reconstructed images in holographic applications. A simple angular multiplexed holographic memory arrangement in a striated strontium barium niobate sample is considered. For forward-propagating reconstruction the striations distort the readout images and introduce ghost images. For counterpropagating readout, inhomogeneous image reconstruction and interimage cross talk can occur. We investigate these effects experimentally for different reconstruction methods and demonstrate the cross talk and the ghost images with a simple model based on coupled-wave theory. Readout with a counterpropagating plane wave eliminates the ghost images and the distortions but suffers degradation from cross talk and inhomogeneous image intensity. Only reconstruction with a phase conjugate of the transmitted reference beam eliminates cross talk and distortions. The model indicates that negligible material dispersion is a necessary requirement for high-quality phaseconjugation readout.