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6 pags., 4 figs., Optical force is a powerful tool to actuate micromachines. Conventional approaches often require focusing and steering an incident laser beam, resulting in a bottleneck for the integration of the optically actuated machines. Here, we propose a linear nanomotor based on a plasmonic particle that generates, even when illuminated with a plane wave, a lateral optical force due to its directional side scattering. This force direction is determined by the orientation of the nanoparticle rather than a field gradient or propagation direction of the incident light. We demonstrate the arrangements of the particles allow controlling the lateral force distributions with the resolution beyond the diffraction limit, which can produce movements, as designed, of microobjects in which they are embedded without shaping and steering the laser beam. Our nanomotor to engineer the experienced force can open the door to a new class of micro/nanomechanical devices that can be entirely operated by light., This work was supported by JST PRESTO grant JPMJPR15PA, Japan, and JSPS KAKENHI grant nos. JP19H02533 and JP19H04670 in Scientific Research on Innovative Areas “nanomaterial optical manipulation.” Preliminary studies were performed at Imperial College London, under support of the EPSRC Active Plasmonics Programme (EP/H00917/2). P.A acknowledges Ramon y Cajal Fellowship RYC-2016-20831 and MICINN project PGC2018- 096649-B100. M.R. acknowledges support from the Royal Society and the Wolfson Foundation.

We present a numerical study of optical torque between two twisted metal nanorods due to the angular momentum of the electromagnetic field emerging from their plasmonic coupling. Our results indicate that the interaction optical torque on the nanorods can be strongly enhanced by their plasmon coupling, which highly depends on not only the gap size but also the twisted angle between the nanorods. The behaviors of the optical torque are different between two plasmon coupling modes: hybridized bonding and anti-bonding modes with different resonances. The rotations of the twisted nanorods with the bonding and anti-bonding mode excitations lead to mutually parallel and perpendicular alignments, respectively. At an incident intensity of 10 mW/μm2, the rotational potential depths are more than 30 times as large as the Brownian motion energy, enabling the optical alignments with angle fluctuations less than ∼±10°. Thus, this optical alignment of the nanoparticles with the plasmon coupling allows dynamic control of the plasmonic characteristics and functions.

I will introduce test items and measuring methods for camera lenses in the “New Face Clinic” of the “Photography Journal, Asahi Camera” that is a monthly magazine for camera and photo enthusiasts. In that article, we introduce the lens configuration, special glass material, aspheric surfaces, coating type first. The main test items are focal length, open aperture value, spherical aberration, curvature of field, astigmatism, distortion, and modulation transfer function (MTF). Also, we actually shoot distant views, point light sources, close-up shots and evaluate images through the eyes. Although the test results are mixture of the effect of the image processing and physical lens performance, we measure Spatial Frequency Response (SFR). Finally, we evaluate the lenses comprehensively from these test results.

A new photovoltaic generation unit based on the application of holographic technologies called a Holo-Window is proposed in this work. The basic principle and the optical configuration used for the basic experimental unit are described. Suitable fabrication technology for a hologram with the broadband spectrum required to provide the appropriate sunlight capture capability is then discussed. Finally, a laboratory-prototype Holo-Window unit was developed and its performance was evaluated.

In holographic data storage systems, the quality of the reconstructed data pattern is decisive and directly affects the system performance. However, noise from the optical component, electronic component and recording material deteriorates reconstruction quality. A high noise margin decoding method developed from compressed sensing technology was proposed to reduce the impact of noise in the decoding process. Compared with the conventional threshold decoding method, the proposed method is more robust to noise and more suitable for multilevel modulation. The decoding performance with five-level amplitude modulation was evaluated by both simulation and experimentation. For the combination of Gaussian noise, Rician noise and Rayleigh noise, the proposed decoding method reduces the BER of the threshold method to one-sixth with an SNR of -1 in the simulation. In the experiment, it behaves up to 8.3 times better than conventional threshold decoding.

High harmonic generation (HHG) in solids has great potential for coherent extreme ultraviolet (EUV) sources, all-optical band-structure reconstruction, and electron dynamics metrology. Solid HHG driven by plasmonic near-fields will open a new paradigm, enabling high repetition-rate HHG with a compact laser, HHG manipulation with meta-surfaces, and precise control over carrier trajectory. In this paper, we demonstrate antenna-enhanced HHG in a wide-bandgap semiconductor ZnO. By exploiting gold nano-antennas resonating at the driver wavelength of 2 μm, we successfully trigger HHG at input intensity of ~0.02 TW/cm2 and observe harmonic radiations up to 9th-order. Orders-of-magnitude enhanced conversion efficiency at the hot-spots brings about ten-fold enhancement in the total yield. The spectral selection rule is found to reflect crystal symmetry, suggesting the possibility of nano-scaled EUV sources and band-structure reconstruction.

We present an experimental study on the optical polarization properties of the phenanthrenequinone-doped poly(methyl methacrylate) photopolymer materials. We discuss the diffraction efficiency with different weight ratios of phenanthrenequinone that was dissolved in the materials. In addition, we observe the diffraction efficiency difference between polarization holography and traditional holography.

The interaction between light and magnetism is considered a promising route to the development of energy-efficient data storage technologies. To date, However, ultrafast optical magnetization control has been limited to a binary process, whereby light in either of two polarization states generates (writes) or adopts (reads) a magnetic bit carrying either a positive or negative magnetization. Here, we report how the fundamental limitation of just two states can be overcome, allowing an arbitrary optical polarization state to be written magnetically. The effect is demonstrated using a three-sublattice antiferromagnet--hexagonal YMnO_3. Its three magnetic oscillation eigenmodes are selectively excited by the three polarization eigenstates of the light. The magnetic oscillation state is then transferred back into the polarization state of an optical probe pulse, thus completing an arbitrary optomagnonic write-read cycle.

Fast non-interferometric phase retrieval is a very important technique for phase-encoded holographic data storage and other phase based applications due to its advantage of easy implementation, simple system setup, and robust noise tolerance. Here we present an iterative non-interferometric phase retrieval for 4-level phase encoded holographic data storage based on an iterative Fourier transform algorithm and known portion of the encoded data, which increases the storage code rate to two-times that of an amplitude based method. Only a single image at the Fourier plane of the beam is captured for the iterative reconstruction. Since beam intensity at the Fourier plane of the reconstructed beam is more concentrated than the reconstructed beam itself, the requirement of diffractive efficiency of the recording media is reduced, which will improve the dynamic range of recording media significantly. The phase retrieval only requires 10 iterations to achieve a less than 5% phase data error rate, which is successfully demonstrated by recording and reconstructing a test image data experimentally. We believe our method will further advance the holographic data storage technique in the era of big data.

Polarization holography is the coherent interference of the beams that can have the different polarized states. The early-stage theory of polarization holography is based on Jones matrix, where the paraxial approximation is assumed, while the theory of polarization holography represented by dielectric tensor can describe the case with a large crossing angle. And it also depicts the relationship between diffraction light and interference light. During the research people find some extraordinary phenomenon, such as null reconstruction and inverse polarizing effect. But there is a disadvantage in this new polarization holography theory, where only under a peculiar circumstance can we get a faithful reconstruction. The circumstance can be expressed as “A+B=0”, where A and B refer to the coefficients for intensity and polarization holograms respectively. In this research, we calculate the formula of diffraction light’s polarization, and extract the A+B factor in it. Then we establish a series of equations which can let the diffraction light faithfully reconstruct, no matter what value of A plus B is. From the result, we can use an artificial reference beam which is corresponding to the signal beam to generate the hologram. Under this condition, the polarization of the diffraction light is similar to the signal. For simplification, we only discuss the signal wave with circular polarization and experimentally verify the result.

Achieving high directionality of scattered light in combination with high flexibility of the direction using plasmonic nanoparticles is desirable for future optical nanocircuits and on-chip optical links. The plasmonic characteristics of nanoparticles strongly depend on their geometry. Here, we studied directional light scattering by a single-element triangular plasmonic nanoparticle. Our experimental and simulation results demonstrated that the triangular nanoparticle spatially sorted the incoming photons into three different scattering directions according to their polarization direction, including circular polarization, despite its compact overall volume of ∼λ3/300. The broken mirror symmetry and rotational symmetry of the triangular nanoparticle enabled such passive tridirectional polarization routing through the constructive and destructive interference of different plasmon modes. Our findings should markedly broaden the versatility of triangular plasmonic nanodevices, extending their possible practi...

Holographic memory is currently attracting attention as a data storage system capable of achieving a data transfer rate of about 105~106105~106 times that of an optical disc such as Blu-ray disc. In conventional holographic memory, data is generally recorded by optical writing using volume holograms. However, a volume hologram has the problem not only that it is required to have high mechanical accuracy of a system and low coefficient of thermal expansion of a recording medium, because reconstruction tolerance is extremely low, but also that duplicating time efficiency is poor because whole data cannot be recorded at once. In this paper we proposed surface holographic memory that achieved a high data transfer rate, stable readout performance, and collective duplication by expressing holograms with fine surface asperity. Furthermore, the theoretical formulas of recording and reconstruction processes in the proposed system were derived and the reconstruction characteristics of the hologram were evaluated by numerical simulation. As a result, the proposed method generated reconstructed image readout with sufficient signal for a single page recording. However, the reconstructed image had noise, which was particular to a surface holographic memory.

Volume polarization holographic recording in phenanthrenequinone-doped poly(methyl methacrylate) (PQ-PMMA) photopolymer with linear polarized light is obtained. The characteristics of the volume polarization hologram are experimentally investigated. It is found that beyond the paraxial approximation the polarization states of the holographic reconstruction light are generally different from the signal light. Based on vector wave theoretical analyses and material properties, the special exposure condition for correctly holographic reconstruction is obtained and experimentally demonstrated.

Recently new photopolymers that does not require scheduled exposure in the multiple page writing for the holographic memory was introduced. Its sensitivity does not decrease throughout the multiple page writing process. We will explain why this photopolymer has such a property with our reaction model. The key is the existence of the molecules which make seeds of the polymers. At the first exposure process to record the page data, seeds are produced corresponding to the light intensity distribution. Then after writing the information of all multiplexed pages, the seeds are growing to be polymers under the spatially uniform light illumination.

We report on the inverse polarizing effect (IPE) of an elliptical-polarization recorded hologram at a large recording angle. The IPE is a polarizing phenomenon in which the reconstructed signal switches the major and minor axes and keeps the original polarization, direction compared, to that of the signal wave. In reviewing the case of a linear-polarization and circular-polarization recorded hologram, we found that the IPE is a unique phenomenon for elliptical polarization. The IPE was observed at the cross angle of 38° experimentally, and was theoretically explained using tensor theory to remove paraxial limitation.

Polarization holography is the superposition of differently polarized beams. Due to its ability to record the polarization states, some extraordinary optical phenomena were found in the polarization holography. For example, the recently reported null-reconstruction phenomenon in polarization volume hologram is odd for the conventional holography which only records the amplitude and phase. In this paper, we perform a thorough investigation of the null reconstruction of polarization hologram recorded by orthogonal circularly polarized waves. To explore the mechanism behind this phenomenon, an interferometry was built to measure the phase difference between the same polarized components within the reconstructed wave. The phase difference of π was secured in our experiment, indicating a destructive interfering effect, which nicely explains the extraordinary null reconstruction observed in the polarization hologram.

Researchers use spatially shaped light to control the direction of spin-wave emission from the ferrimagnetic insulator Gd4/3Yb2/3BiFe5O12. They capture the essential features of the observations by employing a simple model that maps the spatial profile of the pump pulse onto the dispersion relation of the spin wave.

The optical response of polarization sensitive materials is studied on the basis of a model in which the materials consist of rod molecules that are oriented in random directions. The photoinduced change of the dielectric tensor is expressed as a function of the electric field in a general form. Using this expression, we derive the vectorial coupled wave equations in volume polarization holograms. We discuss the basic properties of polarization holograms.

Increasing photosensitizer concentration has been considered as an effective approach to improve the performance of holographic material. In this paper, we report on new method for increasing the saturated dissolvability of photosensitizer PQ within polymeric media by introducing copolymerization monomer into the PQ/PMMA. The photosensitizer concentration of PQ was increased from 0.7wt% to 1.3wt%, compared with the typical PQ/PMMA sample. Besides, we investigated performance of polarization holographic recordings in typical PQ/PMMA and copolymerization monomer-containing PQ/PMMA with the orthogonally polarized signal and reference waves. And the doping of THFMA component resulted in a significant improvement of diffraction intensity and photosensitivity. In addition, high-quality holographic image reconstruction was realized in our home-made material.

Holographic reconstruction with polychromatic light was investigated in terms of the multiplexing capability in a volume holographic memory. The broadband spectrum of the probe beam considerably degrades the angular selectivity and causes a serious deterioration of the multiplexing capability, but this can be overcome if an additional suitable optical component is used in the imaging system. We could selectively detect the signal image alone even though the angular separation between holograms was insufficient to suppress the diffraction from other holograms. We theoretically derived the minimum angular separation for multiplexing and experimentally demonstrated the effectiveness of this selective detection method. 2010 Optical Society of America.

We succeeded to develop a reference light source in the range of very low luminance using a double integrating sphere system, and calibrated a commercial spectrophotometer below 1 × 10-5cd/m2 levels, which is 1/100 lower than the specified limit for measurement. And we improved measurements in the ultra low luminance range of displays using the calibrated commercial spectrophotometer and a dark sphere to suppress the influence of the surround.

The properties of a light-induced absorption and two-color recording were investigated in three kinds of Ru and Fe codoped LiNbO3 crystals with different Fe concentrations. A nonvolatile hologram could be recorded at a wavelength of 633 nm when blue light was simultaneously incident on the crystal. The characteristic feature of these crystals is a high persistence factor , i.e., small loss of the hologram strength at the beginning of the readout process. The sensitivity and the dynamic range increased with increasing Fe concentration, and a high sensitivity of 0.12 cm/J and a large dynamic range of 0.53 were obtained in a 0.5 mm-thick crystal. The recording model of the nonvolatile hologram is discussed from the results of light-induced absorption measurements. 2009 Elsevier B.V. All rights reserved.

We experimentally demonstrate engineerable compression of two-colored pulses in a linearly-chirped quasi-phase-matching grating. Quadratic solitons generated from fundamental input are reshaped through cascaded parametric processes of second-harmonic generation (SHG) and the back-conversion. We use type-I (e: o + o) SHG geometry in a 50-mm-long aperiodically-poled MgO:LiNbO 3 device to satisfy the group-velocity matching condition. Simultaneously compressed fundamental and SH pulses of about 55-fs duration with small pedestal are generated from the fundamental input pulses of 95-fs duration.

We studied the sorting of micro-scale objects by use of an optically induced asymmetric double-well potential. Migration events, driven by Brownian motion, were experimentally investigated for various particle sizes, inter-well distances, and optical intensities. Temporal evolution of the measured sorting efficiency qualitatively agreed well with theoretical predictions. We found that the sorting performance was improved by introducing temporal modulation on the potential landscape.

International audience; A new method to determine complex amplitudes of index and absorption gratings in photorefractive multiple quantum wells is presented. In usual method to evaluate these parameters, spectral measurement of absorption change due to the applied field over a wide wavelength range is required and the index change is calculated through the Kramers-Krönig's relationship. In our method, only a set of measured data for photorefractive two-wave mixing set-up with small sinusoidal phase modulation for fixed wavelength is needed. This is thus well adapted to insitu characterization of systems that operate at a given wavelength. Real and imaginary part of the gains owing to the index and the absorption gratings are calculated from the output signals of the system. We apply this method to our photorefractive multiple quantum devices operating at 1064 nm, and obtain consistent results

Cascaded processes of energy exchanges between optical beams via second-order nonlinearities lead to various phenomena, including Kerr-like nonlinear phase shifts and optical solitons. The cascading nonlinearity, combined with the developments in quasi-phase-matching devices, is becoming more promising for all-optical signal processing and ultrashort pulse generation. Here we review the basic properties of the cascading nonlinearity and present our recent experiments on femtosecond cascading with quasi-phase-matching devices.

The adaptive measurement of nanometre-scale vibrations of diffuse surfaces using the polarization self-modulation effect in a photorefractive GaP crystal is described. The sensitivity is investigated as a function of the applied electric field, the intensity of the incident light, and the mean size of the speckles. Numerical estimation of the sensitivity dependence on the mean size of the speckles shows a good agreement with experimental results.

Cascaded quadratic nonlinearities in periodically poled lithium tantalate are studied with femtosecond pulses by using the spectrally resolved two-beam coupling technique. The nonlinear phase shifts show obvious dependence on the phase mismatch conditions. Relatively large intrinsic nonlinear refraction and its anisotropy are also measured.

Photorefractive response rate in a polymer composite of poly(N-vinylcarbazole)/2,4,7-trinitro-9-fluorenone, doped with 2,5-dimethyl-4-(4'-nitrophenylazo)anisole (DMNPAA) is improved by modifying the structure of DMNPAA. We found that four kinds of modified DMNPAA with alkyl substituents have fast orientational response to an external electric field and keep large anisotropy in polarizability. Among them, 4-butoxy-3-propyl-1-(4'-nitorophenylazo)benzene has the shortest reorientation time constant of 19 ms (E=54 V/µm) and photorefractive time constant of 80 ms (I=2 W/cm2, E=54 V/µm) that are 2,300 times and 63 times faster than those of the DMNPAA composite. The mechanism of the fast reorientation in new polymer composites is investigated. The fast reorientational response is achieved by the improvement of the dispersivity in the polymer composites and the decrease of the glass transition temperature.

Faithful reconstruction, the polarization state of the diffraction wave being identical to that of the signal wave, is the primary target in polarization holography. With the large crossing angle between signal and reference wave, namely, nonparaxial approximation, attaining a balance between intensity and polarization grating is generally known as a prerequisite for faithful reconstruction. However, obtaining such a balance point within polarization-sensitive material has long been known as a challenging job, due to its extremely strict exposure condition. In this paper, we report on a smart method with the well-designed reference and reading wave to faithfully reconstruct the signal wave at an arbitrary exposure energy. Moreover, when the signal is circularly polarized, the diffraction efficiency can be precisely manipulated by changing the reading wave’s polarization state, which is favorable for optical storage and functional diffractive optical elements.

We report on dual-channel recording within polarization holography written by orthogonal linear polarization waves. The null reconstruction effect (NRE) of linear polarization holography was experimentally achieved at a large cross-angle of π/2 inside the polarization-sensitive media. Based on the NRE, two polarization encoded holograms were recorded in a dual-channel recording system with negligible inter-channel crosstalk. The two polarization multiplexed holograms could then be sequentially or simultaneously readout by shifting the polarization state of reference wave with the best signal-to-noise of 18:1 obtained within the experiment.

Unidirectional control of optically induced spin waves in a rare-earth iron garnet crystal is demonstrated. We observed the interference of two spin-wave packets with different initial phases generated by circularly polarized light pulses. This interference results in unidirectional propagation if the spin-wave sources are spaced apart at 1/4 of the wavelength of the spin waves and the initial phase difference is set to pi/2. The propagating direction of the spin wave is switched by the polarization helicity of the light pulses. Moreover, in a numerical simulation, applying more than two spin-wave sources with a suitable polarization and spot shape, arbitrary manipulation of the spin wave by the phased array method was replicated.

The experimental study of the in-plain vibration of the speckle pattern in photorefractive GaP and BTO crystals using the polarization self-modulation effect are presented. The simple model based on the propagation of the Gaussian beam through the crystal with recorded space-charge field is proposed to explain main features of the experiment. It is shown that these results allow designing a linear sensor of the speckle-pattern displacement in an effective way. The optical system is very simple and does not include any reference or readout beam. The sensitivity of this system is the same as that of an interferometer.

A broad-area laser is injection-locked by another broad-area laser that is also injection-locked by a single-mode diode laser. Two double-phase conjugate mirrors of photorefractive BaTaO3 are used to couple the master laser beams to the first slave laser, and the first slave laser output to the second slave laser. One of the double-phase conjugate mirrors is built up with the beams from two broad-area lasers. Two slave lasers are oscillating in single longitudinal mode at 808.5 nm and the spectral width is the same as that of the master laser. Final single-mode output power from the second slave broad-area laser is 840 mW, which is limited by the power of the injection beam. This work verifies the possibility of the multi-stage cascaded injection locking of high-power diode lasers with phase-conjugate injection.

Injection locking of a 1-W broad-area diode laser was achieved using a double phase-conjugate mirror (DPCM). The DPCM was build up for the first time using the broad-area laser beam and it coupled an elliptical-shaped beam of a single-mode master laser efficiently into a broad-area laser's slit-shaped output facet. Single longitudinal mode operation up to 550 mW output was achieved by an injection beam power of no more than 0.6% of the whole output. The lateral beam pattern was also improved and the measured M 2 value of the locked beam greatly decreased compared with that of the free-running beam. M 2 =8 for low-power operation and M 2 =28 for high-power operation were achieved.

The performance of an externally-pumped photorefractive phase conjugator is greatly improved, especially in stability and response to signal change, by the phase control of incident beams. Using piezoelectrically-driven mirrors in all incident optical paths, the backward pump is made incoherent with the other input beams and the fluctuation of mutual mean phases between forward pump and signal beams is canceled.

The photorefractive effect was observed in He-ion-implanted semi-insulating GaN film in the UV spectral region. The photorefractive grating is induced by the electroabsorption effect near the band-edge. Two beam coupling experiments were performed at a wavelength of 363.8 nm. The experimental results indicate that absorption grating mainly contributes to the beam coupling.

A collinear holographic memory system recording multichannel time sequential signal is proposed and demonstrated. Higher recording density and data transfer rate comparing to the page-oriented system are expected because Run-Length Limited code can be used.

A polarization coded holographic memory is demonstrated. Writing and reading characteristics are examined through numerical calculations and experiments. The polarization state of the readout signal is influenced by the ratio of intensity and polarization gratings.

Efficient second-harmonic generation of 95 fs pulses were achieved by simultaneous group-velocity matching and quasi-phase matching in a 10 mm long periodically poled potassium niobate crystal. The quasi-phase-matched bandwidth was 38 nm and second-harmonic pulse duration was 158 fs with a fundamental input pulse of 95 fs without spectral and temporal distortion in the S-band of telecommunication. Utilizing the larger off-diagonal nonlinear coefficient, d31 of potassium niobate, the conversion efficiency was significantly improved from that of periodically poled MgO-doped lithium niobate. [Appl. Phys. Lett. 82, 3388 (2003)]

Efficient pulse compression was achieved by the use of cascaded quadratic nonlinearities in 10-mm-long periodically poled MgO-doped lithium niobate. An off-diagonal component of the nonlinear coefficient was utilized for group-velocity matched cascaded interactions at the fundamental wavelength of 1560 nm. Simultaneously compressed fundamental and second-harmonic pulses of ∼35 fs duration were obtained from the 110-fs-pump pulses.

Polychromatic reconstruction(PCR) is a novel nondestructive readout method that utilizes a spectrally broad light source for the probe beam. The stored image can be completely reconstructed even though the probe wavelength is very different from the recording one. The large spectral width of the polychromatic probe beam also causes adverse effect on the storage density. But this can be overcomed if an additional optical component is inserted in the imaging system. Therefore, PCR has a great potential to achieve nondestructive readout and large storage density simultaneously. In addition, PCR enables us to design a simple memory system because its readout tolerance is quite large as compared to the conventional monochromatic readout. Unique and attractive features of the polychromatic reconstruction method was theoretically and experimentally demonstrated.

We demonstrate efficient frequency doubling of 95-fs pulses with a small temporal broadening in a 10-mm-long, periodically poled, 5-mol % MgO-doped lithium niobate crystal. Simultaneous group-velocity matching and quasi phase matching were achieved by the off-diagonal nonlinear optical coefficient d32 at telecommunications range.

The characteristics of the reconstruction of polarization holography are investigated in order to analyze the fundamental performance of polarization holographic memory system. It is found that, in general, the polarization state of the reconstructed signal is different from that of the recorded signal. This fact makes the design of polarization holographic memory system complicated because we have to retrieve the polarization state of original signal from measured data. In this paper we describe preliminary experimental results of photodissociation-type holographic media.We discuss the special material condition that the signal can be faithfully reconstructed.

In volume holographic memory (van Heerden, 1963), the information is stored as a volume hologram and retrieved through the holographic reconstruction process by illuminating the hologram with a readout probe beam whose wavelength, incident angle, and wavefront should be identical to those of the reference beam used in the recording process. This requirement stems from the fact that diffraction from the volume hologram is restricted by Bragg’s law. While such a restriction is responsible for the large storage density of volume holographic memories, it also causes some obstacles for implementing practical memory systems. For example, in rewritable recording media, like photorefractive materials, illumination with a readout probe beam will rewrite the recorded hologram, destroying the stored information. Even in a photopolymer, some of the storage capacity will be wasted during the readout if some monomers still exist in the readout volume. These issues are obviously caused by the destructive probe beam having the ability to expose the recording medium in a similar manner to the recording beam. To avoid such a problem, several nondestructive readout methods have been proposed so far (Gulanyan et al., 1979; Petrov et al., 1979; Kulich, 1987), where the readout is performed at a longer wavelength, outside the sensitive spectral region of the recording material. These methods can successfully reconstruct the stored image at a wavelength different from the recording one, but most of these methods may not be practical for holographic memory systems because the multiplexing capability is considerably lowered. For example, anisotropic diffraction (Petrov et al., 1979) requires a specific recording configuration and thus limits the number of multiplexed pages. A spherical probe beam method (Kulich, 1987) tends to produce severe crosstalk noise from other multiplexed pages, which demands a large angular separation between two adjacent multiplexed holograms, resulting in a small storage density. Recently, we proposed another way to reconstruct an image at a different wavelength (Fujimura et al., 2007). Our method, which we call polychromatic reconstruction (PCR), utilizes a spectrally broad light source for the probe beam, as shown in Fig. 1. Each angular spectral component of the recorded gratings can be Bragg-matched with one particular wavelength within the broadband spectrum of the probe beam. Thus, the whole image can be reconstructed from the volume hologram even though the probe wavelength is very different from the recording one. On the other hand, analogous to the spherical probe beam

The intrinsic problems, such as beam fanning and internal beam reflection, in measuring the photorefractive coupling constant are discussed. We propose a new technique, the varying interaction length method, to avoid them. In this method, the coupling constant is obtained by differentiating the coupling strength that is measured with changing the interaction length. We applied this method to BaTiO 3 and obtained a 10–20% larger coupling constant compared with that of the conventional method.

InGaAs/GaAs multiple quantum well (MQW) photorefractive devices that have the sensitivity around the wavelength of 1.06 μm are fabricated and some characteristics are measured. Saturation intensity is 3.6 mW / cm 2 and cut off grating pitch is 1.4 μm . These values show that this device is usable in practical applications using Nd:YAG lasers. A vibration measurement system using two-wave mixing is also demonstrated.