Search

Recently, several works have shown that the light group velocity can be reduced using the recording of dynamic refractive index gratings in photorefractive (PR) crystals [1] , [2] , [3] . The slowdown of light is performed using optical beams wave mixing: it yields the output pulse amplification and increases the light dispersion inside the PR crystal. For example, the TWM method demonstrated that the delay time ∆ τ can be varied as a function of the TWM gain values Γ and the input pulse widths T 0 . However, several works have shown that the results achieved by this method are limited in terms of pulse duration. Indeed, only long pulses with time duration of ms−s can be slowdown in a PR crystal both at the visible and the infrared wavelengths [4] .

We present a new, to the best of our knowledge, method to perform acousto-optic imaging based on a spatiotemporal structuration of long-duration acoustic plane waves. This approach is particularly relevant when using detectors with long integration times. We show how it is possible to reconstruct an image by measuring its two-dimensional Fourier components. A proof of concept is presented using a photorefractive detection scheme, demonstrating equal performances to direct imaging. The overall acquisition time is compatible with medical monitoring applications.

International audience; Slowdown of Gaussian "dark pulses" in photorefractive crystal using a two-wave mixing method is shown at room temperature. In agreement with our theoretical model, the experiment shows the dependence of the "dark pulse" delay and the group velocity values on the photorefractive gain Γ and the input pulse width t 0. The use of "dark pulses" allows to improve the time delay values and achieve maximal fractional about 1 for a pulse duration close to the crystal response time.

Ferroelectric tin hypothiodiphosphate (Sn 2P 2S 6) crystals are well-known for their significant piezoelectric, electro-optic, and nonlinear optical properties. These crystals have usually been grown by a vapor transport technique. We report in this paper on the first study of photorefractive nonlinearity in Sn 2P 2S 6 crystals grown by the Bridgman method. Pronounced photorefraction is observed in the near-infrared region of the spectrum even with no preliminary optical sensitizing.

Acousto-optic imaging is a multi-modal imaging technique where coherent light diffusing in a complex medium is ‘tagged’ over time by a ballistic ultrasound pulse of frequency ωus. The photons which paths cross with the ultrasound pulse undergo the acousto-optic effect, resulting in the frequency shift of ωus that can be selectively detected using heterodyne interferometry. Since the ultrasounds propagate at a known velocity, a time-to-space map of the tagged photons results in an image I(x, z), where x is the lateral direction and z the depth direction of the diffuse medium. Images at propagation depths much greater than the average mean free path, typically ~1mm in biological tissue, can be obtained. In most images obtained so far, the ultrasounds are focused line after line to recover an image, and therefore limited by the probe emission rate which is ~1-10 KHz depending on the probe size and the acoustic pulse power. Therefore, in order to acquire acoustic images at frame rates greater than 1 Hz for ‘direct visualization’ of the system under study, it is crucial to minimize the number of individual acquisitions necessary to reconstruct an image. Here, we present an alternative probe configuration where plane waves emitted at various angles are used rather than focused waves to tag the diffuse light. This approach, first proposed by P.Kuchment and L.Kunyansky (2010), is similar to X-ray tomography since the image information is contained in the various angular scans performed for one acquisition. Because the piezo-elements on the acoustic probe are non-isotropic emitters, the angular scan is typically limited to +/20 degrees, which is sufficient to recover information and can be improved using more than one probe. An inversion algorithm based on inverse Radon-transform is than used to reconstruct the image

Ultrasound image (left) and acousto-optic image (right) of a liver biopsy with tumors. Acousto-optic imaging exhibits tumors that are not detected through ultrasound. Biological tissues are very strong light-scattering media. As a consequence, current medical imaging devices do not allow deep optical imaging unless invasive techniques are used. Acousto-optic imaging is a light-ultrasound coupling technique that takes advantage of the ballistic propagation of ultrasound in biological tissues to access optical contrast with a millimeter resolution. We have developed a photorefractive-crystal-based system that performs self-adaptive wavefront holography and works within the optical therapeutic window. As it works at an appropriate wavelength range for biological tissues imaging, it was tested on ex vivo liver samples containing tumors as a pre-clinical study. Optical contrast was obtained even if acoustical one was not significant.

Photochromic effect in nominally pure and doped Sn 2 P 2 S 6 photorefractive crystals is investigated in the temperature range 120–310 K. This effect determines a mechanism of the amplitude hologram formation at low temperatures, and we show that a competition between the photorefractive (phase) and the amplitude gratings occurs at increasing temperature.

International audience; Due to multiple light scattering inside biological tissues, deep non-invasive optical medical imaging is very challenging. Acousto-optic imaging is a technique coupling ultrasound and light that allows recovering optical contrast at depths of few centimeters with a millimeter resolution. Recent advances in acousto-optic imaging are using short focused ultra-sound pulses often averaged over several hundred or thousand pulses. As the pulsing rate of commercial probes is limited to about few ultrasound cycles every 100 µs, acquiring an acousto-optic image usually takes several tens of seconds due to the high number of acoustic pulses excitation. We propose here a new acousto-optic imaging technique based on the use of ultrasound plane waves instead of focused ones that allows increasing drastically the imaging rate.

The current trends in stimulated Brillouin scattering and optical phase conjugation are overviewed. This report is formed by the selected papers presented in the “Fifth International Workshop on stimulated Brillouin scattering and phase conjugation 2010” in Japan. The nonlinear properties of phase conjugation based on stimulated Brillouin scattering and photo-refraction can compensate phase distortions in the high power laser systems, and they will also open up potentially novel laser technologies, e.g., phase stabilization, beam combination, pulse compression, ultrafast pulse shaping, and arbitrary waveform generation.

We propose a fundamental interconnection method using a polarization alignment system for waveguides having different spatial modes. In addition, as an example for the verification of the fundamental operation, we demonstrate an interconnection between a photonic crystal fiber and a laser that have obviously different spatial modes. The polarization alignment system operates synergistically with a self-written waveguide formed with a double phaseconjugate mirror. This technique enables us to interconnect a photonic crystal fiber with a laser source without complicated and time-consuming optical alignment. In this method, although it is not necessary to perform an external control for interconnection, the waveguide most suitable for connection is formed autonomously in a Sn2P2S6:Sb crystal developed for this purpose. There was a marked reduction in the polarization dependence of coupling efficiency, compared with that observed using a stand-alone double phase-conjugate mirror.

We developed photorefractive Sb-doped Sn2P2S6 samples of different crystal thickness to investigate the properties of the media for double phase conjugation. The Sn2P2S6:Sb crystals have high coupling coefficient of 20 cm−1, and show fast response time of 1.3 ms at total incident intensity of 0.3 W/cm2. The maximum phase conjugate reflectivity of 32% was observed with a 4 mm thickness sample. To investigate the characteristics of Sn2P2S6:Sb for double phase conjugation, on the basis of the coupled wave equations, we showed the dependence of phase conjugate reflectivity on incident angle, crystal thickness, total incident intensity and polarization. From these results, the optimum incident condition was clarified for obtaining the highest phase conjugate reflectivity.

We present various optical basic properties of a 1.5% doped Sb:Sn2P2S6 crystal experimentally and demonstrate a double phase conjugate mirror (DPCM). We measured the reflection, absorption and scattering of this crystal. The large coupling strength Γ L = 8.2 was obtained by the experiment of two-wave mixing. In addition, we succeeded in the formation of the DPCM and achieved the high diffraction efficiency of 42%. This crystal was suitable for DPCM applications of high speed response.

Nominally undoped tin hypothiodiphosphate is shown to possess photorefractive sensitivity from its band edge in the visible region of the spectrum up to 1.3 μm in the near infrared. The gain factor decreases with the increasing wavelength while the characteristic rate of photorefractive build-up roughly follows the spectral dependence of photoconductivity.

The electro-optical (EO) properties of monoclinic Sn2P2S6 single crystals are measured by direct interferometric technique for electric fields applied parallel to the crystallographic x-axis that lies near the spontaneous polarization vector. The room temperature free EO coefficient rT111 reaches 174±10 pm/V at λ=633 nm and shows only weak dispersion in the wavelength range between λ≈0.6,…,1.3 μ m. Corresponding values of rT221 and rT331 at λ=633 nm are 92±8 and 140±18 pm/V, respectively. The temperature dependence of the EO coefficients near the structural phase transition at T C =65 ° C is well described by a Curie–Weiss law with a peak value rT111≅4500 pm/V. The ratio between the free and clamped EO coefficients is determined at the wavelength λ=633 nm by applying a fast pulsed electric field instead of an AC field and measuring the temporal evolution of the electrically induced refractive index change. The values are rS111/rT111=0.30±0.02, rS221/rT221=0.12±0.02 and rS331/rT331=0.30±0.09.

We demonstrate a novel scheme for lensless image formation which combines the properties of an amplifying dynamic hologram and a pinhole camera. The scheme is realized on the base of a SPS:Sb1% photorefractive crystal working at 633 nm.

Focusing light deep inside living tissue has not been achieved despite its promise to play a central role in biomedical imaging, optical manipulation and therapy. To address this challenge, internal-guide-star-based wavefront engineering techniques--for example, time-reversed ultrasonically encoded (TRUE) optical focusing--were developed. The speeds of these techniques, however, were limited to no greater than 1 Hz, preventing them from in vivo applications. Here we improve the speed of optical focusing deep inside scattering media by two orders of magnitude, and focus diffuse light inside a dynamic scattering medium having a speckle correlation time as short as 5.6 ms, typical of living tissue. By imaging a target, we demonstrate the first focusing of diffuse light inside a dynamic scattering medium containing living tissue. Since the achieved focusing speed approaches the tissue decorrelation rate, this work is an important step towards in vivo deep tissue noninvasive optical imaging, optogenetics and photodynamic therapy.

Time-reversed ultrasonically encoded (TRUE) optical focusing is an emerging technique that focuses light deep into scattering media by phase-conjugating ultrasonically encoded diffuse light. In previous work, the speed of TRUE focusing was limited to no faster than 1 Hz by the response time of the photorefractive phase conjugate mirror, or the data acquisition and streaming speed of the digital camera; photorefractive-crystal-based TRUE focusing was also limited to the visible spectral range. These time-consuming schemes prevent this technique from being applied in vivo, since living biological tissue has a speckle decorrelation time on the order of a millisecond. In this work, using a Tedoped Sn_2P_2S_6 photorefractive crystal at a near-infrared wavelength of 793 nm, we achieved TRUE focusing inside dynamic scattering media having a speckle decorrelation time as short as 7.7 ms. As the achieved speed approaches the tissue decorrelation rate, this work is an important step forward toward in vivo applications of TRUE focusing in deep tissue imaging, photodynamic therapy, and optical manipulation.

Employing a Te-doped Sn2P2S6 photorefractive crystal, we demonstrated time-reversed ultrasonically encoded optical focusing deep inside fast-decorrelating scattering media with a speckle decorrelation time of 2.8 ms by imaging an absorptive target.

In the communication a significant enhancement of two-beam gain coefficient reached in modified Sn 2 P 2 S 6 photorefractive (PR) crystals is reported. These crystals were obtained using the vapor-transport technique with the modifications including alternate transport agent. The comparative studies of the optical and PR parameters of the nominally pure and modified samples were carried out.

We report new data on the electrooptic behaviour of the semiconductor ferroelectric crystal Sn2P2S6 in the temparature region 295-390 K. At the room temperature the magnitude of the effective longitudinal electro-optic coefficient is about 50 pm/V, much larger than the value reported previously. Corresponding half-wave voltage is less than 500V. We also observed Curie-Weiss type increase of reff near the ferroelectric phase transition at 339 K

It was revealed that specific directional light scattering takes place in ferroelectric semiconductors in polydomain states. This scattering is observed at to the light beam direction. The behaviour of this effect suggests that it is caused by light reflection by layers formed by charge carriers which screen charged domain walls between opposing domains. The orientation of the reflecting layers was determined. This effect allows one to check easily the degree of unipolarity throughout the sample and refine optically the complete crystallographic orientation.

We determine both the lifetime and the mobility of photoexcited charge carriers in Sn2P2S6 using holographic time of flight in diffusion mode. From the build-up dynamics of space charge gratings created by pulsed interband illumination at 532 nm, we find a charge carrier lifetime of 2.6 ± 0.5 ns and a charge carrier mobility along the x-axis of 2.4 ± 0.6 cm2 V−1 s−1 at room temperature. This mobility is assigned to holes as more mobile carriers in tin hypothiodiphosphate (SPS), and it is found to decrease for decreasing temperature, a behavior consistent with small-polaron hopping.

Acousto-optic imaging is based on light interaction with focused ultrasound in a scattering medium. Thanks to photorefractive holography combined with pulsed ultrasound, we perform a time-resolved detection of ultrasound-modulated photons in the therapeutic window (780 nm). A high-gain SPS:Te crystal is used for this purpose and enables us to image through large optical thickness (500 mean free paths). We are able to generate three-dimensional (3D) acousto-optic images by translating a multielement ultrasound probe in only one direction. A 3D absorbing object is imaged through a 3 cm thick phantom. (C) 2012 Optical Society of America

Since the optical inter-satellite communication has attractive advantages such as high-speed transmission with high confidence, almost no electronic magnetic interference, and low power consumption, it has been activity investigated. However, directivity control of the laser beams requires a bulky and complicated system in satellite mobile communications. A more flexible and high accurate system with small and simple mechanism has been desired. In this study, we propose a new method of optical inter-satellite communication with a dynamically reconfigurable optical directional device in which diffraction gratings are automatically rewritten and reorganized in response of incident conditions by moving satellites. For realizing such a device, we have developed Sn2P2S6 crystals which have a high sensitive photorefractivity and dynamic reconfigurable property. Furthermore, this crystal has hundreds times faster response than conventional photorefractive materials such as BaTiO3. These features are extremely advantageous to construct a high-speed and flexible communication system with a large tolerance to displacement of moving satellites. To investigate the possibility of the dynamically reconfigurable optical inter-satellite communication system, we experimentally evaluate the temporal and spatial characteristics of Sn2P2S6 crystals for the variation of the beam incident angle. Moreover, the diffraction beam from the crystal has phase conjugate wavefronts of the beam entering from the counter direction. We try to utilize this behavior to suppress the beam spread and to reduce the background light such as sunlight with a spatial filtering technique that has sensitivity in wavefront differences of the signal and background light.

In order to realize a dynamic reconfiguration technique that automatically switches configurations and functions of an optical device, we need a technique to control freely the connections of light inside and between devices without needing submicron-level alignments. In this study, we investigate the behavior of dynamic index gratings with nanoscale reversible self-organization in Sn 2 P 2 S 6 crystals that we have newly developed so as to realize an autonomous and dynamic reconfigurable optical waveguide by externally controlling its motions with light and examine its basic properties. Experimental results showed autonomous and dynamic reconfigurations of the optical waveguide formed in a Sb doped Sn 2 P 2 S 6 crystal with a 4 mm thickness for variations of an incident light position. We have successfully reconfigured the waveguide by a self-organization based on a photorefractive effect without cutting time series signals flowing through the waveguide, for variations of an incident light position long as approximately 2000 μm. Furthermore, we have recognized tolerance up to around 0.2 degrees for incidence angles in the experiment. This technique allows us to connect light freely without needing spatial adjustments in a nanostructured optical waveguide seen in photonic crystal fibers. Moreover, it is a technique that can be applied to dynamic connections between optical fibers and integrated waveguides accompanied with time variations of spatial modes. We also verified a possibility of removable and replaceable optical connection by utilizing large shift-tolerance of the autonomous and dynamic reconfigurable waveguide.

Acousto-optic imaging is based on ultrasound modulation of multiply scattered light in thick media. We experimentally demonstrate the possibility to perform a self-adaptive wave-front holographic detection at 790 nm, within the optical therapeutic window where absorption of biological tissues is minimized. A high-gain Te-doped Sn2P2S6 bulk crystal is used for this purpose. We image optical absorbing objects embedded within a thick scattering phantom by use of pulsed ultrasound to get a dynamic millimetric axial resolution. Our technique represents an interesting approach for breast cancer detection.

Acousto-optic imaging is based on ultrasound modulation of multiply scattered light in thick media. We experimentally demonstrate the possibility to perform a self-adaptive wavefront holographic detection at 790nm, within the optical therapeutic window where absorption of biological tissues is minimized. A high-gain Te-doped Sn(2)P(2)S(6) crystal is used for this purpose. Optical absorbing objects embedded within a thick scattering phantom are imaged by use of pulsed ultrasound to get a dynamic millimetric axial resolution. Our technique represents an interesting approach for breast cancer detection.

We demonstrated a dynamically reconfigurable optical waveguide based on a double phase conjugate mirror with a Sn 2 P 2 S 6 crystal, which is applicable to future optical communication technologies such as three dimensional optical wiring and free space optics.

International audience; The experimental observation of photorefractive self focusing in Sn2P2S6 : Te bulk crystals at 1.06 mm wavelength is presented. Steady state self focusing is reached as fast as 15 ms for an input peak intensity equal to 160 W/cm2. Self focusing is maximum for input peak intensities around 15 W/cm2 and is decreasing for intensities below and above this value.

We demonstrate self-pumped optical phase conjugation in Te-doped Sn2P2S6, a semiconducting ferroelectric crystal, using a 1.06 microm wavelength cw Nd:YAG laser. The photorefractive gain of this crystal has been increased to Gamma = (3.9+/- 0.4) cm-1 by Te doping. We observed self-pumped optical phase conjugation in a ring cavity scheme with phase conjugate reflectivities of more than 40 percent and a very fast phase conjugate rise time below 100ms at a light intensity of 20 W/cm2. This is more than two orders of magnitude faster than in any other photorefractive crystal, as e.g. in Rh-doped BaTiO3.

We develop a splicing device for photonic crystal fibers (PCFs) based on a double phase conjugate mirror (DPCM) using a novel photorefractive (PR) Sn 2 P 2 S 6 :Sb 1.5% crystal. This PR splicer has many attractive characteristics including modal field compensation and the automatic reconfiguration of the optical path. Utilizing a DPCM as the splicer, our device can adapt to misalignments automatically since the incident beams continuously rewrite an index grating which formed in the crystal. By the implementation of the Sn 2 P 2 S 6 :Sb crystal, the response time for the characteristic of dynamic reconfiguration is improved several-hundred-fold compared with conventional materials, e.g. BaTiO 3 . We demonstrate that the high angular tolerance is provided using the DPCM with the Sn 2 P 2 S 6 :Sb crystal. When the misalignment of the incident angle is from -7° to 8°, the increment of coupling loss is less than 0.6dB. This is several-ten-fold compared with the fusion splicing. We reveal the dependence of the coupling loss on the position of the incident beams and also the dependence of the energy flow on the propagation distance for the first time with the two-dimensional finite-difference beampropagation method. Using our numerical simulation tool, we can visually investigate the beam propagation property considering the influence of the fanning effect in the Sn 2 P 2 S 6 crystals.

International audience; Enhanced photorefractive properties of tin hypothiodiphosphate (Sn2P2S6) crystals as a result of Bi doping are presented. These new crystals were obtained by the vapor-transport technique using stoichiometric Sn2P2S6 composition with an additional amount of Bi up to 0.5 mol. % in the initial compound. The bandgap edges of the obtained crystals are located at ~750 nm and shift toward the red wavelengths with increasing Bi concentration. Sn2P2S6:Bi crystals are found to exhibit larger two-beam coupling gain coefficients (up to 17 cm−1 at a wavelength of 854 nm) as compared to (i) pure Sn2P2S6 (2.5 cm−1 at 854 nm), (ii) Sn2P2S6 crystals modified by the growth conditions (14 cm−1 at 860 nm), and (iii) Te-doped Sn2P2S6 (8 cm−1 at 860 nm). At the same time, for an intensity of 1.3 W/cm2 at 854 nm, buildup times of 0.9 and 2.5 ms at grating spacings of Λ=9.8 and 1.3 μm, respectively, are found; Bi-doped Sn2P2S6 crystals are the fastest among all the presently known Sn2P2S6 crystals operating at near-infrared wavelengths.

We demonstrated for what is the first time to our knowledge photorefractive two-wave mixing in a bulk ferroelectric crystal using cw light at the telecommunication wavelength 1.55 microm. In the Te-doped ferroelectric semiconductor Sn2P2S6 with absorption constant0.1 cm(-1) at 1.55 microm, grating recording times of 10 ms and a two-beam coupling gain of 2.8 cm(-1) have been measured at 350 mW power (intensity 440 W/cm(2)) without a necessity to apply an external electric field. With a moving grating technique, a maximal gain of 6.0 cm(-1) has been obtained.

International audience; The photorefractive properties of Sn2P2S6 crystals doped with Te and Sb in the near-infrared wavelength range up to 1064 nm are reported. The main photorefractive parameters, i.e., two-wave mixing gain, effective electro-optic coefficient, diffusion length, concentration of traps, and response time, are compared with conventional nominally pure Sn2P2S6. Te-doped Sn2P2S6 shows the fastest response with the smallest decrease of the photorefractive efficiency with increasing wavelength in the near infrared. Sb doping, on the other hand, inhibits photorefraction in the near infrared. Sn2P2S6:Te and Sn2P2S6:Sb crystals both show a high two-wave mixing gain Γ at 633 nm, and 10 and 20 cm−1. Te-doped Sn2P2S6 shows a photorefractive gain of 4.5 cm−1 at 1064 nm. Response times at 1064 nm of 20 ms have been measured for the intensity 6 W⁄cm2, which is 2 orders of magnitude shorter than in Rh-doped BaTiO3.

http://www.opticsinfobase.org//abstract.cfm?URI=PR-2007-TuB3; International audience; Light induced waveguides are demonstrated in Sn2P2S6by two alternative methods allowing a waveguide formation time of the order of 1 ms. The techniques are based on shaped visible lateral illumination and near infrared self-focusing.

We investigate the photorefractive properties of Sn2P2S6 in the near infrared wavelength range. Conventional ”yellow” and modified ”brown” crystals are examined, which were grown by vapor-transport technique using respectively SnI4 and SnI2 as a transporter agent. We measured an increase of the photorefractive gain from 3 cm−1 in yellow samples to 18 cm−1 in brown samples at 780 nm. Self-pumped phase conjugation was studied in a ring cavity scheme. Using yellow sample phase conjugate reflectivity at 780 nm is about 55% and the response time in the order of 5 s at 20 mW. The phase-conjugate response in brown samples, however, is observed in only about 20 ms, which is two-orders of magnitude faster.

The purpose of this paper is to find the polarizations and spatial orientations of the two interacting counterpropagating coherent light waves which ensure the largest beam coupling in monoclinic photorefractive crystal. The results of calculations are presented that are verified experimentally with Sn₂P₂S₆.

Light pulse group velocity manipulations due to the specific dispersion of a medium (so-called “slow” and “fast” light phenomena) can be obtained on the basis of several mechanisms. One of these techniques is two-wave mixing in a photorefractive crystal. This work presents a modification of this method, exploiting the strong beam fanning in Sb-doped Sn2P2S6 crystals. Our experimental results demonstrate a “fast light” behavior of Gaussian pulses transmitted through a Sn2P2S6:Sb sample. The phenomenon is due to the beam fanning (i.e., the self-diffraction of the incident beam on self-induced noisy photorefractive gratings) that ensures a significant depletion of the input beam. Due to the relatively fast photorefractive response of the Sn2P2S6:Sb crystals this depletion occurs with times in the range of 10–100 ms, depending on the beam intensity, and the “fast light” feature is observed. The temporal and amplitude characteristics of the output beam are measured in function of the intensity and polarization azimuth of the incident beam. Besides, a negative phase shift of the periodical output beam relative to a sinusoidal intensity-modulated input beam is also obtained experimentally. It is shown that the phase and amplitude relation between the input and output periodic signals are described by a simple analytical expression that takes into account the beam fanning strength (depletion factor) and its dynamics (depletion time constant). It is also demonstrated that the pulse advance (or phase shift of the modulated signal) can be regulated by the light polarization azimuth. The advantages of the proposed method are discussed.

Self-diffraction with the appearance of higher diffraction orders is discovered when writing a grating with a single sub-100 fs pulse in a nominally undoped Sn(2)P(2)S(6) sample. The short time of grating development, dependence of diffraction efficiency on the recording light intensity, correlation of wavelength dependence of efficiency with the spectrum of the two-photon absorption (TPA) constant, and a π phase shift of the diffracted beam allow for attributing the recorded grating to a dynamic amplitude grating of TPA.

We have demonstrated for the first time, to the best of our knowledge, photorefractive two-wave mixing in He+ implanted waveguides in one of the most promising materials for infrared photorefractive applications, the ferroelectric semiconductor Sn2P2S6. The high optical nonlinearity is preserved after implantation and at the telecommunication wavelength λ=1.55 μm, a maximal two-wave mixing gain of 2.5 cm−1 has been measured in Te-doped waveguides. In the nominally pure material an increase of the effective number of traps after implantation has been observed, resulting in an increase of the two-beam coupling gain by a factor of almost 2 in the 633-1064 nm spectral range. In 1% Te-doped Sn2P2S6 the effect of ion implantation to the photorefractive response is completely different than in pure materials. While the dominant contribution by holes is not considerably affected, a strong, thermally induced charge compensation is observed in the He+ implanted Te-doped waveguides.

The photorefractive properties of Sn2P2S6 crystals in the wavelength range from 633to1064 nm are investigated. Conventional yellow and modified brown crystals with a variation of nonstoichiometric defects are examined. Brown crystals respond much faster and exhibit substantially higher photorefractive gain than yellow crystals, up to 18 cm−1 at 780 nm and 8.5 cm−1 at 1064 nm. Ring-cavity self-pumped phase conjugation is demonstrated using both types of crystal. The phase-conjugate response of brown Sn2P2S6 is 2 orders of magnitude faster than in yellow Sn2P2S6 or Rh-doped BaTiO3, with a grating recording time below 50 ms for 30 mW power at 860 nm. Various thresholding effects are analyzed to determine the optimal wavelength range for the two types of crystal. We find an optimum in the range of 650–950 nm for yellow and in the range of 850–1100 nm for brown Sn2P2S6.

We present a joint Fourier-transform correlator working at a 10-kHz repetition rate. It is based on a photorefractive Sn(2)P(2)S(6) crystal operated in the pulsed direct band-to-band photoexcitation regime at a wavelength of 532 nm and a pulse length of 50 ns. The intersection of two plane waves with a total pulse fluence of 100muJ/cm(2) results in a buildup of thin dynamic holograms to a typical diffraction efficiency of 10(-4) in a time of ~1mus and decay again in less than 10mus . The correlator was tested by a fast image sequence generated by the pulsed readout of a holographic memory system. Successful correlation at a rate of 10 kHz has already been achieved for a pulse energy of only 200 nJ in the template images.

Ferroelectric photorefractive Sn2P2S6 crystals become more sensitive to cw radiation of a Nd3+: YAG laser (λ = 1.06 µm) by preexposure with incoherent white light. Space-charge formation and hologram recording are dominated by a diffusionlike charge transport, leading to an ultimate gain factor for transmission gratings that exceeds 6 cm−1 at a laser intensity of approximately 50 W/cm2.

Infrared recording of phase gratings is investigated in Sn(2)P(2)S(6) crystals by radiation of a Nd(3+):YAG laser (lambda = 1.06 microm) Space-charge formation and hologram recording are dominated by diffusion charge transport. Without an applied electric field, but with preexposure of the sample to incoherent white light, the gain factor exceeds 6 cm(-1) at laser intensities above 50 W/cm(2).

Results of experimental investigation of the non-steady-state photoelectromotive-force effect in ferroelectric photorefractive Sn2P2S6 crystals at a wavelength of 633 nm are reported. The dominating photoconductivity proved to be of the p type, with a diffusion length of photocarriers of ∼1 µm. This characteristic length proved to be shorter than the Debye-screening length evaluated as ∼3 µm in this crystal. The millisecond characteristic time of the space-charge grating formation was determined by the photoconductivity-relaxation time, which was found to be a little bit longer than the dielectric-relaxation time. Both of them dropped sublinearly with increasing average light intensity in the investigated region I0≃0.5–0.01 mW/mm2.

We show that by doping Sn2P2S6 crystals, the near-infrared photorefractive sensitivity can be enhanced or suppressed. In Te-Sn2P2S6, fast self-pumped optical phase conjugation with a rise time below 100 ms at 1.06 µm has been demonstrated.

Single bean dynamic double interferometer with optical (beam coupling) and electrical (holographic current) response rate in millisecond was realized using CW HeNe laser (P=15mW) with ferroelectric crystal Sn2P2S6.

A significant improvement of the ring-cavity self-pumped phase conjugation response time in the near-infrared cw regime with a photorefractive brown Sn2P2S6 crystal is reported. Brown Sn2P2S6 is grown by the vapor-transport technique with SnI2 as the transport agent. With our Sn2P2S6 samples response times below 50 ms and phase-conjugate reflectivities of 25% can be achieved at a wavelength of 780 nm with a moderate light intensity of 1 W/cm2. We show that the measured reflectivity represents the limit given by the transmission of the ring-cavity arrangement. Reaching this limit is possible because of the large two-wave mixing gain of 18 cm-1 measured in this crystal.