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For the first time, we demonstrated successful post-synthesis incorporation of metal dopants at elevated temperature into a host structure of Sn2P2S6, known as the grandfather of dichalcogenide ferroelectrics with a formula M2P2X6 (M = metal and X = chalcogen). With the example of Cu, we show that the integration of dopant atoms into the bulk of an already grown crystal could be easily tuned up to 0.5 at% affecting the structural, optical, vibrational, electrical, and ferroelectric properties. Thermally diffused copper atoms in Sn2P2S6 bulk are in the metallic state, inducing a multiaxial expansion of the Sn2P2S6 unit cell for 2–3.4%. The energy reduction between indirect and direct optical transitions was observed, combined by small hardening for acoustic and soft optical vibrational modes originating in the partial substitution of Sn by Cu in the Sn2P2S6 crystal lattice. Similar to hydrostatic pressure, the structurally bonded copper initiates a small downward shift in the critical temperature Tc. The presence of copper has a substantial impact on the shape smearing of the ferroelectric domains as well as on the behaviour of the dielectric permittivity. The real part of the dielectric constant e′ reaches its maximal value at intermediate concentrations of Cu 0% < x < 0.5%, showing an ∼20% increase with respect to the parent structure. The incorporated metal atoms provoke a monotonous expansion of the ferroelectric P–E loops along the increased dopant content direction. An increase in the electrical conductivity at higher Cu concentration reveals a trend for inducing a metal-like behaviour.

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.

Electron paramagnetic resonance (EPR) is used to monitor photoinduced changes in the charge states of sulfur vacancies and Cu ions in tin hypothiodiphosphate. A Sn2P2S6 crystal containing Cu+ (3d10) ions at Sn2+ sites was grown by the chemical vapor transport method. Doubly ionized sulfur vacancies ( V S 2 +) are also present in the as-grown crystal (where they serve as charge compensators for the Cu+ ions). For temperatures below 70 K, exposure to 532 or 633 nm laser light produces stable Cu2+ (3d9) ions, as electrons move from Cu+ ions to sulfur vacancies. A g matrix and a 63,65Cu hyperfine matrix are obtained from the angular dependence of the Cu2+ EPR spectrum. Paramagnetic singly ionized ( V S +) and nonparamagnetic neutral ( V S 0) charge states of the sulfur vacancies, with one and two trapped electrons, respectively, are formed during the illumination. Above 70 K, the neutral vacancies ( V S 0) are thermally unstable and convert to V S + vacancies by releasing an electron to the conduction band. These released electrons move back to Cu2+ ions and restore Cu+ ions. Analysis of isothermal decay curves acquired by monitoring the intensity of the Cu2+ EPR spectrum between 74 and 82 K, after removing the light, gives an activation energy of 194 meV for the release of an electron from a V S 0 vacancy. Warming above 120 K destroys the V S + vacancies and the remaining Cu2+ ions. The photoinduced EPR spectrum from a small concentration of unintentionally present Ni+ ions at Sn2+ sites is observed near 40 K in the Sn2P2S6 crystal.

Electron paramagnetic resonance (EPR) is used to establish the role of iodine as an electron trap in tin hypothiodiphosphate (Sn2P2S6) crystals. Iodine ions are unintentionally incorporated when the crystals are grown by the chemical-vapor-transport method with SnI4 as the transport agent. The Sn2P2S6 crystals consist of Sn2+ ions and (P2S6)4- anionic groups. During growth, an iodine ion replaces a phosphorus in a few of the anionic groups, thus forming (IPS6)4- molecular ions. Following an exposure at low temperature to 633 nm laser light, these (IPS6)4- ions trap an electron and convert to EPR-active (IPS6)5- groups with S = 1/2. A concentration near 1.1 × 1017 cm-3 is produced. The EPR spectrum from the (IPS6)5- ions has well-resolved structure resulting from large hyperfine interactions with the 127I and 31P nuclei. Analysis of the angular dependence of the spectrum gives principal values of 1.9795, 2.0123, and 2.0581 for the g matrix, 232 MHz, 263 MHz, and 663 MHz for the 127I hyperfine matrix, and 1507 MHz, 1803 MHz, and 1997 MHz for the 31P hyperfine matrix. Results from quantum-chemistry modeling (unrestricted Hartree-Fock/second-order Moller-Plesset perturbation theory) support the (IPS6)5- assignment for the EPR spectrum. The transient two-beam coupling gain can be improved in these photorefractive Sn2P2S6 crystals by better controlling the point defects that trap charge.

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 present experimental results for the semilinear oscillator geometry based on Sn2P2S6:Sb photorefractive crystals. The samples with strong two-wave mixing gain demonstrate degenerate oscillation regime. A mirrorless oscillation is also observed.

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.

We have investigated the ferroelectric domain structure a Sn2P2S6 (SPS) single crystal using a chemical etching technique. For this purpose new etching compound was elaborated. The results demonstr...

It was found that doping of tiny ferroelectric particles to a nematic liquid crystal(LC)strongly affects dielectric properties of the system. The doping increases dielectric response of the LC due to interaction between LC molecules and particles, which possess large dipole moment and high polarisability. The suspension reveals high dielectric response on either side of the Curie temperature, TCurie, and peculiarity at the vicinity of TCurie. This result points at retention of dipole ordering above Curie temperature that may be a result of a memory effect.

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.

Doping photorefractive single crystals of Sn2P2S6 with antimony introduces both electron and hole traps. In as-grown crystals, Sb3+ (5s2) ions replace Sn2+ ions. These Sb3+ ions are either isolated (with no nearby perturbing defects) or they have a charge-compensating Sn2+ vacancy at a nearest-neighbor Sn site. When illuminated with 633 nm laser light, isolated Sb3+ ions trap electrons and become Sb2+ (5s25p1) ions. In contrast, Sb3+ ions with an adjacent Sn vacancy trap holes during illumination. The hole is primarily localized on the (P2S6)4− anionic unit next to the Sb3+ ion and Sn2+ vacancy. These trapped electrons and holes are thermally stable below ∼200 K, and they are observed with electron paramagnetic resonance (EPR) at temperatures below 150 K. Resolved hyperfine interactions with 31P, 121Sb, and 123Sb nuclei are used to establish the defect models.

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.

Electron paramagnetic resonance (EPR) is used to identify the singly ionized charge state of the Sn vacancy ( VSn−) in single crystals of Sn2P2S6 (often referred to as SPS). These vacancies, acting as a hole trap, are expected to be important participants in the photorefractive effect observed in undoped SPS crystals. In as-grown crystals, the Sn vacancies are doubly ionized ( VSn2−) with no unpaired spins. They are then converted to a stable EPR-active state when an electron is removed (i.e., a hole is trapped) during an illumination below 100 K with 633 nm laser light. The resulting EPR spectrum has g-matrix principal values of 2.0079, 2.0231, and 1.9717. There are resolved hyperfine interactions with two P neighbors and one Sn neighbor. The isotropic portions of these hyperfine matrices are 167 and 79 MHz for the two 31P neighbors and 8504 MHz for the one Sn neighbor (this latter value is the average for 117Sn and 119Sn). These VSn− vacancies are shallow acceptors with the hole occupying a diffuse wave...

Single crystals of Sn${}_{2}$P${}_{2}$S${}_{6}$ are both ferroelectric and photorefractive. Antimony (Sb) ions are optically active in this material and play an important role in optimizing the photorefractive response. Electron paramagnetic resonance (EPR) is used to determine the site and charge states of the Sb ions in Sn${}_{2}$P${}_{2}$S${}_{6}$ and to illustrate the photocharging behavior of these ions. In as-grown crystals, Sb${}^{3+}$ ions substitute for Sn${}^{2+}$ ions. A multiline EPR spectrum from Sb${}^{2+}$ ions is observed after exposing a crystal at 30 K to either 633- or 442-nm laser light. These Sb${}^{2+}$ ions are thermally stable at low temperature after the light is removed. They revert back to Sb${}^{3+}$ ions when the crystal is warmed above 250 K for a few minutes. The EPR spectrum has $S$ $=$ 1/2 and consists of well-resolved sets of hyperfine lines from ${}^{121}$Sb and ${}^{123}$Sb nuclei. Spin Hamiltonian parameters are obtained from the angular dependence of the spectrum (principal values are 1.810, 1.868, and 1.887 for the $g$ matrix and 1404, 1687, and 1849 MHz for the ${}^{121}$Sb hyperfine matrix). These parameters provide evidence that the wave function for the unpaired spin has significant $p$-like character and has overlap with neighboring ions.

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.

The light-induced charge transfer in ferroelectric tin hypothiodiphosphate Sn 2 P 2 S 6 is investigated by means of optical absorption and EPR spectroscopy and their combination. Light-induced metastability at 298 K, known to affect the holographic sensitivity, is observed via optical absorption. EPR measurements support the recent identification of holes as the dominating charge carriers. For excitation energies exceeding the band gap of 2.5 eV at 10 K, EPR reveals that the following processes are likely to occur: a hole is captured at one of two different Sn 2+ sites, creating Sn 3+ . At an energy of 1.5 eV the hole is first transferred to the other Sn 2+ and for excitation of 2.0 eV to a further center. Since these defects are intrinsic and therefore not limited in quantity, the light-induced sensitisation is a very effective way to improve the holographic performance.

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.

The electronic properties of the ferroelectric compound ${\mathrm{Sn}}_{2}{\mathrm{P}}_{2}{\mathrm{S}}_{6}$ are investigated by x-ray photoelectron spectroscopy and soft x-ray fluorescence spectroscopy. Excellent agreement between theoretical calculations and experimental data for the electronic structure of the investigated compound is achieved. With help of the Sn core level spectra it is confirmed that the compound contains ${\mathrm{Sn}}^{2+}$ ions. The valence band mainly consists of five resolvable bands between 3.3 eV and 14.5 eV. Consistent with the results of band-structure calculation and the soft x-ray fluorescence spectra, ${\mathrm{Sn}}_{2}{\mathrm{P}}_{2}{\mathrm{S}}_{6}$ can be viewed as an ionic crystal, built of ${\mathrm{Sn}}^{2+}$ and the $({\mathrm{P}}_{2}{\mathrm{S}}_{6}{)}^{4\ensuremath{-}}$ fragments. Within the latter, P-P and P-S bonds are largely covalent and characterized by $\mathrm{sp}$ hybridization.

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.

A photoinduced electron paramagnetic resonance (EPR) spectrum in single crystals of Sn2P2S6 (SPS) is assigned to an electron trapped at a sulfur vacancy. These vacancies are unintentionally present in undoped SPS crystals and are expected to play an important role in the photorefractive behavior of the material. Nonparamagnetic sulfur vacancies are formed during the initial growth of the crystal. Subsequent illumination below 100 K with 442 nm laser light easily converts these vacancies to EPR-active defects. The resulting S = 1/2 spectrum shows well-resolved and nearly isotropic hyperfine interactions with two P ions and two Sn ions. Partially resolved interactions with four additional neighboring Sn ions are also observed. Principal values of the g matrix are 1.9700, 1.8946, and 1.9006, with the corresponding principal axes along the a, b, and c directions in the crystal. The isotropic parts of the two primary 31P hyperfine interactions are 19.5 and 32.6 MHz and the isotropic parts of the two primary Sn...