Tin Hypothiodiphosphate Research Articles

Photoinduced trapping of charge at sulfur vacancies and copper ions in photorefractive Sn2P2S6 crystals

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 (VS2+) 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 (VS+) and nonparamagnetic neutral (VS0) charge states of the sulfur vacancies, with one and two trapped electrons, respectively, are formed during the illumination. Above 70 K, the neutral vacancies (VS0) are thermally unstable and convert to VS+ 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 VS0 vacancy. Warming above 120 K destroys the VS+ 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.

Charge trapping by iodine ions in photorefractive Sn2P2S6 crystals.

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 Møller-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.

Near-infrared-sensitive photorefractive Sn2P2S6 crystals grown by the Bridgman method

Ferroelectric tin hypothiodiphosphate (Sn2P2S6) 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 Sn2P2S6 crystals grown by the Bridgman method. Pronounced photorefraction is observed in the near-infrared region of the spectrum even with no preliminary optical sensitizing.

Optical determination of the charge carrier mobility in Sn2P2S6

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.

Tin hypothiodiphosphate: nonlinear response in the sub-100 fs time domain

The interaction of sub-100 fs light pulses (τp ≲ 75 fs) with single crystals of nominally undoped tin hypothiodiphosphate, Sn2P2S6, is studied in the near-infrared spectral range (590 – 1630 nm). A predominant contribution of the two-photon absorption (TPA) is verified in the measurements of the sample transmission as a function of pulse intensity and of the time delay between pump and probe pulses. Scans over the photon energy show that the two-photon absorption coefficient β increases in a superlinear way for photon energies hω exceeding Eg/2; for any quantum energy it is nearly independent of propagation direction and polarization of the incident beam. Such a behavior is qualitatively similar to that predicted by perturbation theory within models with allowed-forbidden transitions. The TPA coefficient saturates at a maximum value of β ≈ 8 cm GW−1 at hω ≈ 1.80 eV. It drops when reaching the bandgap Eg. Using pump-probe measurements at 626 nm, a transient absorption is verified that persists for probe pulse delays much longer than the pump pulse duration, up to 2.5 ns. We discuss our results in the framework of the microscopic structure of Sn2P2S6 with emphasis on the optical generation of S− small hole polarons.

Coupling of counterpropagating light waves in low-symmetry photorefractive crystals

Coupling of two counterpropagating coherent light waves in tin hypothiodiphosphate (Sn2P2S6, SPS) is usually strongly inhibited because of severe space charge limitations. Analysis of two-beam coupling gain is performed for different orientations of the grating vector in this monoclinic crystal that takes into account the anisotropy of dielectric permittivity. Gain enhancements are predicted and demonstrated experimentally for several nontraditional orientations of counterpropagating waves.

Enhanced photorefractive properties of Bi-doped Sn_2P_2S_6

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.

Photorefraction of Pb-doped tin hypothiodiphosphate

Photorefractive properties of deliberately lead-doped tin hypothiodiphosphate (Sn2P2S6, SPS) are investigated. In contrast to nominally undoped crystals, the space-charge gratings are formed by charge species of only one type, holes, what ensures a high beam-coupling gain in the steady-state. No optical sensitization is needed for the recording with near infrared light, but a photorefractive sensitivity does not extend beyond a wavelength of 0.9μm. Like for some other SPS crystals, in SPS:Pb the effective trap density was found to be intensity dependent in the red spectral range.

Spectral sensitivity of nominally undoped photorefractive Sn2P2S6

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.

Reflection-type photorefractive gratings in tin hypotiodiphosphate

Reflection photorefractive gratings recorded by nearly counterpropagating light waves in the near infrared are studied in tin hypothiodiphosphate. The ratios are established for certain electrooptic tensor components responsible for reflection grating recording, and the Debye screening length is evaluated. Reflection holograms of binary objects are recorded.

Studies of light-induced charge transfer in Sn2P2S6 by combined EPR/optical absorption spectroscopy

The light-induced charge transfer in ferroelectric tin hypothiodiphosphate Sn2P2S6 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 Sn2+ sites, creating Sn3+. At an energy of 1.5 eV the hole is first transferred to the other Sn2+ 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.

Multiline coherent oscillation in photorefractive crystals with two species of movable carriers

The coherent oscillation, because of nearly degenerate four-wave mixing in photorefractive crystals with two types of movable charge carriers, occurs at two spectral lines symmetrically shifted with respect to the pump frequency. Consequently the output oscillation exhibits the high contrast intensity modulation. The frequency separation of two oscillation modes (and modulation frequency of the output intensity) depend on the incident light intensity and spatial frequency of the developing grating. A model is presented explaining this type of oscillation by the two-maxima shape of the gain spectrum in crystals with sufficiently different relaxation times of two space-charge gratings, one formed by movable electrons and the other one by movable holes. The experimental data for coherent oscillator with tin hypothiodiphosphate (Sn2P2S6) are in reasonable quantitative agreement with the calculations.

Nearly degenerate two-beam coupling in photorefractive crystals with two species of movable carriers

Two-beam intensity coupling is calculated for photorefractive crystals with two types of movable charge carrier in the undepleted-pump approximation. The analytical expressions are derived for the temporal evolution of the space-charge field; for weak coupling they are used for calculation of the transmitted beam intensities. The results of the calculation are compared with the experimental observations in photorefractive tin hypothiodiphosphate (Sn2P2S6). All experimental data are in reasonable quantitative agreement with the calculations.

Enhancement of beam coupling in the near infrared for tin hypothiodiphosphate

By using the moving grating technique, without an applied electric field, it is possible to significantly increase the steady-state two-beam coupling gain in photorefractive Sn2P2S6. Another technique of gain enhancement consists of cooling of the sample to −30 °C. The measured data confirm the existence of two out-of-phase gratings in Sn2P2S6 generated by charge carriers of different sign, with relaxation times of 70 ms and 500 s.

Photorefraction in tin hypothiodiphosphate in the near infrared

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.

New Photorefractive Ferroelectric Material for Red and NearInfraRed: Tin Hypothiodiphosphate

New photorefractive crystal is described which can be easily sensitized to near infrared radiation by pre-illumination to incoherent white light with moderate intensity. The gain factor up to 7 cm-1 and the response time down to 10 ms are measured for unexpanded beams of CW radiation of a 1.06 µm Nd3+:YAG laser with 0.5 W output power.

Photorefractive beam coupling in tin hypothiodiphosphate in the near infrared

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).