Secondary photorefractive centers in Sn2P2S6:Sb crystals

Secondary photorefractive centers in Sb-doped Sn2P2S6 have a lifetime comparable to the formation time of the space-charge grating. This considerably affects the dynamics of two-beam coupling and results in a new type of transient gain enhancement for preilluminated samples.     

Thermal expansion of Sn2P2S6 crystals

We present the results for thermal expansion coefficients of Sn₂P₂S₆ crystals determined both in the crystallographic system and the system based on eigenvectors of thermal expansion tensor. Peculiarities of temperature evolution of the indicative surface of thermal expansion tensor for Sn₂P₂S₆ are discussed, including the region of their ferroelectric phase transition.     

Electric-field enhancement of beam coupling in Sn2P2S6

The influence of an external field on photorefractive recording in Sn₂P₂S₆ (SPS) crystals is studied. A large gain factor of more then 15 cm^-1 is achieved for a grating spacing of 12 μm at λ=0.9 μm. For an applied field exceeding ±200 V/cm a switching of the beam coupling direction is detected, exhibiting a pronounced hysteresis. Received: 25 October 2000 / Revised version: 18 January 2001 / Published online: 21 March 2001     

Critical behaviour of Sn2P2S6 and Sn2P2(Se0.28S0.72)6 crystals under high hydrostatic pressures

Basing on the temperature dependences of optical birefringence for Sn₂P₂S₆ and Sn₂P₂(Se_0.28S_0.72)₆ crystals subjected to hydrostatic pressures, we prove unambiguously that Sn₂P₂S₆ reveals a tricritical point on its (p, T)-phase diagram with the coordinates (p, T) = (4.3 kbar, 259 K), so that the second-order phase transition transforms into the first-order one whenever the pressure increases above 4.3 kbar. We also find that increasing hydrostatic pressure applied to Sn₂P₂(Se_0.28S_0.72)₆ leads to the change in the phase transition character from tricritical to first order. Further increase in the pressure up to ~2.5 kbar imposes splitting of the first-order paraelectric-to-ferroelectric phase transition into two phase transitions, a second-order paraelectric-to-incommensurate one and a first-order incommensurate-to-ferroelectric transition.     

New phase-transformation line and intermediate phase in Sn2P2S6

A study is made of the effect of temperature and hydrostatic pressure on the permittivity, forbidden-band width, electrical conductivity, and photoconductivity of crystals of Sn₂P₂S₆. The empirical results obtained indicate that a new phase-transformation line exists on the p-T diagram of Sn₂P₂S₆ p > 0.17 GPa. This phase transformation is evidently a second-order transition, separating a paraelectric, proportionate phase from an intermediate, possibly nonproportionate phase. The results also indicate similarity of the p-T diagram of Sn₂P₂S₆ to the x-T diagrams of ferroelectric solid solutions of Sn₂P₂(Se_xS_1–x)₆.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 57–60, September, 1985.     

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. PACS 42.65.Hw; 42.70.Nq     

Transient short-circuit photocurrent in ferroelectric semiconductor Sn2P2S6 films

Transient short-circuit photocurrents in films of the ferroelectric semiconductor Sn₂P₂S₆ are studied in a temperature range that includes the phase transition point. The influence of an external electric field and white-light illumination on the photoelectric response of samples is discussed.     

GaAs photonic crystal cavity with ultrahigh Q: microwatt nonlinearity at 1.55 microm

We haves realized and measured a GaAs nanocavity in a slab photonic crystal based on the design by Kuramochi et al. [Appl. Phys. Lett. 88, 041112 (2006)]. We measure a quality factor Q=700,000, which proves that ultrahigh Q nanocavities are also feasible in GaAs. We show that owing to larger two-photon absorption in GaAs nonlinearities appear at the microwatt level and will be more functional in gallium arsenide than in silicon nanocavities.     

Microwave study of the soft ferroelectric mode in Sn2P2S6 crystals

This paper presents the results of the investigation of dielectric dispersion in semiconductive ferroelectric Sn₂P₂S₆ crystals over the frequency range 1 kHz to 78.5 GHz. The main dielectric dispersion in Sn₂P₂S₆ is caused by the soft ferroelectric B_u mode and in the vicinity of the Curie point it occurs in the millimeter region. The frequency of the soft mode in the paraelectric phase varies according to v_s = 35 (T-T_c )^½ GHz on approaching the Curie point. The soft mode is strongly overdamped. Close to T_c the relative damping is y/v_s = 14. The dielectric contribution of the soft mode is equal to the static dielectric permittivity and it explains its whole temperature-dependence.     

Ferroelectricity, nonlinear dynamics, and relaxation effects in monoclinic Sn2P2S6

An ab initio-based model of the temperature-induced ferroelectric phase transition in Sn2P2S6 (SPS) as a prototype of an unconventional ferroelectric is developed. The order parameter in SPS is found as the valley line on a total-energy surface of the zone-center fully symmetrical Ag and polar Bu distortions. Significant nonlinear coupling between order parameter and strain is observed. Monte Carlo simulations describe the additional low-temperature rearrangement in polar structure, which appears in domain boundaries, and describe the relaxation phenomena near the ferroelectric phase transition.     

Electronic and spatial structure of nanoclusters of Sn2P2S6 ferroelectric crystals

The results obtained from ab initio calculations of the optimized configuration and electronic structure of the cluster models of Sn₂P₂S₆ ferroelectric crystals are presented. The calculations have been performed using the spin-restricted Hartree-Fock method and the density functional theory in the DZVP basis set. It has been shown that the clusters are stable and retain the topology of the simulated crystal. The influence of the cluster sizes on the properties under investigation has been analyzed.     

CO and CO2 spectroscopy using a 60 nm broadband tunable MEMS-VCSEL at approximately 1.55 microm

The spectroscopic application of a new broadband microelectromechanical-system-tunable vertical cavity surface-emitting laser with single-mode coverage of 60 nm (245 cm(-1)) in a single, continuous sweep is described. The operation of the device is illustrated with high-resolution spectra of CO and CO2 over 110 cm(-1) (27 nm) and 67 cm(-1) (17 nm), respectively, with the CO band shown for high-pressure scans between 1 and 3 bars (0.1-0.3 MPa). The achieved tuning range opens up new opportunities for tunable diode laser absorption spectroscopy. The spectra were compared with HITRAN-derived model calculations. The benefits of a sensor based on this laser are greater speed, laser power, and tuning range.     

Temperature dependence of Mn2+ EPR in Sn2P2S6 near the phase transition

The X-band EPR spectrum of Mn²⁺ in Sn₂P₂S₆ was studied in the temperature rangeT=223–363 K. At room temperature the spin-Hamiltonian constants areg=2.00±0.01,B ₂ ⁰ =(163±3)·10^?4 cm^?1,B ₂ ² =(159±3)·10^?4 cm^?1,A=?(75±1)·10^?4 cm^?1. The effect of the invariance in temperature of the resonance magnetic fields in the narrow temperature rangeT=337–340 K and the model of the paramagnetic centre are discussed. According to EPR data a phase transition occurs atT=337 K. This transition from the paraelectric phase to the ferroelectric one is accompanied by a dramatic change in value of the spin-Hamiltonian constantB ₂ ⁰ .     

Band structure and model of the paraelectric structure of the Sn2P2S6 crystal

Since a variety of choices exist in the literature for the symbol of the space group of crystals of the type Sn₂P₂S₆, hindering the use of a group-theoretic analysis to determine the properties of the band spectrum and phonon spectrum, identified this symbol is identified with the known coordinates of the atoms of the given crystals. It is shown that the different arrangements P2₁/c and P2₁/n lead to permutations of the points in the Brillouin zone and correspondingly to a change in the irreducible representations. Calculating the spectra of Sn₂P₂S₆ and Sn₂P₂Se₆ by the method of pseudopotentials fitted to the width of the bandgap confirms the double degeneracy at the X, Y, Z points of the Brillouin zone, which was not expected for the X point in the arrangement P2₁/c. We have detected the previously predicted minimal band complexes in the band spectrum, which are the same as for In₄Se₃ of the group D _2h ¹² . Fiz. Tverd. Tela (St. Petersburg) 39, 1219–1222 (July 1997)     

Dielectric properties of Sn2P2S6 crystals as a function of their growth conditions

The behavior of the permittivity near a phase transition in Sn₂P₂S₆ crystals of different technological quality is studied. It is established that, in high-resistance crystals, where an internal electric field is formed by the screening of spontaneous polarization in the polar phase, long-time relaxation of ɛ is observed in a temperature range ∼2 K above T _max. This relaxation and change in the form of the maximum of ɛ′(T) at a phase transition are attributed to an internal electric field induced by the volume space charge formed in regions near the surface. It is established that the existing differences in the properties of Sn₂P₂S₆ crystals are due to deviations from stoichiometry, arising during growth and synthesis of the crystals. Fiz. Tverd. Tela (St. Petersburg) 41, 1456–1461 (August 1999)     

Thermal expansion of the Sn2P2(Se0.28S0.72)6 solid solutions

In the present work, we have experimentally studied and analysed the dilatometric properties of Sn₂P₂(Se_0.28S_0.72)₆ crystals during ferroelectric phase transition. A complete matrix of thermal expansion tensors as well as the characteristic surface of the thermal expansion tensor has been obtained. The critical exponent for the volume thermal expansion coefficient in Sn₂P₂(Se_0.28S_0.72)₆ crystals has been determined to be equal to 0.50 ± 0.06, which is peculiar to tricritical behaviour. Non-agreement in the values of the critical exponents evaluated from the temperature dependencies of relative elongation (0.17 ± 0.01) and volume thermal expansion coefficient (0.50 ± 0.06) is shown to be due to the possible temperature rotation of the spontaneous polarisation vector.     

Temperature behavior of the photovoltaic and pyroelectric responses of Sn2P2S6 semiconductor ferroelectric films

The electric response of Sn₂P₂S₆ semiconductor ferroelectric films to focused laser radiation (λ = 6328 Å) with a modulation frequency of 24 Hz is studied experimentally. It is shown that the signal involves an initial spike of current (an unsteady component) followed by the relaxation decrease to a certain value (a quasisteady component). The most significant changes in the shape and amplitude of the film response is observed near the Sn₂P₂S₆ monocrystal phase transition point of T _C = 66°C. The behavior of the unsteady component of the response is related to the behavior of the spontaneous polarization. This component decreases with increasing temperature. The quasi-steady component reaches its maximum at T = C _C. It is revealed that the behavior of relaxation processes corresponding to the decreasing response undergoes a change near the phase transition temperature.     

Photosensitivity of As2S3 chalcogenide thin films at 1.5 microm

Chalcogenide glasses are promising candidates for all-optical switching and various nonlinear applications. However, we show that As2S3 thin films are photosensitive at wavelengths in the 1.5-microm telecommunication window. This sensitivity is evidenced by the formation of self-written waveguides in slabs, where channels as narrow as 1 microm are created. We also show the detrimental effects of such photosensitivity in ridge waveguides. This photosensitivity seems to occur only in thin-film form and not in bulk samples or fibers.