Temperature dependent polariton emission from strongly coupled organic semiconductor microcavities

We investigated the absorption and photoluminescence (PL) of J-aggregates of a cyanine dye both in a thin film format and when used as the active layer in a strongly-coupled microcavity. We show that as temperature is reduced, the absorption linewidth of the J-aggregates narrows and shifts to higher energy. When the J-aggregate is placed in a microcavity we find that the energy of the polariton modes also shifts to higher energies as temperature is reduced. We compare the intensity of PL emission from the upper and lower branches at resonance as a function of temperature, and find that it can be described by an activation energy of 25 meV. PL emission spectra at resonance also suggest that uncoupled excitons inside the microcavity populate the upper polariton branch states.     

Temperature dependent of thermal expansion and ferroelectric properties for BiAlO3-modified BaTiO3 lead-free ceramics

Perovskite structure (1 − x)BaTiO₃–xBiAlO₃ (BT–xBA, x = 0.05, 0.08 and 0.1, corresponding to BT–5BA, BT–8BA and BT–10BA, respectively) ceramics were prepared by a solid state reaction route. Crystal structure of BT–xBA ceramics was determined by using XRD. Dielectric properties were studied as a function of temperature at different frequencies, which indicated that the phase transition temperature (T_m) decreased with increasing x. With increasing temperature, thermal expansion dl/l increased while thermal expansion coefficient α had weak concentration dependence. The Burns temperature was much higher than the transition temperature T_m for the BT–5BA and BT–10BA ceramics. Polarization hysteresis loops (P–E), electrostrictive (S–E) and S versus P² (S–P²) at different temperatures were displayed. P_max slightly decreased due to thermally activated leakage current, meanwhile, S_max decreased at above T_m with the contribution of ferroelectric domain reorientation decreased. The electrostrictive coefficient Q value was clearly larger than that of lead-based electrostrictors at room temperature. Furthermore, it exhibited a slim loop with negligible hysteresis and a subtle linear feature at higher temperature.     

Temperature dependence of flexoelectricity in BaTiO3 and SrTiO3 perovskite nanostructures

Piezoelectricity is a property of non-centrosymmetric crystals. In most typically used ferroelectrics, this property is lost as the temperature is increased beyond the Curie point thus strongly reducing the availability of efficient materials that can be used for high temperature energy harvesting. Flexoelectricity, as can be shown from simple symmetry arguments, is a universal and linear electromechanical coupling that dictates the development of polarization upon application of inhomogeneous strains. The implications of this phenomenon become amplified at the nanoscale. In this communication, we develop a molecular dynamics approach predicated on a specially tailored interatomic force-field to extract the temperature dependence of flexoelectricity. Surprisingly, we find that it, at least for Barium Titanate and Strontium Titanate nano structures, increases with temperature. Apart from cataloging this interesting observation for the future use in high temperature energy harvesting, we also examine the physical mechanisms that lead to the observed temperature dependence.     

Polarization dependent chemistry of ferroelectric BaTiO3(001) domains

Recent works suggest that the surface chemistry, in particular the presence of oxygen vacancies, can affect the polarization in a ferroelectric material. This should, in turn, influence the domain ordering driven by the need to screen the depolarizing field. Here we show using density-functional theory that the presence of oxygen vacancies at the surface of BaTiO(3)(001) preferentially stabilizes an inward pointing, P-, polarization. Mirror electron microscopy measurements of the domain ordering confirm the theoretical results.     

Temperature dependence of anthracene crystal polariton luminescence

Lifetimes and luminescence spectra of anthracene crystal in the temperature range 4–80 K have been investigated. It has been shown that the increase of the luminescence decay time with temperature is due to the polariton antistokes scattering by phonons and the estimate of the scattering probability has been given.     

Temperature dependence of two-beam coupling and dark decay in photorefractive BaTiO3

The temperature dependence of two-beam coupling and dark decay in photorefractive BaTiO₃ is reported. We show that the competition between deep and shallow traps depends on temperature and writing intensity, and influences two-beam coupling and dark decay. The dynamics of dark decay, characterized by a fast decay of partial erasure and a subsequent slow decay, is influenced by the presence of deep and shallow traps. Partial erasure, due to thermal excitation of charges from the shallow traps, decreases with temperature and increases with writing intensity. The time constant of the slow decay, due to thermal excitation of charges from the deep traps, depends strongly on temperature, but not on the writing intensity. At room temperature, the existence of deep and shallow trap leads to intensity-dependent photorefractive gains. As temperature increases, the influence from the shallow trap decreases, and the photorefractive gain becomes independent of the intensity. However, at much higher temperatures (100°C), the photorefractive gain resumes its dependence on intensity due to an increase in dark conductivity at elevated temperature.     

Temperature dependent epr measurements of Mn2+ in diamagnetic garnets

EPR measurements have been made of the spin Hamiltonian parameters of Mn²⁺ at the three different sites in each of the four diamagnetic garnets LuAlG, YAlG, LuGaG and YGaG. The temperature variation of all the second order crystal field parameters together with those of the fourth order crystal field parameter and the hyperfine splitting parameter for octahedral sites in YGaG have been followed over a range 4·2-550°K. The magnitude of the various parameters and their temperature variations are discussed.     

Temperature shift of the maximum of critical X-ray scattering in a BaTiO3 monocrystal

The temperature shift of the maximum of critical scattering in BaTiO₃ monocrystal was found to depend on the direction of the scattering vector with respect to the crystalline axes.     

Anisotropic selfdiffraction in BaTiO3

A comparison of the operating characteristics of the high-voltage regime of the discharge in a hollow-cathode tube, the hollow-canthode discharge (HCD) and the discharge to a plane cathode are presented. The disappearnce of the hollow-cathode effect and the transition to a high-voltage discharge after inserting several anode rods into the cathode cylinder is exhibited. The similarity between the operating characteristics of such a high-voltage discharge and of a plane cathode discharge is shown. The loss of ions at the anode rods, as well at insulators or floating conductors is believed to be the reason behind the increase of the operating voltage and the disappearance of HCD characteristics. Practical means of increasing the operating voltage are mentioned.On leave from the Institute of Fluid Flow Machines, Polish Academy of Sciences, Gdansk-Wrzeszcz, Fiszera 14, Poland. This work was supported by the Alexander-von-Humboldt Foundation and the Deutsche Forschungsgemeinschaft (DFG)     

Four-wave polariton scattering of light in LiNbO3

Four-wave Stokes k-spectra for light scattering on polaritons in lithium niobate crystals with an Mg impurity are studied experimentally. The mechanisms for direct, cascade, coherent, and incoherent four-wave mixing of light are discussed in the course of interpreting the angular dependences of the scattered light intensity. It is shown that the dispersion of the real part of the polariton wave vector and the refractive index of the crystals at the polariton frequencies can be measured with an order of magnitude greater accuracy than by spontaneous three-wave polariton light scattering. A significant discrepancy is found between determinations of the polariton absorption coefficient from the angular spectra of three-wave scattering and four-wave scattering in terms of the model employed here. Zh. éksp. Teor. Fiz. 112, 441–452 (August 1997)     

Photorefractive detection of antiparallel ferroelectric domains in BaTiO3 and BaTiO3:Co crystals

3 ). Rod-shaped domains with square cross sections are revealed. Received: 11 July 1997/Accepted: 19 November 1997     

Twin-seeded BaTiO3 ceramics

Grain size is important in the barium titanate ceramics used as capacitors and thermistors. Normally this is controlled by chemical additives and firing schedules, but growth twins also lead to increased grain size. Seeding experiments described in this paper demonstrate how the twins can be used to produce large-grained BaTiO₃ ceramics.     

Temperature study of the polariton associated with the 248 cm soft mode in LiNbO3

The temperature dependence of the dispersion curve of the polariton associated with the 248 cm^-1 mode in LiNbO₃ is measured by means of near-forward Raman scattering, and described through the classical oscillator dispersion theory. The measured intensity and line-width of the Raman line are compared to those expected from theory.     

Mirrorless parametric oscillation in BaTiO3

We report on the first observation of mirrorless oscillation in BaTiO₃ pumped by two nearly counterpropagating mutually incoherent light waves, both containing an ordinary and an extraordinary component. The oscillation waves appear at a certain angle defined by the phase matching condition for four-wave mixing of orthogonally polarized waves.     

Soft phonon dispersion in BaTiO3

We treat a simple lattice dynamical model for the critical modes in BaTiO₃. The Ti-ions move in an anharmonic single particle potential formed by the frame of Ba- and O-ions that are bound more tightly to their equilibrium positions. The dipole interaction of displaced Ti-ions is enhanced by polarization effects due to all electronic polarizabilities and to an additional ionic polarizability of the oxygen ions. The transverse optical mode of this model is low lying and softening as temperature decreases until its frequency for zero wavevector vanishes. This leads to a ferroelectric instability. The dispersion of this critical mode for small wavevectors was found to be highly anisotropic. So long this prediction has not been tested experimentally.     

Stoichiometry of BaTiO3 nanoparticles

Barium titanate nanoparticles with various nominal Ba/Ti ratios were prepared through direct synthesis from solution (DSS) and further annealed at different temperatures. Their deviation from stoichiometry was studied through XRD analysis, and a large deviation from stoichiometry has been observed. The grain size we studied ranges from 50 nm to 1 μm. For the as-prepared particles, the grain size is about 50 nm, and the maximum excess of Ti is over 15%. For the samples annealed at 800 °C, the grain size is increased to 100 nm, and the maximum excesses of Ba and Ti are 8 and 9%, respectively. The defects formed during synthesis and surface effect of nanoparticles are both estimated for their contributions to the deviation from stoichiometry, and the defects have been found to be mainly responsible for the large deviation observed.     

Multiwave-pumped mirrorless parametric oscillation in BaTiO3

We report on the observation of mirrorless oscillation in photorefractive BaTiO₃ pumped by two pairs of two nearly counterpropagating light waves, each of them containing an ordinary and an extraordinary component of polarization. The oscillation waves are located on a cone with an apex angle defined by the phase-matching conditions of this process.