Temperature Dependence Of Optical Birefringence Research Articles

Phase Transitions and Nonlinear Optical Property Modifications in BaGa4Se7.

Phase transitions can change the crystal structure and modify the physical properties of crystals. In this work, we investigate the phase transition behavior in BaGa4Se7, an important middle infrared (mid-IR) nonlinear optical (NLO) crystal, in the temperature range from room temperature to 1173 K. Interestingly, the BaGa4Se7 crystal undergoes a reversible ferroelastic phase transition at T = 528 K, resulting in the presence of a newly discovered phase (γ-phase) at the higher temperature. The experimental temperature dependence of optical birefringence, as well as the first-principles birefringence and NLO coefficients, reveals that the γ-phase exhibits larger birefringence and better NLO properties compared with those of the low-temperature phase (α-phase). This work demonstrates that phase-transition-induced structural modification can improve the mid-IR NLO properties, which would provide an effective avenue to obtain materials with good optoelectronic performance.

Optical anisotropy, order parameter and its critical behavior in temperature-dependent refractive indices of nematic liquid crystals

The temperature dependence of optical birefringence, order parameter, effective geometry parameter, and normalized polarizability was determined for nematic liquid crystal mixtures by measuring the refractive indices. The effects of the adjustable parameters obtained from temperature-dependent refractive indices were analyzed appropriately for the crossover temperature and order parameter values. Besides, a four-parameter power law expression method was used for the optical birefringence measurement to address inappropriate fits in the weak first-order phase transition region. From this method, the effective second order phase transition temperature T⁎⁎ was determined by showing the slightly higher values than phase transition temperature. The value of the order parameter critical exponent λ obtained from this method showed a good agreement with the mean field theory by demonstrating closeness to the tricritical hypothesis (β = 0.25). As a consequence, the measurement of the temperature-dependent optical birefringence was successfully verified the pre-transitional phenomena accurately, and it can facilitate an insightful description of that region.

Effect of the hockey-shaped mesogen as an additive on the critical behaviour at the smectic A to nematic phase transition

We present systematic experimental data on high resolution measurements of temperature dependence of optical birefringence (Δn) in a binary liquid crystalline system comprising calamitic 4-cyano-4′-octyloxybiphenyl (8OCB) and a hockey stick-shaped compound, 4-(3-decyloxyphenyliminomethyl) phenyl 4-decyloxycinnamate (SF7). The hockey stick-shaped mesogen shows a smectic A (SmA) phase in addition to two polymorphic tilted smectic phases. From an analysis of the Δn data, a quotient Q(T) has been derived which exhibits power law divergences both above and below the smectic A-nematic (SmA-N) phase transition. A noticeable shrinkage in width of the nematic range has been observed for a small increase in concentration of the angular mesogenic dopant. In the vicinity of the SmA-N phase transition, the values of effective critical exponent (α′), characterizing the critical fluctuation at this transition, have appeared to be in between those predicted by the 3D-XY and tricritical hypothesis. Furthermore, with increasing hockey stick-shaped molecular concentration, the SmA-N transition shows a tendency to be driven towards a first order nature.

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(Se0.28S0.72)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(Se0.28S0.72)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.

On the tricritical behaviour of the Sn2P2S6 crystals

Based on the results for temperature dependences of optical birefringence and optical indicatrix rotation derived for Sn2P2S6 crystals, we show that at the atmospheric pressure, those crystals are under the conditions of tricritical point, with the critical exponent of the order parameter . At the same time, we have found that the temperature behaviour of the rotation angle of spontaneous electric polarization obeys the law . The action of hydrostatic pressure leads to only small changes in the critical exponent β, thus suggesting that the tricritical point on the P, T-diagram of Sn2P2S6 should occur in the pressure range .

Structural Phase Transition in Lysozyme Crystals

We show that the electrostatic surface properties of substrates affect the conditions of growth for biological lysozyme crystals. It follows from our results that negatively charged substrates, including the untreated washed glasses, adsorb selectively the ionic lysozyme residues and so they are applicable for growing the biocrystals in flat cells. The largest lysozyme crystals we have obtained have the dimensions about 0.7 mm. The results of studies for the temperature dependences of optical birefringence in single lysozyme crystals inside the 3–40°C region are presented. It is revealed that the lysozyme crystals undergo a first-order structural phase transition at T c = 11°C.

Behaviour of the Optical Birefringence in Conditions of the "Viscous" Interaction

The anomalies on temperature dependences of optical birefringence [1] and dielectric permittivity [2] are observed in conditions of the "viscous" interaction. Their shape vIew has been discussed in different works [3, 4]. Moreover, the magnitude of δ(Δn) anomalies is considerably changed depending from the investigated crystal [5-7] and the sample of the same crystal [8]. In accordance with Refs. [3, 4] the "viscous" interaction is affected by the interaction of the movable defects with the soliton structure that causes the appearance of the region of the interaction of two commensurate phases. Such anomalous behaviour of the physical parameters in a transition region relates with the interaction of the space-modulated oscillations causing an appearance of the resulting oscillation. Temperature behaviour of the resulting oscillation should be affected by residual defects concentration around the soliton [8, 9]. At the first approach the residual concentration may be considered as an action of electric field intensity of definite value on the soliton structure [2].

On the role of charge carriers in the thermooptical memory effect FOR Sn2P2Se6ferroelectric-semiconductor in the incommensurate phase

Abstract Non-equilibrium of incommensurate (IC) phase in proper Sn2P2Se6 ferroelectric with semiconductor properties is studied by measuring the temperature dependences of optical birefringence–-δ(δn)(T). Typical manifestations of these non-equilibrium conditions: an anomalous temperature hysteresis, a cross-over from continuous dependence to stepwise dependence at low temperature variation rate, a thermooptical memory effect are similar to those observed in the IC-phases of dielectric crystals. At the same time, a considerable influence of illumination on thermooptical memory record and clear operations is found. It is suggested that a significant role in the IC-phase of Sn2P2Se6 semiconductor is played by a square-law interaction of polarization wave with mobile defects. The efficiency of this interaction varies noticeably with charge state change in defects upon specimen illumination.

Optical studies of chain orientation by thermal treatment and mechanical stretching of Teflon PFA foil

AbstractThe effect of thermal treatment and uniaxial stretching on the temperature dependence of optical birefringence, degree of crystallinity, thermally stimulated depolarization, and isothermal discharge of electrets obtained of Teflon PFA foil has been studied. The increase in stability of charge is related to changes in the polymer structure.

On the optical birefringence of TGS-type crystals

Abstract The temperature dependence of optical birefringence for TGS, DTGS, TGSe and DTGSe with different degrees of deuterium has been measured between 12K - 370K. Three characteristic temperatures for all crystals and a discontinuity in Δn at Tc for DTGSe are observed.