Effect of quenched disorder on the RI-RV, RII-RI, and liquid-RII rotator phase transitions in alkanes.

The ability of disorder to reduce the coupling between the distortion and tilt angle order parameters was tested over the R(I)-R(V) phase transition by measuring the specific heat in alkanes with different R(I) phase temperature range mixed with various concentrations of silica nanoparticles. It was found that the disorder significantly affects the character of the R(I)-R(V) transition, driving it toward a second-order character. The features about the R(II)-R(I) transition were progressively attenuated for increasing disorder in both alkanes, becoming very faint for the largest particle concentration, but the first-order character was maintained. Over the liquid-R(II) transition, the single peak observed in both the specific heat and the latent heat in the pure materials splits into two features, at different temperatures, as over the isotropic-nematic transition in liquid crystals.

Invited review article: Photopyroelectric calorimeter for the simultaneous thermal, optical, and structural characterization of samples over phase transitions.

The study of thermophysical properties is of great importance in several scientific fields. Among them, the heat capacity, for example, is related to the microscopic structure of condensed matter and plays an important role in monitoring the changes in the energy content of a system. Calorimetric techniques are thus of fundamental importance for characterizing physical systems, particularly in the vicinity of phase transitions where energy fluctuations can play an important role. In this work, the ability of the Photopyroelctric calorimetry to study the versus temperature behaviour of the specific heat and of the other thermal parameters in the vicinity of phase transitions is outlined. The working principle, the theoretical basis, the experimental configurations, and the advantages of this technique, with respect to the more conventional ones, have been described and discussed in detail. The integrations in the calorimetric setup giving the possibility to perform, simultaneously with the calorimetric studies, complementary kind of characterizations of optical, structural, and electrical properties are also described. A review of the results obtained with this technique, in all its possible configurations, for the high temperature resolution studies of the thermal parameters over several kinds of phase transitions occurring in different systems is presented and discussed.

Strain effects at the hexatic-B-smectic-A transition in the 65OBC liquid crystal.

We have studied the hexatic-B-smectic-A (HexB-SmA) transition in n -hexyl- 4'-n -pentyloxybiphenyl-4-carboxylate (65OBC) by means of a high-resolution ac photopyroelectric (PPE) calorimetric technique. A procedure for the interpretation of the PPE data, which allows the detection of an internal heating source due to strain annealing and/or latent heat, has been applied. We have found that the strain present in the sample depends on the kinetics of formation of the smectic phase once the sample is cooled from the isotropic one. The strain field keeps memory of this kinetics and can be only partially annealed on decreasing the temperature or cycling it around the Hex-SmA transition. A reversible ordering-disordering process has been found at T(c) and has been explained in terms of the competition between the order parameter variation with temperature and the constraints imposed by the disorder. The results confirm that the transition has a weakly first-order character with a specific heat critical exponent that disagrees with the available theoretical predictions. Our data show the importance of the disorder in 65OBC and we tried to clarify what would be the consequence of this result in theoretical modeling devoted to solving the puzzle of the HexB-SmA transition in this compound.

Thermal and optical study of the kinetics of the nematic-isotropic transition in octylcyanobiphenyl.

It is shown in this paper that the kinetics of the nematic-isotropic (NI) transition in 30-microm-thick octylcyanobiphenyl liquid crystal films is strongly affected by the cell walls. We have found that this is due to the modification of the wetting properties of the liquid crystal depending on the surface treatment. The results have been obtained with an experimental setup which allows the simultaneous high resolution study of the sample thermal properties and texture evolution with temperature. An accurate determination of the width of the two phase coexistence region associated with the NI transition has been possible and it is shown that it is affected by the presence of a surfactant. The results have been interpreted with a simple model which is based on the possible presence of a well-defined interface between the nematic and the isotropic phases in the coexistence region, which depends not only on the thermal gradients in the sample but also on the surface treatments.

Study of the refractive index of microscopic glass beads by light-refraction analysis.

The refractive index of submillimeter glass beads has been measured by means of a novel, to our knowledge, procedure with reference liquids that does not require close index matching and therefore avoids the use of toxic compounds for high-index glasses (i.e., n >/= 1.8). The method is based on the analysis of the light refracted by a monolayer of beads in comparison with ray-tracing simulations. For the three different types of glass beads investigated a satisfactory fit is achieved by the assumption of a radial variation of the refractive index inside the beads. This is ascribed to the tensile and compressive stresses originating inside the beads during rapid solidification of the glass.

Third-order nonlinearity enhancement in an artificial Kerr medium through bulk intrinsic birefringence.

Laser-induced circular birefringence measurements have been performed on a liquid suspension of polymer microellipsoids in order to study its optical nonlinearity as a function of the host liquid's refractive index n(h). The results show that the large values obtained for the chi1221((3)) term (1.6 x 10(-8) esu for n(h) = 1.330) can be explained with an intrinsic birefringence of the microellipsoids relative to their geometrical axes. The obtained values for the refractive indices are n || = 1.391 and n perpendicular = 1.368.

Polarization-resolved optical phase conjugation in an artificial anisotropic Kerr medium.

Optical phase conjugation by degenerate four-wave mixing has been achieved in a liquid suspension of polymer microellipsoids as a result of light-induced rotational effects. Different polarization configurations of the pump and probe beams have been used, and the resulting conjugate polarization states have been discriminated. The results are discussed in terms of different kinds of particle orientational gratings.

Nematic liquid crystals: interferometric determination of the elastic constant ratio K(B)/K(S).

An optical interference method has been developed to measure the distribution of the molecular director in an oriented cylindrical sample of nematic liquid crystal. The ratio of the elastic constants K(B)/K(S) can be obtained. Unlike other methods this one does not require independently measured quantities or the application of an external field. One interferogram is sufficient to determine refractive indexes and the elastic constant ratio. The hypothesis of strong anchoring can be tested directly.

Differential interferometric method for optical fiber testing.

A new differential interferometric method of determining the optical characteristics of cylindrical systems is presented. In particular it is applied to the study of a step-index optical fiber where small changes in geometric and optical characteristics have been introduced.

Differential interferometry of gamma-irradiated optical fibers.

An interferometric method is discussed which enables changes in refractive index of irradiated optical fibers to be determined. The method was used to determine the changes of step-index fibers irradiated with 1 krad of gamma rays from a (60)Co source. In one case, relative changes in the core and cladding radii of 3.9% and 1.8%, respectively, and in the core and cladding refractive indices of 2.8% and 2.26%, respectively, have been determined.