Adaptive tuning of infrared emission using VO2 thin films.

Phase-transition materials provide exciting opportunities for controlling optical properties of photonic devices dynamically. Here, we systematically investigate the infrared emission from a thin film of vanadium dioxide (VO2). We experimentally demonstrate that such thin films are promising candidates to tune and control the thermal radiation of an underlying hot body with different emissivity features. In particular, we studied two different heat sources with completely different emissivity features, i.e. a black body-like and a mirror-like heated body. The infrared emission characteristics were investigated in the 3.5-5.1 µm spectral range using the infrared thermography technique which included heating the sample, and then cooling back. Experimental results were theoretically analyzed by modelling the VO2 film as a metamaterial for a temperature range close to its critical temperature. Our systematic study reveals that VO2 thin films with just one layer 80 nm thick has the potential to develop completely different dynamic tuning of infrared radiation, enabling both black-body emission suppression and as well as mirror emissivity boosting, in the same single layer device. Understanding the dynamics and effects of thermal tuning on infrared emission will benefit wide range of infrared technologies including thermal emitters, sensors, active IR filters and detectors.

Communication: Optically induced decoupling of the smectic-A and nematic order parameters at the AN phase transition in phototropic liquid crystals.

In the present work, by means of the photopyroelectric calorimetric investigations in liquid crystal/naphtopyran (NP) molecule mixtures, the effect of the photoisomerization of NP by UV light on the critical behavior of specific heat could be studied over the smectic-nematic phase transition. It is shown that the UV light irradiation of the sample can shift the critical exponent from a non-universal value typical of the non-irradiated mixture to one close to that of the 3D-XY universality class, predicted for such a transition. This witnesses the optically triggered reduction of the coupling between the smectic and nematic order parameters responsible for the deviation from the XY-like behavior. A tentative explanation is provided based on the weakening of the order parameter coupling caused by the NP molecules entering the smectic layers following the UV light illumination.

Simultaneous specific heat, thermal conductivity and imaging evaluations in thin samples of 8CB liquid crystal dispersed with microemulsion of DDAB/water micelles.

In this work we use a photopyroelectric calorimetric set up to analyze the transformations occurring in thin samples of liquid crystal-microemulsion system by combining the results of the simultaneously performed high temperature resolution polarization microscopy observations and the temperature dependence of the specific heat, and of the thermal conductivity. This enabled an accurate direct correlation of the observations concerning the different measured quantities for a most comprehensive interpretation of the obtained results. It is found that the I-N conversion progresses in the entire nematic range because of the micelles expelled from the nucleating nematic volumes remaining confined in the residual isotropic material. A hysteretic behavior is observed in the nematic range between cooling and heating run because of the different micelle content in the material during the cooling and heating runs. A moderate hysteresys also over the N-A transition indicated an additional material refinement from micelles during the transition. Finally in samples with different micelle concentration the same maximum micelle content is retained in the nematic and smectic phases.

Simultaneous calorimetric and polarization microscopy investigations of light induced changes over phase transitions in a liquid crystal-napthopyran mixture.

We have studied the specific heat and the thermal conductivity in a 4-(n-octyl)-4'-cyanobiphenyl liquid crystal (LC)-photochromic molecules mixture, before, during, and after the photo-activation of the dispersed photochromic molecules, over both the smectic A-nematic and the nematic-isotropic phase transitions. The evaluation of the specific heat has enabled the determination of the changes of the phase transition characteristics induced by the photochromic molecules photoisomerization, while that of the thermal conductivity could be used to monitor the modifications induced in the average LC molecular orientation. The polarization microscopy imaging of the sample texture constituted a valuable support for the interpretation of the obtained thermal conductivity results.

Dual beam differential photopyroelectric setup for broadband thermal effusivity investigation of glass transitions in polymers.

The behaviour of the thermal effusivity squared over the glass transition in polystyrene is measured over a considerable frequency range using a novel dual beam front detection photopyroelectric configuration, which ensures a very effective processing of the obtained signal. This has enabled the detection of the very small changes taking place in such a parameter, despite the non-favourable sensitivity conditions due to the large mismatch between the thermal effusivities of the pyroelectric transducer and of the sample. The effect of the thermal contact resistance between the sample and the pyroelectric detector is also considered and discussed.

Study of the smecticA-hexaticB phase transition in homeotropic single domain samples of 65OBC liquid crystal by photopyroelectric calorimetry.

The smecticA-hexaticB phase transition was studied in a homeotropic single domain sample and in a non-aligned sample of n-hexyl-4'-n-pentyloxybiphenyl-4-carboxylate liquid crystal compound to probe the effect of different amount of defects on the phase transition. The specific heat, the thermal diffusivity and the enthalpy exchange were monitored over the transition and, at the same time, polarization microscopy observations could be carried out. The transition during the first cooling run was found to be accompanied by a considerably larger defect annealing in the non-aligned sample than in the homeotropic one, but the critical behaviour of the specific heat remained substantially the same.

Trace gas analysis from glazes by means of a compact photothermal deflection spectroscopy apparatus.

A portable photothermal deflection spectroscopy apparatus for accurate analysis of gas mixtures containing volatile organic substances is presented. The main technical aspects of the photothermal deflection cell, and of the gas sampling equipment are discussed in details. Finally, we present a straightforward industrial application of the apparatus concerning the qualitative and quantitative analysis of the gas emissions from thermal treatment of ceramic glazes.

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.

Analysis of the order character of the R(II)-R(I) and the R(I)-R(V) rotator phase transitions in alkanes by photopyroelectric calorimetry.

The character of the R(II)-R(I) and the R(I)-R(V) transitions was studied in alkane samples by analyzing the hysteresis the specific heat shows between heating and cooling measurements, using photopyroelectric calorimetry. The hysteresis systematically shows up in all the investigated samples over the R(II)-R(I) transition, similarly to what also observed over the liquid-R(II) and the R(V)-crystal phase transitions, thus confirming the first order character of all such phase transitions. Over the R(I)-R(V) transition a hysteresis is also present but with varying characteristics depending on the width of the temperature range of the R(I) phase, the first order character being smaller for decreasing R(I) range. The observed results were explained in terms of possible varying strength of the coupling term in the free energy between the distortion and tilt angle order parameters in the different samples, a variation which can induce a change in the order character of the transition. The results are compared to those obtained in an alkane sample under increasing ambient pressure, which turned the transition from first to second order. In both cases it was suggested that the variations of the coupling strength between the two order parameters could be associated with changes in the effectiveness of the molecular interaction between the layers relative to that occurring within each layer.

Effect of confinement and strain on the specific heat and latent heat over the nematic-isotropic phase transition of 8CB liquid crystal.

Simultaneous measurements of the specific heat, latent heat, light scattering, and polarization microscopy were carried out in 8CB liquid crystal confined in Aerosil and Millipore membranes networks, to assess the different properties of the nematic material nucleating over the nematic-isotropic transition under different strain conditions. In Aerosil networks, it was found that the higher temperature peak of the double-peaked structure observed in the specific heat survived up to silica concentration values considerably larger than previously reported. Moreover, the first-order character of the transition occurring over the lower temperature peak, where the involved strain is larger, was considerably smaller than that observed over the larger temperature one as observed in the latent heat detection and hysteretic behavior. The dynamics of the nucleated nematic, characterized by the frequency dependence of the real and imaginary parts of specific heat and of the detected latent heat, was also shown to be markedly different over the lower temperature peak with respect to that observed over the higher temperature peak where it was similar to that of bulk material. Corresponding investigations carried in LC confined in Millipore membranes show that the nucleated nematic possesses characteristics similar to the ones observed over the higher temperature peak in Aerosils in terms of the dependence of the latent heat and of the specific heat from increasingly confining conditions and frequency, consistently with a less strain inducing environment with respect to Aerosils.

Simultaneous characterization of optical and thermal parameters of liquid-crystal nanocolloids with high-temperature resolution.

We report on the high-temperature resolution measurements of the optical and thermal parameters of a liquid-crystal-silica nanoparticle colloid, as well as its video inspection, simultaneously performed in an upgraded photopyroelectric calorimeter. Over the nematic-isotropic coexistence region, the determined nematic correlation length, obtained from turbidity measurements, showed the characteristic two-step nematic nucleation process previously reported only for the specific heat.

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.

Dynamics of nematic liquid crystal with quenched disorder in the random dilution and random field regimes.

By means of specific heat spectroscopy, we have studied the dynamics of nematic 8CB nucleated, in the presence of random disorder, as the temperature is lowered across the isotropic to nematic phase transition such that the crossover region of random dilution to random field regimes of disorder strength is explored. We show, in terms of frequency dependence, hysteretic behavior, and aging effects, that the out-of-equilibrium properties of the nematic nucleated in the random disorder regime are strongly reminiscent of the bulk material while the one grown in the random field regime shows a glassy behavior with aging leading to slower dynamics. The aging effect has been explained in terms of strain relaxation in analogy with observations in other soft glassy colloidal systems.