Photostimulated Phase Research Articles

Unraveling the Role of Thyroid Hormones in Seasonal Neuroplasticity in European Starlings (Sturnus vulgaris).

Thyroid hormones clearly play a role in the seasonal regulation of reproduction, but any role they might play in song behavior and the associated seasonal neuroplasticity in songbirds remains to be elucidated. To pursue this question, we first established seasonal patterns in the expression of thyroid hormone regulating genes in male European starlings employing in situ hybridization methods. Thyroid hormone transporter LAT1 expression in the song nucleus HVC was elevated during the photosensitive phase, pointing toward an active role of thyroid hormones during this window of possible neuroplasticity. In contrast, DIO3 expression was high in HVC during the photostimulated phase, limiting the possible effect of thyroid hormones to maintain song stability during the breeding season. Next, we studied the effect of hypothyroidism on song behavior and neuroplasticity using in vivo MRI. Both under natural conditions as with methimazole treatment, circulating thyroid hormone levels decreased during the photosensitive period, which coincided with the onset of neuroplasticity. This inverse relationship between thyroid hormones and neuroplasticity was further demonstrated by the negative correlation between plasma T3 and the microstructural changes in several song control nuclei and cerebellum. Furthermore, maintaining hypothyroidism during the photostimulated period inhibited the increase in testosterone, confirming the role of thyroid hormones in activating the hypothalamic–pituitary–gonadal (HPG) axis. The lack of high testosterone levels influenced the song behavior of hypothyroid starlings, while the lack of high plasma T4 during photostimulation affected the myelination of several tracts. Potentially, a global reduction of circulating thyroid hormones during the photosensitive period is necessary to lift the brake on neuroplasticity imposed by the photorefractory period, whereas local fine-tuning of thyroid hormone concentrations through LAT1 could activate underlying neuroplasticity mechanisms. Whereas, an increase in circulating T4 during the photostimulated period potentially influences the myelination of several white matter tracts, which stabilizes the neuroplastic changes. Given the complexity of thyroid hormone effects, this study is a steppingstone to disentangle the influence of thyroid hormones on seasonal neuroplasticity.

Photo-stimulated phase and anchoring transitions of chiral azo-dye doped nematic liquid crystals

We report concurring phase and anchoring transitions of chiral azo-dye doped nematic liquid crystals. The transitions are induced by photo-stimulation and stable against light and thermal treatments. Photochromic trans- to cis-isomerization of azo-dye induces an augmented dipole moment and strong dipole-dipole interaction of the cis-isomers, resulting in the formation of nano-sized dye-aggregates. Consequent phase separation of the aggregates of a chiral azo-dye induces phase transition from a chiral to nonchiral nematic phase. In addition, the deposition of dye-aggregates at the surfaces brings about anchoring transition of LC molecules. The stability and irreversibility of the transition, together with no need of pretreatments for LC alignment, provide fascinating opportunity for liquid crystal device applications.

The effects of fluorescent probes on model membrane organization: photo-induced lipid sorting and soft structure formation

Liquid crystalline phase behaviour in model cell membranes prepared in-vitro has generated an enormous amount of interest in recent years and the phase diagrams of various bio-mimetic mixtures explored. Most recently photo-induced oxidation in fluorescently labelled membranes has been shown to influence these phase diagrams and can be used to generate interesting membrane-based geometries based on differences in membrane composition. In this article the different phases observed in model membranes will be discussed. Methods used to study these systems and recent developments in photo-stimulated phase changes are described along with areas for future investigation.

Effect of aerosil dispersions on the photoinduced nematic–isotropic transition

We report differential scanning calorimetric (DSC) and dielectric measurements on thenematic–isotropic transition in the bulk and aerosil composites of a liquid-crystal mixturehaving a photoactive guest azobenzene compound in a non-photoactive host, 4-n-heptylcyanobiphenyl (7CB). The DSC scans taken at different cooling rates show that, at slowerrates, the bulk displays a single peak across the transition, whereas the composites in thesoft gel regime exhibit a double-peak profile. Such a double-peak profile, although seen inhigh-resolution ac calorimetric studies, has been observed for the first time inDSC experiments. The temperature range of the region between the two peaks iscomparable to that seen in ac calorimetric experiments and has similar features.This observation is significant since the appearance of the low-temperature peakin ac calorimetric data has been explained to be due to a crossover from therandom-dilution to the random-field limits. This work also constitutes the firstexperiments on the photoisomerization driven isothermal phase transitions inliquid-crystal–aerosil composites. The studies carried out in the absence and presence of alow-magnitude UV radiation not only bring out the standard features now establishedfor such photostimulated phase transitions, but display a few surprises. Notableamong them are that (i) the photoinduced shift in the transition temperature is anon-monotonic function of the aerosil composition and appears qualitatively similar to thedependence of the transition temperature itself, and (ii) the thermal anomaly mentionedabove characterizing the crossover is also seen in the temperature-dependentas well as the temporal variation of the sample capacitance for a composite inthe soft gel regime. We have also evaluated, using the temporal variation of thecapacitance, the different response times associated with the UV-on photochemicalprocess as well as the UV-off thermal back-relaxation process; the response timesappear to have a similar dependence on the aerosil concentration as the transitiontemperature.

High pressure investigations of the photo-stimulated orientational ordering transition in a liquid crystal with photoactive dimeric molecules

High pressure investigations have been carried out on the photo-stimulated phase transition exhibited by a guest–host system, where the host is a non-photoactive liquid crystal. As guest molecules we chose successive homologues of a dimeric photoactive azobenzene liquid crystalline molecule to understand the influence of the length and parity of the alkylene spacer on such a transition. Our studies show that application of pressure as well as the parity of the spacer has a drastic influence both on the static and dynamic characters of the photo-stimulated phase transition.

Effect of pressure on the dynamics of the photostimulated orientational ordering transition in a liquid crystal

We have investigated the influence of the applied pressure on the shift in the photostimulated phase transition temperature and the dynamics of the system to undergo such a phase transition. The shift is seen to decrease with increasing pressure, a feature that can be countered, to a certain extent, by increase in the intensity of the light used. Whereas the time scales of the system to undergo such a transition increases with pressure, the thermal back relaxation process to recover the original phase occurs faster at elevated pressures. We propose a simple argument to explain these observations.

Optical and Physical Properties of Silver Colour Centres in Microcrystals in Silver Bromide Films

AbstractResults are presented of investigations of silver colour centres of molecular and colloidal types arising from the deep photolysis of AgBr microcrystals in ultra-fine-grain emulsions. Using experimental data connecting the position of the absorption maximum of first, secondary and “limited” colour centres, their dimensions and the sizes of the AgBr crystals, values of solubility, partial degree of solubility and surface tension ofAg in AgBr are calculated. Dependence of these values on the dimensions of the AgBr crystals are studied. It is shown that the surface tension increases ivilh the number of atoms in the colloidal particles. The solubility of Ag changes by 6 orders oj magnitude as the colloidal particles decrease from massive to atomic size. Good agreement is shown between the experimental results and the predictions of thermodynamic theory for the nucleation and growth of a new phase. The possibility of a photostimulated phase transition from molecular centres to colloidal ones is considered.

Kinetics of structural phase transition in semiconductors caused by redistribution of electrons on trapping levels

An approach is proposed for describing photostimulated phase transition kinetics. This approach takes into account the dynamics of the electron density on trapping levels in the semiconductor. A correlation between the abnormal physical properties of crystals and characteristics of the semiconducting subsystem is established.

Photostimulated Phase Transitions in High‐Tc Superconductors and the Origin of the Superconductivity

AbstractThe photostimulated phase transition from the semiconducting to the superconducting phase in YBa2Cu3O7−δ (δ > 0.5) single crystals is revealed. The investigations are performed using the time‐resolved dependences of photoresistivity, photovoltage, and optical second harmonic generation. At the same time the time‐resolved Raman and UV‐visible reflection spectra are monitored. A time delay between the second harmonic generation and photocurrent dependences is found. The intrinsic role of two different mechanisms in the occurrence of the photoinduced phase transition is unambiguously shown. To clarify the main features of this transition it is necessary to account for the specific electron‐phonon interaction together with the existence of the Anderson metal‐insulator transition. The performed band energy calculations show the essential role of photoinduced changes in the Fermi energy level location and the total density of states distribution. The intrinsic role of van Hove singularities in the formation of the superconducting gap is shown.

The photostimulated phase transitions in ferroelectrics

Abstract The effect of nonequilibrium electrons (or holes) on phase transitions and properties of ferroelectrics is of great interest. Phase transitions due to such electrons are described as photostimulated in the literature [1]. While photostimulated phase transitions (PPT) can occur in all semiconductors that undergo phase transitions, investigation of photoferroelectrics has the advantage that it permits photostimulated phase transitions to be correlated with the basic phenomenological parameters of ferroelectrics, such as the Curie-Weiss constant, the spontaneous polarization, the heat capacity discontinuity and other independently measured parameters.

Properties and applications of photoresponsive polymers

Abstract