Macroscopic polarization and thermal conductivity of GaN

We theoretically investigated the effect of macroscopic polarization (sum of spontaneous and piezoelectric polarization) on the thermal conductivity of wurtzite GaN. Macroscopic polarization contributes to the effective elastic constant of the GaN and thus modifies the phonon group velocity. We used the revised phonon velocity to estimate the Debye frequency and temperature. Different phonon scattering rates were calculated as functions of the phonon frequency. The thermal conductivity of GaN was estimated using revised parameters such as the phonon velocity and phonon relaxation rate. The revised thermal conductivity at room temperature increased from 250 to 279 W m⁻¹ K⁻¹ due to macroscopic polarization. The method we developed can be used for thermal budget calculations for GaN optoelectronic devices.     

Influence of the spatial rotation of a polarization plane on the radiative friction force in light fields with polarization gradients

Simple atomic models (1/2→1/2 and 1/2→3/2 transitions) were taken as an example to consider, in the sub-Doppler cooling approximation, influence of the spatial rotation of a polarization plane on the radiative friction force at arbitrary field configurations of dimensionality D<1. Spatial gradients of the angles determining this rotation additionally contribute to the friction force. This contribution is comparable in magnitude with other forces if the detuning δ is on the order of the radiative relaxation constant γ. For the j→j+1 transitions, the contribution promotes sub-Doppler cooling at δ<0, whereas for the j→j transitions (half-integer j) it induces anisotropic heating and cooling processes.     

DNA transport and delivery in thermal gradients near optofluidic resonators

Heat generation and its impact on DNA transport in the vicinity of an optofluidic silicon photonic crystal resonator are studied theoretically and experimentally. The temperature rise is measured to be as high as 57 K for 10 mW of input power. The resulting optical trapping and biomolecular sensing properties of these devices are shown to be strongly affected by the combination of buoyancy driven flow and thermophoresis. Specifically, the region around the electromagnetic hot spot is depleted in biomolecules because of a high free energy barrier.     

Reflection of ion acoustic waves in thermal and density gradients

It is shown that ion acoustic waves can be refracted in thermal and density gradients. Possible implications of the mechanism for turbulent heating experiments are pointed out.     

Circular polarization of the spectral thermal emission from ferromagnets

An experimental study on a new magneto-optic effect, the partially circular polarization of the spectral thermal i.r. and vis emission from the ferromagnets iron, cobalt and nickel is presented. Measurements have been performed at photon energies from 0.2 to 2.0 eV and at temperatures between 600 and 1100 K. Results are correlated to parameters of the magneto-optic Kerr effect. The circular polarization is proportional to the magnetization, vanishes above the Curie temperature and changes sign at 1.3eV for iron, at 1.8eV for cobalt and at 0.3 eV for nickel. In addition, the circular polarization of the spectral thermal emission was studied at the hcp-fcc phase transition of cobalt at 700 K, where no Kerr-effect data are available. Finally, another new magneto-optic effect on the spectral thermal emission is suggested on the basis of the present investigation.     

光的偏振对热光关联成像的影响

根据完全偏振光的不同偏振态和部分偏振光的不同偏振度对二阶关联函数的影响,利用统计光学理论研究了光的不同偏振性质对关联成像的可见度和信噪比的影响.研究表明,光的偏振对热光关联成像的影响体现在部分偏振光的不同偏振度上,关联成像的可见度是偏振度的二次函数,由二阶关联函数方法得到的信噪比随着偏振度增大而提高,而利用减去背景项的二阶关联函数以及二阶相干度方法得到的信噪比均与偏振度无关. 关键词： 关联成像 偏振 可见度 信噪比     

热红外偏振成像技术在目标识别中的实验研究

简要分析了热红外偏振探测与热红外强度探测在物理含义方面的区别,由此提出了一种利用偏振信息在红外图像中识别目标的新方法。通过热红外偏振成像系统获得了目标的偏振图像,并由计算机提取出了图像中的偏振信息。由于目标与自然背景的热红外偏振特性有较大的差异,所以通过分析这些信息,可以更好的识别目标。实验结果表明,该方法不仅可以很好地识别自然背景中的人造目标,而且对热红外伪装目标的识别也很有效。     

Water thermal storage with solidification

This paper investigates the thermal performance of an ice-water cold storage system theoretically and experimentally. The proposed cold storage system can be used to shift electric demand from periods of high demand to ones of low demand. A general lump model to determine the thermal storage characteristics is developed. The solution of the system equations is found through a Laplace transformation method. Experimental data of temperature profiles obtained for various flow rates and different inlet coolant temperatures confirm the validity of the theory. Effects of several parameters on the charge characteristics are quantitatively made clear in the present study.     

Partial circular polarization of the spectral thermal emission from ferromagnetic iron

A partial circular polarization of the spectral thermal emission from a single crystal of ferromagnetic iron is observed. The circular polarization changes sign at a wavelength of 1 μm.     

Analysis of thermal property in hollow-core polarization maintaining photonic crystal fibers

We study the thermal-induced variance of effective refractive indices (ERIs) and birefringence in several kinds of polarization maintaining fibers (PMF) and carry out numerical simulations by utilizing the finite element method (FEM). Responses under varying temperatures in these fibers are analyzed thoroughly. According to our computational results, hollow-core photonic crystal fibers (HC-PCFs) exhibit much more stable temperature-dependent ERIs and birefringence among these PMFs.     

Intensity fluctuations and degree of polarization in three-dimensional thermal light fields

The normalized intensity fluctuations of arbitrary electromagnetic wave fields obeying Gaussian statistics are expressed in terms of the three-dimensional degree of polarization. This general formulation implies an important physical result concerning the polarization of planar fields and the dimensionality of the formalism. The results are expected to be particularly useful in intensity interferometry.     

Suppression of ice nucleation in supercooled water under temperature gradients

Understanding the behaviours of ice nucleation in non-isothermal conditions is of great importance for the preparation and retention of supercooled water. Here ice nucleation in supercooled water under temperature gradients is analyzed thermodynamically based on classical nucleation theory (CNT). Given that the free energy barrier for nucleation is dependent on temperature, different from a uniform temperature usually used in CNT, an assumption of linear temperature distribution in the ice nucleus was made and taken into consideration in analysis. The critical radius of the ice nucleus for nucleation and the corresponding nucleation model in the presence of a temperature gradient were obtained. It is observed that the critical radius is determined not only by the degree of supercooling, the only dependence in CNT, but also by the temperature gradient and even the Young's contact angle. Effects of temperature gradient on the change in free energy, critical radius, nucleation barrier and nucleation rate with different contact angles and degrees of supercooling are illustrated successively. The results show that a temperature gradient will increase the nucleation barrier and decrease the nucleation rate, particularly in the cases of large contact angle and low degree of supercooling. In addition, there is a critical temperature gradient for a given degree of supercooling and contact angle, at the higher of which the nucleation can be suppressed completely.     

Formation of complex structures in dusty plasmas under temperature gradients

Thermophoretic effects on dust structures under temperature gradients in glow and radio-frequency discharge plasmas are studied experimentally. The geometry of dust structures consisting of micrometer-sized polydisperse grains depends on heat release in the plasma. Thermophoretic forces associated with heat release can control the formation of dust structures of different geometries. A theoretical model is proposed to describe dust separation with respect to grain size caused by the effects of radial electrostatic and thermophoretic forces. The glow discharge currents under critical conditions for grain separation predicted by the model agree with those observed experimentally.     

Dynamic nuclear polarization by thermal mixing of cyclohexane in its plastic phase

A new dynamic nuclear polarization effect, probably due to thermal mixing with the electronic spin-spin reservoir, has been observed in plastic cyclohexane doped with the free radical TANO. This probably reflects the existence of regions in the plastic phase where the radical concentration is high.     

Electric polarization in crystals and its response to thermal and elastic perturbations

The basic properties of electric polarization response in crystalline dielectrics are theoretically analysed. Spontaneous polarization, pyroelectricity, piezoelectricity, flexoelectricity (polarization response to strain gradient) and thermopolarization effect (polarization response to temperature gradient) are discussed on the basis of a unified approach. Special attention is paid to the problem of the adequate theoretical definition of these phenomena. The effect of the sample termination on the polarization response is considered in detail.     

A thermal stimulated current investigation of the polarization of NH4Cl

Thermal stimulated currents of undoped, of NH₃ and of NiCl₂ doped crystals were measured and the obtained thermograms decomposed into individual first order kinetics and space charge peaks. Details concerning the procedure of decomposition are given. The results confirm the formation of anion and cation vacancies respectively in consequence of the doping and show that the charge transport is realized in several thermal stimulated steps. Below zero centigrade it is realized most probably by a sequence of proton transfers and restricted in distance. Above this temperature the vacancies become mobile and allow a steady d.c. current. The processes are explained by a catalytic NH ₄ ⁺ Cl^–NH₃ + HCl reaction.