Low frequency dielectric spectroscopy of two room temperature chiral liquid crystal mixtures

Dielectric spectroscopy of two room temperature chiral liquid crystal mixtures (W-96 and W-97) have been carried out in the frequency range of 100?mHz–10?MHz. Low frequency dielectric relaxation modes corresponding to collective behavior of molecules (Goldstone- and soft-modes) in the SmC* phase have been found to be masked by the ionic conductance. Two slow modes of dielectric relaxation due to the ionic conductance have been detected (below 15?Hz) in planar-aligned samples. It has been observed that the effect of ionic conductance decreases with the number of thermal annealing cycles on the materials. With large number of thermal annealing cycles it has been possible to wipe out the effects of ionic conductance and then to detect other weak modes of dielectric relaxation which are otherwise masked.     

Hydrodynamic modes,soft modes and fluctuation spectra near the threshold of a current instability

We give a full threedimensional treatment of the stability and the fluctuations of the uniform stationary current state in a voltage-controlled current instability. We consider a model which exhibits bulk negative differential conductivity due to Bragg scattering of hot electrons. The model consists of Langevin equations for the mean momentum and the mean energy of the charged carriers, coupled to Maxwell's equations. We investigate the normal modes and the fluctuation spectra of this system, in particular the occurrence of soft modes and of critical fluctuations at the stability limit of the uniform current state. It is shown that the nature of the normal modes is strongly determined by the electromagnetic interactions between the carriers, giving rise to hydrodynamic flux modes and to dielectric relaxation modes. As the threshold field is approached, the dielectric relaxation modes soften and couple strongly to the flux modes. It is shown that as a consequence of this coupling the exponential decay of the correlation functions due to ordinary dielectric relaxation is followed at very long times by a power law decay due to the hydrodynamic modes.Work supported by the Swiss National Science Foundation     

Splitting the surface wave in metal/dielectric nanostructures

We investigate a modified surface wave splitter with a double-layer structure, which consists of symmetrical metallic grating and an asymmetrical dielectric, using the finite-difference time-domain (FDTD) simulation method. The metal/dielectric interface structure at this two-side aperture can support bound waves of different wavelengths, thus guiding waves in opposite directions. The covered dielectric films play an important role in the enhancement and confinement of the diffraction wave by the waveguide modes. The simulation result shows that the optical intensities of the guided surface wave at wavelengths of 760-nm and 1000-nm are about 100 times and 4～5 times those of the weaker side, respectively, which means that the surface wave is split by the proposed device.     

Quantum Thermal Transport through Extremely Cold Dielectric Chains

In the framework of Green＇s function theory out of equilibrium, a Landauer-Buttiker （LB） formula for thermal conductance is derived. A simplified model for describing extremely cold dielectric chains is proposed for the first time. Further we apply the present LB formula for studying thermal conductance at low-lying modes, emerging in dielectric atom chains. We find that quantum thermal conductance undergoes an anomalous transition due to new quasiparticle excitations, resulting from nonlinear atom-atom interactions. This theoretical prediction is in excellent agreement with a high-accuracy measurement to thermal conductance quantum.     

First principles study of the vibrational,dielectric and thermal properties of SrClF

SrClF is an important optical crystal and has many technological applications. In this work, vibrational, dielectric and thermal properties of SrClF were investigated by density functional perturbation calculation. The calculated Born effective charges are close to their nominal ionic charges, revealing the ionic characteristic of SrClF. Group theory analysis indicates that there are two E _u and A _2u infrared modes at the Brillouin zone center of SrClF. The LO-TO splitting of these infrared modes were calculated and discussed and their vibrational modes were sketched. Static dielectric constants were studied, which show that SrClF has a larger ionic dielectric contribution than its electrons. Its birefringence was calculated and the infrared reflectance spectra were simulated, which can be used to explain the experimental findings. Based on the computed phonon dispersion curves, the lattice heat capacity, the Grüneisen parameter and the thermal expansion coefficient as functions of temperature were predicted.     

热变形谐振腔的激光模式理论分析

 应用微扰理论,对微热变形腔的激光模式进行了理论计算与分析;给出了激光模式之间的耦合以及等效光束发散角与镜面变形量之间的关系。结果表明：镜面中心处热变形量越大,模式之间的耦合越强,激光腔模式发生畸变就越严重;当镜面中心处热变形量为0.5λ时,基模TEM₀的中心光强将下降50%,其等效光束发散角将增大到镜面无变形时的2倍。     

The lattice dynamics of an anharmonic crystal

The theory of the physical properties of an anharmonic crystal is discussed by using the thermodynamic Green's functions for the phonons. A perturbation procedure is developed to obtain the Green's functions and it is shown that for some purposes a quasi-harmonic approximation is useful, in which the frequencies of the normal modes are those determined by infra-red or neutron spectrometry. The thermodynamic, elastic, dielectric and scattering properties of an anharmonic crystal are discussed in terms of the Green's functions, and detailed expressions are given for the more important contributions. Detailed numerical calculations are presented of the thermal expansion, dielectric properties and shapes of some of the inelastically scattered neutron groups, for sodium iodide and potassium bromide. The calculations, which give reasonable agreement with experiment, show that even at quite low temperatures, the lifetimes of some of the normal modes can be quite short. By using the quasi-harmonic approximation it is shown that the large temperature dependence of the normal modes in a ferroelectric crystal can be treated adequately.     

金属/介质薄膜中声子热辐射的空间和各向异性研究

基于理论、实验和仿真相结合的方式,着重研究了金属/介质(MD)薄膜中声子热辐射的空间特性和各向异性。声子是由于晶格振动产生的元激发,是物质的内在属性。尽管声子不易调控,但是声子与其他光学激发的耦合会产生奇异的光学现象。特别是红外到太赫兹范围内的光子与极性介质中的声子强耦合产生表面声子激元（SPhP）。SPhP具有强局域、低损耗等特点,与等离子体（plasmon polaritons）形成互补,使得深亚波长光学成为可能。为了进一步了解声子吸收的内在理论基础,首先通过黄昆方程和超晶格连续介电模型在理论上分析了声子吸收。实验上,主要以SiO₂声子作为研究对象,利用等离子体增强化学气相沉积(PECVD)方法,分别在Si/Al（150 nm）薄膜和Si衬底上制备出500 nm厚的SiO₂薄膜。基于傅里叶红外光谱仪(FTIR),在垂直入射下得到热辐射光谱,通过热辐射光谱分析,并结合由时域有限差分算法（finite-difference time-domain,FDTD）计算得出的仿真光谱图,对比了MD薄膜结构和非MD薄膜结构中声子的热辐射,发现MD薄膜结构更能够有利于声子和SPhP的激发。根据Berreman效应,纵光学波（LO）声子只在倾斜入射时产生。光谱线没有呈现洛伦兹线型,因此,虽然LO声子在垂直入射时测得的热辐射图中不辐射,但同样影响横光学波（TO）声子辐射谱的线型。另外,利用FTIR对金属（Si/Al）/介质（SiO₂薄膜）进行热辐射转角测试,对热辐射转角图分析证明,Si/Al/SiO₂薄膜中SiO₂声子遵循LST(lyddano-sachs-teller)关系,纵横声子成对出现,且两种声子的空间辐射特性不同。改变偏振,发现在S偏振和P偏振下,声子热辐射呈现不同的模式, 体现出声子的空间各向异性。并且,声子与光子耦合可以激发SPhP,反过来,SPhP可以增强声子的吸收。基于MD结构,能够激发并调控SPhP和声子辐射行为,为红外器件的实现奠定了基础。     

Combined dielectric and plasmon resonance for giant enhancement of Raman scattering

Combined dielectric/metal resonators for colossal enhancement of inelastic light scattering are developed and their properties are investigated. It is shown that a record enhancement factor of 2 × 10⁸ can be obtained using these structures. The dielectric resonators are fabricated on Si/SiO₂ substrates where periodic arrays of square 10- to 200-nm-high dielectric pillars are produced via electron-beam lithography and plasma etching. The lateral size a of the pillars varies between 50 and 1500 nm, and their period in the array is 2a. To make a combined dielectric/metal resonator, a nanostructured layer of silver is deposited onto the fabricated periodic dielectric structure by thermal evaporation. It is established that, for a fixed height of the dielectric pillars, the Raman scattering enhancement factor experiences pronounced oscillations as a function of the period (and size) of the pillars. It is shown that these oscillations are determined by the modes of the dielectric resonator and governed by the relation between the excitation laser wavelength and the planar size of the dielectric pillars.     

Nonlinear dielectric properties of planar structures based on ferroelectric betaine phosphite films

Ferroelectric films of partly deuterated betaine phosphite are grown on NdGaO₃(001) substrates with an interdigitated system of electrodes on their surfaces by evaporation at room temperature. These films have a high capacitance in the ferroelectric phase transition range. The dielectric nonlinearity of the grown structures is studied in small-signal and strong-signal response modes and in the intermediate region between these two modes by measuring the capacitance in a dc bias field, dielectric hysteresis loops, and the Fourier spectra of an output signal in the Sawyer-Tower circuit. In the phase transition range, the capacitance control ratio at a bias voltage U _bias = 40 V is K ? 7. The dielectric nonlinearity of the structures in the paraelectric phase is described by the Landau theory of second-order phase transitions. The additional contribution to the nonlinearity in the ferroelectric phase is related to the motion of domain walls and manifests itself when the input signal amplitude is higher than U _st ～ 0.7–1.0 V. The relaxation times of domain walls are determined from an analysis of the frequency dependences of the dielectric hysteresis.     

Effect of dispersion degree of orientation on dielectric properties of (100)-oriented PST thin film

(100) Oriented (Pb_xSr_1−x)TiO₃ (PST) thin films were prepared on indium tin oxide coated glass substrates by sol–gel technique with rapid thermal processing. The dielectric permittivity and tunability of the thin films with different dispersion degrees of orientation were investigated in detail by characterizing the full width at half maximum of their (100) peak based on rocking curves at different annealing temperatures. Influence of orientation dispersion on dielectric properties was exhibited in the tunable dielectric thin films. It shows that the dielectric constant and hence the tunability of the sol–gel derived PST thin films are improved with the decrease in the dispersion degree of orientation of the perovskite phase other than the increase in the content of crystalline phase in the thin films. The dielectric constant (capacitance) and figure of merit of the oriented thin films are 3–6 times and 1 times higher than that of randomly oriented thin film respectively.     

Plasma-filled dielectric Cherenkov maser

The effect of a background plasma on a dielectric Cherenkov maser is examined by solving the linearized, beam-plasma, dielectric-lined waveguide dispersion relation. The results indicate that introduction of the background plasma can produce a higher spatial growth rate for the beam-waveguide instability and can reduce the electric field at the dielectric surface when compared to the system with no background plasma present. It is also found that for some sets of waveguide parameters, the TM_0n electromagnetic modes can propagate frequencies that are below the background plasma frequency in the system. These modes have a finite off frequency and are different from the Trivelpiece-Gould modes for the system     

Experimental observation of left-handed behavior in an array of standard dielectric resonators

We demonstrate that by utilizing displacement currents in simple dielectric resonators instead of conduction currents in metallic split-ring resonators and by additionally exciting the proper modes, left-handed properties can be observed in an array of high dielectric resonators. Theoretical analysis and experimental measurements show that the modes, as well as the subwavelength resonance, play an important role in the origin of the left-handed properties. The proposed implementation of a left-handed metamaterial, based on a purely dielectric configuration, opens the possibility of realizing media at terahertz frequencies since scaling issues and losses, two major drawbacks of metal-based structures, are avoided.     

Low-frequency surface waves in a cylindrical nonisothermal plasma column

The dispersion relation of surface waves propagating on a homogeneous Maxwellian plasma column with sharp boundary and surrounded by dielectric and vacuum is investigated in the low-frequency domain. The thermal motion of ions is also taken into account. The spectra and damping rates both of short and longwavelength surface modes in thick and thin columns are obtained.     

介电常数近零模式与表面等离激元模式耦合实现宽带光吸收

介电常数为零或近零模式在微纳结构中提供了一个新的方式调控光与物质的相互作用.本文首先利用金属圆盘阵列结构激发了表面等离激元共振,在共振频率处实现了光的局域效果;然后通过在金属-绝缘体-金属超表面微纳结构中加入掺杂半导体材料,利用上层金属圆盘阵列激发的表面等离激元共振诱导介电常数近零模式的产生,从而使得介电常数近零模式与表面等离激元模式发生耦合,在中红外波段实现了一个470 nm的宽带吸收效果;数值模拟结果显示,在宽带吸收处存在光场的强局域效果.与窄带吸收相比,宽带吸收有更广泛的应用,比如吸收器、传感器、滤波器、微测辐射热计、光电探测器、相干热发射器、太阳能电池、指纹识别和能量收集装置等.     

Tunnelling leaky modes on optical waveguides

The lossless dielectric waveguide of circular cross section with an index of refraction greater than its surround supports trapped modes in addition to two subclasses of leaky modes: refracting and tunnelling modes. Refracting modes leak because their fields are formed by waves that undergo refraction at the core-cladding interface. Tunnelling modes leak because their fields are formed by waves that undergo a form of electromagnetic tunnelling at the core-cladding interface due to the curvature of its cross section. Tunnelling modes have a very slow leakage compared to refracting modes and are therefore important for the understanding of propagation in multimode, optical waveguides of circular cross section.     

一种计算和分析二维光子晶体缺陷模式的方法

通过改进时域有限差分(FDTD)法,计算和分析了二维光子晶体的缺陷模式。运用一维时域有限差分算法和线性插值法在总场散射场(TF-SF)连接边界引入入射平面波,采用完全匹配层(PML)技术对外行波进行了有效吸收。计算和分析结果表明,在光子晶体非对称方向入射的平面波能激发所有的缺陷模式,选取合适的探测点位置收集电场值,经快速傅里叶变换(FFT)能得到所有的共振峰值。另外,采用该方法研究了二维正方介质柱光子晶体缺陷模的共振频率与缺陷介质柱半径和介电常量之间的关系。结果表明通过改变缺陷的半径和介电常量大小可以在光子晶体禁带中一定的范围内调节缺陷模式的共振频率大小。     

Lattice thermal conductivity of skutterudite compounds

A generalized expression is used on the basis of relaxation time approximation to facilitate calculation of lattice thermal conductivity of dielectric materials as well as skutterudite family consists of compounds of the form AB₃. It is assumed that phonon scattering processes are independent and is represented by frequency dependent relaxation times. The contributions of normal three phonon scattering processes are included explicitly as redistribution of phonon momentum between two oscillation branches is considered. Magnitudes of relaxation times are estimated from the experimental data. The result for CoSb₃ is in reasonably good agreement with the experimental result in the temperature range 1–1000°K. It is observed that redistribution of phonon momentum between two oscillation branches leads to a significant suppression of thermal conductivity maximum and it is observed that for unfilled skutterudite the main dominant mechanism at the thermal conductivity maximum is three phonon normal scattering process.     

Femtosecond laser induced desorption of water from silver nanoparticles

The photodesorption mechanism of H₂O from quartz-supported silver nanoparticles has been studied by femtosecond laser two-pulse correlation and fluence dependence measurements. With the laser wavelength close to the maximum of the (1,1) plasmon resonance of the nanoparticles, the desorption was found to be purely thermal, i.e., induced by coupling of the desorption coordinate to the nanoparticle lattice temperature, both in the low- and the high-coverage regimes. The lattice cooling times of the nanoparticles are in the range of several hundred ps, in accordance with recent time-resolved X-ray measurements. Also observed is a reversible red-shift of the nanoparticle plasmon modes with increasing H₂O coverage which is attributed to dielectric screening. PACS 78.67.-n; 82.53.-k; 65.80.+n     

Raman spectra of ZnSe nanoparticles synthesized by thermal evaporation method

ZnSe nanoparticles were synthesized by using a thermal evaporation method for a vapour phase reaction of zinc and selenium sources. The sample was characterized by XRD, TEM, HRTEM and Raman spectroscopy. Raman spectra of longitudinal-optical (LO) like mode was analysed by a macroscopic continuum dielectric model. Large broadening of LO-mode is homogeneous and comes from the relaxation of the phonons at the interface and contribution of defects. Surface modes give the main contribution to the asymmetry of the lineshape.