Frequency dependence of the phase shift in optical-fibre thermal modulators

The optical phase shift as a function of frequency was determined in the range from DC to >10 kHz for some single-mode optical-fibre phase modulators, formed by depositing a metal layer on the cladding surface by vacuum evaporation. A thermal analysis is presented in which the fibre surface and core temperatures are determined as a function of the frequency of the heating due to an electric current passing through the coating. The phase modulation arises from a combination of the temperature change at the core (predominant at low frequencies) and strains produced by the thermal expansion of the metal coating (predominant at the high frequencies). Applications include phase and birefringence control in optical-fibre interferometers.     

Higher harmonic thermal wave detection in thermoreflectance microscopy with combined electrical and optical heating

Insulating lines and channels prepared by focused ion beam implantation on SIMOX wafers have been investigated by thermally modulated optical reflectance microscopy using optical and electrical excitation. Continuous hot lines and hot spots can be visualized by modulated electrical heating, whereas insulating lines forming channels can be imaged only with simultaneous optical and electrical excitation. Three different schemes have been investigated for the combined excitation: (i) optical modulation with additional DC voltage, (ii) optical and electrical pumps modulated at the same frequency with the detection effected at higher harmonics and (iii) two different modulation frequencies used for optical and electrical excitation with the detection effected at sum and difference frequencies. When detecting at higher frequencies, best contrast for the observation of the insulating lines adjacent to a channel is achieved by recording the modulated reflectance signal at the fourth harmonic 4f where f is the modulation frequency of the optical and electrical pump. The observed contrast enhancement of the double excited thermoreflectance signal is found to be mainly of a thermal origin.     

Probing thermal properties of vanadium dioxide thin films by time-domain thermoreflectance without metal film

Vanadium dioxide(VO_2) is a strongly correlated material, and it has become known due to its sharp metal–insulator transition(MIT) near room temperature. Understanding the thermal properties and their change across MIT of VO_2 thin film is important for the applications of this material in various devices. Here, the changes in thermal conductivity of epitaxial and polycrystalline VO_2 thin film across MIT are probed by the time-domain thermoreflectance(TDTR) method.The measurements are performed in a direct way devoid of deposition of any metal thermoreflectance layer on the VO_2 film to attenuate the impact from extra thermal interfaces. It is demonstrated that the method is feasible for the VO_2 films with thickness values larger than 100 nm and beyond the phase transition region. The observed reasonable thermal conductivity change rates across MIT of VO_2 thin films with different crystal qualities are found to be correlated with the electrical conductivity change rate, which is different from the reported behavior of single crystal VO_2 nanowires. The recovery of the relationship between thermal conductivity and electrical conductivity in VO_2 film may be attributed to the increasing elastic electron scattering weight, caused by the defects in the film. This work demonstrates the possibility and limitation of investigating the thermal properties of VO_2 thin films by the TDTR method without depositing any metal thermoreflectance layer.     

Thermal characterization of micro-structured NiTi samples by 3ω scanning thermal microscopy

The lateral modification of the thermal conductivity of a NiTi sample have been measured by scanning thermal microscope using the 3ω-technique. Squares of lateral length in the micrometer range had been drawn in a polycrystalline NiTi sample by a focussed ion beam of Ga. Amplitude and phase of the 3ω-signal have been recorded at some selected positions as a function of frequency between 10 Hz and 10 KHz and as a function of position at selected modulation frequencies. The 3ω-signals are modified inside the squares as well as the border lines and also change when the temperature is increased above the martensite-austenite transition temperature.     

Thermal properties of compressed expanded graphite: photothermal measurements

The thermal diffusivity and the thermal conductivity of compressed expanded graphite (CEG) samples were investigated by photothermal measurements in two geometries differing by a place of temperature disturbance detection. This disturbance can be detected on a surface opposite to the one at which the disturbance was generated (rear detection) or on the same surface (front detection). A measurement based on the rear detection allowed us to determine the effective thermal diffusivity of the sample, while the method with front detection gives the possibility of analysis of homogeneity of the sample. It is shown that the thermal diffusivity of CEG strongly depends on its apparent density. Moreover, CEG samples reveal anisotropy of the thermal properties. The thermal diffusivity in the direction parallel to the compacting axis is lower than the one in the direction perpendicular to it. The parallel thermal diffusivity decreases with growing apparent density, while the perpendicular thermal diffusivity significantly grows when the apparent density grows. The perpendicular thermal conductivity exhibits the same behavior as the perpendicular thermal diffusivity. The parallel thermal conductivity slightly grows with growing density and then reaches a plateau. The anisotropy of CEG samples grows with growing apparent density and vanishes for low-density samples. The photothermal measurement with front signal detection revealed that the CEG samples are non-homogeneous in the direction of the compacting axis and can be modeled by a two-layer system.     

Frequency-dependent deformation of liquid crystal droplets in an external electric field

Nematic droplets suspended in the isotropic phase of the same substance were subjected to alternating electrical fields of varying frequency. To keep the system at a constant nematic/isotropic volume ratio with constant droplet size, we carefully kept the temperature in the isotropic/nematic coexistence region, which was broadened by adding small amounts of a non-mesogenic liquid. Whereas the nematic droplets remained spherical at low (in the order of 10 Hz) and high frequencies (in the order of 1 kHz), at intermediate frequencies we observed a marked flattening of the droplets in the plane perpendicular to the applied field. Droplet deformation occurred both in liquid crystals (LCs) with positive and negative dielectric anisotropy. The experimental data can be quantitatively modelled with a combination of the leaky dielectric model and screening of the applied electric field due to finite conductivity.     

Amplitude-modulation-free optoelectronic frequency control of laser diodes

A novel method is described for fast frequency modulation or frequency control of diode lasers that avoids problems associated with bias current modulation, namely, amplitude modulation and thermal phase delays. The method is based on amplitude-modulated, noninterfering control light with a wavelength near the transparency region of the laser diode, which specifically modifies the spectral gain profile to yield a constant gain but a controllable refractive index at the lasing wavelength. This permits amplitude-modulation-free frequency modulation at modulation frequencies up to the relaxation oscillation frequency. A phase lock between the emissions of two extended-cavity diode lasers that could not be achieved with bias current modulation was achieved by this method.     

Thermal conductivity degradation induced by heavy ion irradiation at room temperature in ceramic materials

The thermal conductivity degradation induced by irradiation with energetic heavy ions at room temperature is studied and quantified. Three semi-metallic systems: titanium and zirconium carbides, titanium nitride, as well as a covalent compound: 6H silicon carbide were irradiated by 25.8 MeV krypton ions at 10¹⁶ and 6 . 10¹⁶ ions.cm^-2 doses to produce defects. During ion irradiation, inelastic collisions and elastic collisions occur at a different depth in a material. Two collision domains can be defined. Modulated thermoreflectance microscopy measurements were performed at differing frequencies to characterize the thermal conductivity degradation in these two domains for each of the investigated materials. Our results reveal a significant thermal conductivity degradation in the two collision domains for all materials. Elastic collisions are shown to degrade more strongly the thermal properties than inelastic ones. Scattering of thermal energy carriers is larger in elastic collision domain because displacement cascades produce a very high concentration of point defects: vacancies, interstitials and implanted Kr ions. The degradation coming from electronic interactions that seems to be more important in SiC can be explained by the presence of large populations of generated extended defects, facing to generated individual point defects in TiC, TiN or ZrC.     

Potentialities of photothermal displacement experiments in micro-scaled systems

The phase data of the photothermal signals measured at the surface of two-layer systems are interpreted in the framework of 3-D thermal wave propagation, taking into account the finite size of the heating spot and detection spot. In the first approach, concentric heating and detection spots are considered. In this configuration, information on the thermophysical properties of the surface layer is obtained in the range of intermediate and high modulation frequencies of heating. Additionally information on the thermal diffusivity of the subsurface material can be obtained in the limit of low modulation frequencies. Applying pump–probe offsets and controlled displacements between the excitation spot and detection spot provides the possibility of localizing heat sources. On the basis of numerical simulations using displacement distances from the millimeter to the micrometer range, the scaling of the thermal localization of hot spots from macroscopic to microscopic is studied with respect to experimental parameters, such as the heating and detection spot size, and the optimal range of the heating modulation frequencies.     

Thermal conductance of hydrophilic and hydrophobic interfaces

Using time-domain thermoreflectance, we have measured the transport of thermally excited vibrational energy across planar interfaces between water and solids that have been chemically functionalized with a self-assembled monolayer (SAM). The Kapitza length--i.e., the thermal conductivity of water divided by the thermal conductance per unit area of the interface--is analogous to the "slip length" for water flowing tangentially past a solid surface. We find that the Kapitza length at hydrophobic interfaces (10-12 nm) is a factor of 2-3 larger than the Kapitza length at hydrophilic interfaces (3-6 nm). If a vapor layer is present at the hydrophobic interface, and this vapor layer has a thermal conductivity that is comparable to bulk water vapor, then our experimental results constrain the thickness of the vapor layer to be less than 0.25 nm.     

Resistance of thin films with edge superconductivity in strong magnetic fields

It is shown that, in an edge superconducting layer of a thin film in a magnetic field perpendicular to the film plane, phase slip centers are formed. The centers arise below the superconducting transition temperature because of the thermal fluctuations of the order parameter and lead to the suppression of superconductivity. The resistance corresponding to such fluctuations is determined, and the contribution of the Aslamazov-Larkin correction to the conductivity of a thin film in magnetic fields slightly exceeding the critical field that breaks the surface superconductivity is calculated.     

GaN薄膜的热导率模型研究

准确预测GaN半导体材料的热导率对GaN基功率电子器件的热设计具有重要意义.本文基于第一性原理计算和经典Debye-Callaway模型,通过分析和完善Debye-Callaway模型中关于声子散射率的子模型,建立了用于预测温度、同位素、点缺陷、位错、薄膜厚度、应力等因素影响的GaN薄膜热导率的理论模型.具体来说,对声子间散射项和同位素散射项基于第一性原理计算数据进行了系数拟合,讨论了两种典型的处理点缺陷和位错散射的散射率模型,引入了应用抑制函数描述的各向异性边界散射模型,并对应力的影响进行了建模.热导率模型预测值和文献中典型实验数据的对比表明,基于第一性原理计算数据拟合的热导率模型和实验测量值总体符合较好,300 K温度附近热导率数值及其随温度变化的趋势存在20%左右的偏差.结合实验数据和热导率模型进一步确认了第一性原理计算会高估同位素散射的影响,给出了薄膜热导率随薄膜厚度、位错面密度、点缺陷浓度的具体变化关系,同位素和缺陷散射会减弱薄膜热导率的尺寸效应,主要体现在100 nm附近及更小的厚度范围.     

Non-equilibrium molecular dynamics simulation study of the frequency dependent conductivity of a primitive model electrolyte in a nanopore

The frequency dependence of electrical conductivity in a 0.1 molar univalent restricted primitive model electrolyte confined in cylindrical pores is studied by non-equilibrium molecular dynamics simulations. At high frequencies, conductivity is independent of pore size and approaches the zero value limit. The phase lag is independent of pore size and approaches the value π/2 at high frequency. At low frequencies, the conductivity is relatively constant and approaches the zero frequency (dc) conductivity value. For pores with radius smaller than 3 times the ion diameter, severe confinement effects lead to different low frequency behaviour. In these very small pores, axial collisions increase at low frequency and lead to much lower conductivity and a negative phase shift. The current response in severely confined electrolytes can be analogous to an LRC circuit with resonance at a characteristic frequency.     

Electrical Impedance Spectroscopic Study of CNT/Ethylene-alt-CO/Propylene-alt-CO Polyketones Nanocomposite

Impedance spectroscopy was utilized to investigate the dielectric properties, ac conductivity and charge transport mechanisms in propylene-alt-CO/ethylene-alt-CO (EPEC) random terpolymer filled with multi-walled carbon nanotubes (MWCNT) as a function of nanofiller content, frequency, and temperature. Equivalent resistor-capacitor (RC) circuit models were proposed to describe the impedance characteristics of the unfilled terpolymer and the nanocomposite at different temperatures. For the nanocomposites, the ac conductivity tended to be frequency independent at low frequencies. At high frequencies, the ac conductivity increased with frequency. The dc conductivity (i.e., plateau of the ac conductivity at low frequencies) at room temperature increased from 10^?9 (Ω·m)^?1 for the unfilled polymer to l0^?3 (Ω·m)^?1 for the 6 wt% MWCNT/EPEC nanocomposite. At low temperatures, the equivalent RC model for EPEC-0 and EPEC-2 was found to consist of a parallel RC circuit. However, for 6 wt% MWCNT/EPEC nanocomposite, an RC model consisting of an R/constant phase element (CPE) circuit and a resistor in series was required to describe the impedance behavior of the nanocomposite.     

Measurements of electron-phonon coupling factor and interfacial thermal resistance of metallic nano-films using a transient thermoreflectance technique

Using a transient thermoreflectance (TTR) technique,several Au films with different thicknesses on glass and SiC substrates are measured for thermal characterization of metallic nano-films,including the electron-phonon coupling factor G,interfacial thermal resistance R,and thermal conductivity K s of the substrate. The rear heating-front detecting (RF) method is used to ensure the femtosecond temporal resolution. An intense laser beam is focused on the rear surface to heat the film,and another weak laser beam is focused on the very spot of the front surface to detect the change in the electron temperature. By varying the optical path delay between the two beams,a complete electron temperature profile can be scanned. Different from the normally used single-layer model,the double-layer model involving interfacial thermal resistance is studied here. The electron temperature cooling profile can be affected by the electron energy transfer into the substrate or the electron-phonon interactions in the metallic films. For multiple-target optimization,the genetic algorithm (GA) is used to obtain both G and R. The experimental result gives a deep understanding of the mechanism of ultra-fast heat transfer in metals.     

A high-precision magnetoencephalographic study of human auditory steady-state responses to amplitude-modulated tones

The cerebral magnetic field of the auditory steady-state response (SSR) to sinusoidal amplitude-modulated (SAM) tones was recorded in healthy humans. The waveforms of underlying cortical source activity were calculated at multiples of the modulation frequency using the method of source space projection, which improved the signal-to-noise ratio (SNR) by a factor of 2 to 4. Since the complex amplitudes of the cortical source activity were independent of the sensor position in relation to the subject's head, a comparison of the results across experimental sessions was possible. The effect of modulation frequency on the amplitude and phase of the SSR was investigated at 30 different values between 10 and 98 Hz. At modulation frequencies between 10 and 20 Hz the SNR of harmonics near 40 Hz were predominant over the fundamental SSR. Above 30 Hz the SSR showed an almost sinusoidal waveform with an amplitude maximum at 40 Hz. The amplitude decreased with increasing modulation frequency but was significantly different from the magnetoencephalographic (MEG) background activity up to 98 Hz. Phase response at the fundamental and first harmonic decreased monotonically with increasing modulation frequency. The group delay (apparent latency) showed peaks of 72 ms at 20 Hz, 48 ms at 40 Hz, and 26 ms at 80 Hz. The effects of stimulus intensity, modulation depth, and carrier frequency on amplitude and phase of the SSR were also investigated. The SSR amplitude decreased linearly when stimulus intensity or the modulation depth were decreased in logarithmic steps. SSR amplitude decreased by a factor of 3 when carrier frequency increased from 250 to 4000 Hz. From the phase characteristics, time delays were found in the range of 0 to 6 ms for stimulus intensity, modulation depth, and carrier frequency, which were maximal at low frequencies, low intensities, or maximal modulation depth.     

High-temperature phase transitions in the improper ferroelectric KIO3

The ac conductivity (σ_ac) and dielectric permittivity (?) are determined in the temperature range 300?K?T3 compound. The results indicated that the compound behaves as an improper ferroelectric and undergoes a ferroelectric phase transition from a high temperature rhombohedral phase I to a low temperature monoclinic phase II at T _c?=?(486?±?1)?K. A second structural phase transition was observed around 345?K. The conductivity varies with temperature range and for T?>?428?K intrinsic conduction prevails. Different activation energies in the different temperature regions were calculated. The frequency dependence of σ(ω) was found to follow the universal dynamic response [σ(ω)∝(ω) ^s(T)]. The thermal behaviour of the frequency exponent s(T) suggests the hopping over the barrier model rather than the quantum mechanical tunneling model for the conduction mechanism.     

Ac conductivity in boron doped amorphous carbon films

Experimental results on frequency and temperature dependence of ac conduction in boron doped amorphous carbon films are analyzed in the framework of available microscopic models. Depending on the response, the conductivity plot is divided into three regimes (low frequency high temperature; moderate frequency intermediate temperature; high frequency low temperature) and the data in the respective regimes are corroborated with the various theoretical models accordingly. The conductivity data at high frequency and low temperature suggests that relaxation via quantum mechanical tunneling might be the dominant conduction mechanism. At intermediate temperatures and moderate frequencies, the conductivity data is in good agreement with extended pair approximation model with interaction correction. Signature of enhanced interaction effect is observed at low temperature.     

太赫兹波段金属线栅的紫外光控特性研究

基于氧化铟纳米薄膜及金属线栅的特性,利用紫外激光诱导以金属线栅为衬底的氧化铟纳米结构,研究其对于太赫兹偏振透射的调制特性。实验中在金属线栅上滴入溶于乙醇的氧化铟溶液,并使溶液恰好浸润在金属线栅缝隙中,同时将加热台的温度调至340 ℃,对金属线栅中的氧化铟进行热退火。结果表明,氧化铟-金属线栅线长方向与太赫兹电场偏振方向垂直时,在低强度紫外光的照射下,该样品对太赫兹的透射强度有较为明显的衰减,当紫外光功率密度为7 mW·cm-2时,样品对太赫兹的调制深度可达71%;当氧化铟-金属线栅线长方向与太赫兹电场偏振方向平行时,紫外光激发下的样品对太赫兹的调制效果明显减弱,当紫外光功率密度为7 mW·cm-2时,调制深度约为20%。氧化铟纳米薄膜中存在的氧空位,使该材料对紫外光具有特殊响应。在无紫外光照射下,样品环境中的氧气分子被吸附到氧化铟表面,由于化学反应生成O2-离子态。当用光子能量大于氧化铟禁带宽度的紫外光激发样品时,在氧化铟表面激发出电子空穴对,空穴会被氧化铟表面的O2-离子态和缺陷态束缚,从而释放电子到导带,增强了样品的电导率。在太赫兹波频段内,透过氧化铟样品的太赫兹强度与氧化铟电导率有很好的相关性。金属线栅利用金属表面可存在的自由电子的振荡, 使电场方向与线栅方向平行的太赫兹偏振光激发电子沿线栅方向振荡,当电子与金属晶格中的原子碰撞时,此偏振光发生衰减并伴随辐射;而电场方向与线栅方向垂直的太赫兹偏振光,由于周期性结构的限制,无法激发自由电子振荡, 主要表现出透射特性。结合氧化铟的表面缺陷特性,紫外光可实现作为氧化铟-金属线栅结构的光控偏振开关作用,氧化铟-金属线栅结构偏振器能很好地应用于太赫兹波频段的光控偏振调制。     

三氨基三硝基苯基高聚物粘结炸药热力学性质的理论计算研究

高聚物粘结炸药(PBX)的热力学性质是用于炸药结构响应、安全性评估、数值模拟分析等的重要参数.由于PBX结构的多尺度特性,完全采取实验方法精细表征这些参数存在巨大的挑战.本文运用第一性原理和分子动力学计算的方法,系统研究了三氨基三硝基苯(TATB)基高聚物粘结炸药的热力学参数和界面热传导性质.利用散射失配模型研究了TATB与聚偏二氟乙烯(PVDF)界面的热传导过程,发现热导率随温度升高而上升,并且在高温情况下接近于定值.基于分子动力学获得的TATB热导率并结合界面热导率,分析了PBX炸药的热导与颗粒尺寸的关系,当颗粒尺寸大于100 nm时,界面热阻对于PBX热导率的影响有限.