General expression for the depolarization ratio for first‐order collision‐induced light scattering

A general expression that involves only the ranks of the molecular polarizabilties of the two molecules involved in a pair interaction yielding translational and rotational collision‐induced light scattering (CILS) is obtained. It may be used to investigate the behaviour of individual interactions that contribute to the incremental collision‐induced pair polarizability. Copyright © 2010 John Wiley & Sons, Ltd.     

A multispectroscopic investigation of the electronic structure of luminescent,tetrahedrally coordinated vanadium-oxygen compounds: Case of YVO4

Photoelectron (XPS) and core ionization loss (CILS) spectra of YVO₄ have been measured and compared to our earlier appearance potential (APS) data on the same compound, literature data on X-ray emission of YVO₄, X-ray emission and absorption of Na₃VO₄, and the energy level diagram of molecular orbitals of the VO₄^3- cluster. Strong many-body effects, with participation of a 2p core hole on vanadium atoms, are found to lead to a drastic local perturbation of the outer molecular orbitals of the anionic cluster VO₄^3-, at the site of the excited vanadium atoms. Identification of the spectroscopic features in XPS on one hand and the edge spectroscopies CILS and APS on the other hand, enables determination of an empirical energy level diagram of excited configurations, which is different in different spectroscopies used. As a byproduct of our experiment, data on the excitation of oxygen (within the VO₄^3- cluster), yttrium (outside the VO₄^3- cluster), and carbon impurity are presented and discussed.     

Unified approach to short optical pulse recording and spatial imaging with subwavelength resolution

We propose and numerically demonstrate a simple method for measuring waveforms of optical pulses that have spectral bandwidths much larger than the passband of the measuring system, thus enabling a kind of temporal superresolution. The technique is based on pulse intensity modulation that contains high-order harmonics. Parts of the pulse intensity spectrum that are shifted as a result of the modulation, are moved over (“umklapped”) to the center of the passband, transmitted and then recorded by an oscilloscope. The pulse intensity spectrum is restored by parts from the Fourier transform of a few oscillograms, measured after performing the temporal shifts between the pulse train and the modulation. A similar approach is applied for achieving subwavelength spatial resolution in far -field microscopy. The spatial modulation is performed by a diffraction grating. The method allows one to restore a subwavelength object in a single measurement.     

Emission processes accompanying deformation and fracture of metals

Present-day physical methods of investigation reveal that the fracture and plastic deformation of metals is accompanied by emission processes, in particular, by luminescence and emission of electrons. All the metals studied thus far exhibit a capability of luminescence. The intensity, duration, and spectrum of mechanoluminescence are different for different metals. The intensity is determined by the mechanical and thermal characteristics. For a given metal, the intensity depends on dislocation density in the structure and the sample loading rate. The spectrum of noble metals is governed by the electronic structure of surface states. The dynamics of mechanoluminescence and electron emission (exoemission) depends on the rate of stress variation in the sample under study. This permits one to consider the mechanoluminescence and exoemission not only as physical characteristics but also as a potential tool for probing surface states in metals and the kinetics of emergence of mobile dislocations on the surface with a high time resolution. Fiz. Tverd. Tela (St. Petersburg) 41, 841–843 (May 1999)     

Coherent control of interacting electrons in quantum dots via navigation in the energy spectrum

Quantum control of the wave function of two interacting electrons confined in quasi-one-dimensional double-well semiconductor structures is demonstrated. The control strategies are based on the knowledge of the energy spectrum as a function of an external uniform electric field. When two low-lying levels have an avoided crossing, our system behaves dynamically to a large extent as a two-level system. This characteristic is exploited to implement coherent control strategies based on slow (adiabatic passage) and rapid (diabatic Landau-Zener transition) changes of the external field. We apply this method to reach desired target states that lie far in the spectrum from the initial state.     

Direct photoexcitation of appreciable size of domains without lattice motion in neutral-ionic transition systems

In a recent ultrafast pump-probe experiment on a typical neutral-(N-)ionic (I) transition material, tetrathiafulvalene-p-chloranil [S. Iwai, Phys. Rev. Lett. 96, 057403 (2006)10.1103/PhysRevLett.96.057403], it was claimed that an I-phase domain in the N-phase background was very quickly formed even before the lattice started to move. This study proves this idea in terms of an extended Hubbard model that has site-potential alternation and has been frequently used so far. Our focus is on the photoexcited states of the N phase and the optical conductivity spectrum. As a result of accurate spectrum calculation based on a dynamical density-matrix renormalization group technique, we clearly observe photoexcited ionic domains accompanied by no lattice dimerization. Their contribution almost dominates the spectrum near the N-I phase boundary, and we relate this property to the long-tailed spectrum observed in experiments.     

Electronic quasiparticle renormalization on the spin wave energy scale

High-resolution photoemission data of the (110) iron surface reveal the existence of well-defined metallic surface resonances in good correspondence to band calculations. Close to the Fermi level, their dispersion and momentum broadening display anomalies characteristic of quasiparticle renormalization due to coupling to bosonic excitations. Its energy scale exceeds that of phonons by far, and is in striking coincidence with that of the spin wave spectrum in iron. The self-energy behavior thus gives spectroscopic evidence of a quasiparticle mass enhancement due to electron-magnon coupling.     

Ultrabroadband strong light absorption based on thin multilayered metamaterials

Light absorbers have drawn intensive attention as crucial components for solar‐energy harvesting, thermal emission tailoring, modulators, etc. However, achievement of light absorbers with wide bandwidth remains a challenge thus far. Here, a thin, unprecedentedly ultrabroadband strong light absorber is proposed and experimentally demonstrated, which consists of periodic taper arrays constructed by an alumina–chrome multilayered metamaterial (MM) on a gold substrate. This MM can change from a hyperbolic material to an anisotropic dielectric material at different frequency ranges and the special material features are the fundamental origins of the ultrabroadband absorption. The absorber is quite insensitive to the incident angle, and can be insensitive to the polarization. One two‐dimensional periodic array of 400‐nm height MM tapers is fabricated. The measured absorption is over 90% over almost the entire solar spectrum, reaching an average level of 96%, and remains high (above 85%) even in the longer‐wavelength range till 4 μm. The proposed absorbers open up a new avenue to realize broadband thin light‐harvesting structures.     

Green's functions for a volume source in an elastic half-space

Green's functions are derived for elastic waves generated by a volume source in a homogeneous isotropic half-space. The context is sources at shallow burial depths, for which surface (Rayleigh) and bulk waves, both longitudinal and transverse, can be generated with comparable magnitudes. Two approaches are followed. First, the Green's function is expanded with respect to eigenmodes that correspond to Rayleigh waves. While bulk waves are thus ignored, this approximation is valid on the surface far from the source, where the Rayleigh wave modes dominate. The second approach employs an angular spectrum that accounts for the bulk waves and yields a solution that may be separated into two terms. One is associated with bulk waves, the other with Rayleigh waves. The latter is proved to be identical to the Green's function obtained following the first approach. The Green's function obtained via angular spectrum decomposition is analyzed numerically in the time domain for different burial depths and distances to the receiver, and for parameters relevant to seismo-acoustic detection of land mines and other buried objects.     

New evidence for lack of CMB power on large scales

A digitalized temperature map is recovered from the first light sky survey image published by the Planck team, from which an angular power spectrum of the cosmic microwave background (CMB) is derived. The amplitudes of the low multipoles (low-l) measured from the preliminary Planck power spectrum are significantly lower than those reported by the WMAP team. Possible systematical effects are far from enough to explain the observed low-l differences.     

Analytical structure of an apertured vector Gaussian beam in the far field

Based on the angular spectrum representation of the Maxwell’s equations and the complex Gaussian expansion of the aperture function, the structure of an apertured vector Gaussian beam in the far field is presented in the integral form. By means of the method of stationary phase, the analytical vectorial structures are obtained. According to the analytical expressions, the characteristics of vectorial structure of an apertured Gaussian beam are investigated in the far field. The influence of a linearly polarized angle on the vectorial structure is also studied in the far field. This research provides a novel approach to further comprehend the vectorial property of an apertured Gaussian beam.     

舰船螺旋桨轴频估计中线谱要素提取算法

舰船辐射噪声DEMON谱(调制解调谱)线谱要素提取是舰船螺旋桨轴频估计中的关键步骤。现有的线谱提取技术主要用于功率谱线谱检测,侧重于线谱频率的确定,不能满足舰船螺旋桨轴频估计需求。本文根据DEMON谱线谱的特点及人工提取线谱的过程,设计了一种新的线谱要素提取算法,改进了谱峰筛选技术,将多时刻的DEMON线谱基于最大似然估计和谱峰序列相似度加以融合决策,最终确定了线谱频率和幅值(峰值)及边界。通过实际数据验证,此线谱要素提取算法更适用于螺旋桨轴频估计,可提高轴频估计的准确率。     

产生正弦光栅的二值化面积编码新方法

基于面积编码的二值化算法,提出了一定改进,即在对二值化模板进行填充时,用分散的、尽可能等间距的填充方法代替分块的、非等间隔的填充方法,并将此光栅制作出来用于三维形貌测量.利用二元编码光栅的频谱分布特征和光学系统固有的低通滤波特征,可以将二元分布的模板转化为测量区域的正弦分布,同时又滤去了量化噪声,得到质量较好的正弦光场...     

Far field spectrum in surface plasmon-assisted Young's double-slit interferometer

We derive an expression for the resultant spectral density (spectrum) at a point in the far zone for the surface plasmons modulated Young's double-slit interference setup. The resultant spectral interference law has the same form as the standard spectral interference law for the scalar fields. This resemblance in turn provides a means for determination of the modified spectral degree of coherence at the two slits. The mathematical results also show that in an interesting situation when the field is incident at one slit only, the interference can still be observed at the observation plane. These findings are verified theoretically using a wide-band source, i.e. a black-body, having a spectrum following Planck's radiation law.     

Vectorial structure of the far field of an elegant Hermite–Gaussian beam

Based on the vector angular spectrum method and the method of stationary phase, an analytical expression for the vectorial structure of the far field of an elegant Hermite–Gaussian beam is derived. The analytical formulae of the energy flux distributions of the TE term, the TM term, and the whole beam are presented in the far field. Analytical expressions for the ratios of the powers of the TE and TM terms to those of the elegant Hermite–Gaussian beam are obtained without any approximation. The physical pictures of the far field of an elegant Hermite–Gaussian beam are demonstrated and compared with those of the far field of the corresponding standard Hermite–Gaussian beam. This research reveals the internal vectorial structure of the far field of an elegant Hermite–Gaussian beam from an alternative viewpoint.     

Line shape distortion in a nonlinear auto-oscillator near generation threshold: application to spin-torque nano-oscillators

The power spectrum of an auto-oscillator with a large frequency nonlinearity in a noisy environment is calculated. The power spectrum becomes strongly non-Lorentzian, broadened, and asymmetric near the generation threshold. A Lorentzian spectrum is recovered far below and far above the threshold, which suggests that line shape distortions provide a signature of the threshold. We show that the developed theory adequately describes the observed behavior of a strongly nonlinear spin-torque nano-oscillator.     

Gauge unification in type I/I′ models

We discuss whether the (MSSM) unification of gauge couplings can be accommodated in string theories with a low (TeV) string scale. This requires either power law running of the couplings or logarithmic running extremely far above the string scale. In both cases it is difficult to arrange for the multiplet structure to give the MSSM result. For the case of power law running there is also enhanced sensitivity to the spectrum at the unification scale. For the case of logarithmic running there is a fine tuning problem associated with the light closed string Kaluza Klein spectrum which requires gauge mediated supersymmetry breaking on the “visible” brane with a dangerously low scale of supersymmetry breaking. Evading these problems in low string scale models requires a departure from the MSSM structure, which would imply that the success of gauge unification in the MSSM is just an accident.     

光谱特性对半导体泵浦碱蒸气激光器性能的影响

以速率方程模型为基础研究了光谱特性对半导体泵浦碱蒸气激光器性能的影响。结果表明:碱蒸气激光器的运行存在最优温度,温度和长度对碱蒸气激光器性能的影响是等价的,可以定义一个与温度和长度无关的效率描述其它参量一定时激光器的最优效率;泵浦谱宽对阈值泵浦强度的影响是线性的,由于碱金属原子在谱线远翼具有较大吸收截面,即使泵浦谱宽几倍或十几倍于原子吸收谱宽时也可能获得较高的效率;原子吸收谱宽对阈值泵浦强度的影响是非线性的,随着原子吸收谱宽的增加激光器光-光效率出现饱和下降;泵浦谱宽和原子吸收谱宽越宽,半导体激光器由于温度或电流导致的中心波长漂移的影响越小,实际中采用外腔压窄的半导体激光器阵列(谱宽小于200 GHz)可以控制其温度或电流漂移,使得碱蒸气激光器的性能基本不受影响。     

激光等离子体光丝中太赫兹频谱的调控

研究了双色激光场激发空气成丝产生太赫兹辐射频谱的变化规律.实验观察到随驱动光功率和光丝长度增加,太赫兹光谱主要发生红移的现象.分析表明,由于等离子体密度的增加,太赫兹辐射的趋肤深度减小,等离子体吸收主导了红移的发生.在光丝足够短的条件下,趋肤深度远大于光丝长度,从而产生等离子体振荡主导的太赫兹辐射光谱蓝移.本研究为超快宽带太赫兹辐射的频谱调控提供了新思路.