Anomalous behaviours of terahertz reflected waves transmitted from GaAs induced by optical pumping

Femtosecond pump-terahertz probe studies of carrier dynamics in semi-insulating GaAs have been investigated in detail for various pump powers. It is observed that, at high pump powers, the reflection peaks flip to the opposite polarity and dramatically enhance as the pump arrival time approaches the reflected wave of the terahertz pulse. The abnormal polarity-flip and enhancement can be interpreted by the pump-induced enhancement in the photoconductivity of GaAs and half-wave loss. Moreover, the carrier relaxation processes and surface states filling in GaAs are also studied in these measurements.     

Reversed Doppler effect in photonic crystals

Nonrelativistic reversed Doppler shifts have never been observed in nature and have only been speculated to occur in pathological systems with simultaneously negative effective permittivity and permeability. This Letter presents a different, new physical phenomenon that leads to a nonrelativistic reversed Doppler shift in light. It arises when light is reflected from a moving shock wave propagating through a photonic crystal. In addition to reflection of a single frequency, multiple discrete reflected frequencies or a 10 GHz periodic modulation can also be observed when a single carrier frequency of wavelength 1 microm is incident.     

Quantum superarrivals and information transfer through a time-varying boundary

The time-dependent Schrödinger equation is solved numerically for the case of a Gaussian wave packet incident on a time-varying potential barrier. The time evolving reflection and transmission probabilities of the wave packet are computed for several different time-dependent boundary conditions obtained by reducing or increasing the height of the potential barrier. We show that in the case when the barrier height is reduced to zero, a time interval is found during which the reflection probability is larger (superarrivals) compared to the unperturbed case. We further show that the transmission probability exhibits superarrivals when the barrier is raised from zero to a finite value of its height. Superarrivals could be understood by ascribing the features of a real physical field to the Schrödinger wave function which acts as a carrier through which a disturbance, resulting from the boundary condition being perturbed, prpagates from the barrier to the detectors measuring reflected and transmitted probabilities. The speed of propagation of this effect depends upon the rate of reducing or raising the barrier height, thus suggesting an application for secure information transfer using superarrivals.     

Nonlinear Amplitude-Phase Effects in Selective Reflection from a Resonance Gas

Selective reflection of a laser wave from a resonance gas is considered for arbitrary intensity, polarization, and angle of incidence on the interface. Variations of the reflection spectra nonlinear in the wave amplitude are studied taking into account the nonlinear increment of the optical phase upon reflection. The result of this increment is a quadrature component of the reflected wave shifted by a quarter period, which may be observed using the homodyning technique (optical mixing). The regularities studied are of interest both for spectroscopy and for nonlinear transformations in the quantum statistics of reflected light.     

井外溶洞储层的偶极横波反射特征研究*

溶洞型储层是油气田中比较典型的储层之一,其非均质性强,勘探开发难度较大,偶极横波远探测技术是目前探测井外溶洞的有效方法。碳酸盐岩溶洞模型正演模拟对于现场的油气勘探和资料解释具有指导性作用。该文建立了几种比较典型的溶洞模型,并通过有限差分方法对溶洞储层的偶极三维反射声场进行了研究,发现在小源距范围内可以接收到来自溶洞后壁较强的反射波,随着源距的增大,该反射波逐渐减弱至消失;对于缝洞结合体的反射波场,来自裂缝的反射波能量要强于来自溶洞的反射波,且来自裂缝的反射波会掩盖溶洞前壁的反射波信息,这对后期的成像处理带来了困难。最后进一步研究了偶极远探测技术在溶洞方位识别以及尺寸大小估算中的应用。     

Acousto-optic study of the unusual cases of reflection of bulk elastic waves in a paratellurite crystal

The propagation and reflection of bulk ultrasonic waves in a paratellurite crystal have been investigated by the acousto-optic method. Simultaneous excitation of two acoustic waves by one piezoelectric transducer has been observed in a cell of complex, specially chosen configuration. Maximally efficient conversion of the energy of a quasi-longitudinal wave into a single reflected quasi-shear wave with a walkoff angle 72° is implemented for the reflection from a free crystal boundary. The wave beam compression by a factor of 7 is observed for this reflection. A reflection (close to backward) of waves with an angle of spatial separation of the incident and reflected beams equal to 8° is also implemented. The acousto-optic figure of merit values of the medium are calculated for arbitrary propagation and polarization directions of the interacting light and ultrasonic waves. Possible applications of the effects studied in acousto-optics and acoustoelectronics are indicated.     

平面电磁波在任意方向运动的介质-介质界面上的反射和透射

研究平面电磁波从一种介质入射到另一种以任意方向运动的介质时,在介质界面上发生的反射和透射现象,从Maxwell方程组、运动界面的边值关系和运动介质的本构关系出发,得到了反射波和透射波电磁场矢量与入射波电磁场矢量的关系的表达式、反射和透射系数,讨论了反射波、透射波与入射波之间的能量关系,电磁波对运动介质作用力的性质,并得出一些新的结论． 关键词：     

Reflection of a focused light beam in a medium with Kerr nonlinearity. I. Estimates in the paraxial-beam approximation

The reflection intensity in a nonlinear dielectric layer is calculated in the approximation of paraxial incident and reflected light beams. It is shown that reflection is mainly determined by discontinuities of the refractive-index on the boundaries. Unlike reflection by a linear dielectric layer, reflection by a nonlinear layer occurs with a three-time decrease in the effective reflection area and the corresponding broadening of the angular spectrum. The intensity of the reflected wave is comparable to that of an incident wave with nearly critical beam power.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 39, No. 7, pp. 867–875, July, 1996.This work was supported by the International Science Foundation and the Russian Federation Government (projects MR1000 and MR1300).     

Resonant excitation of intense acoustic waves in crystals

The resonant excitation of an intense elastic wave through nonspecular reflection of a special pump wave in a crystal is described. The choice of the plane and angle of incidence is dictated by the requirement that the excited reflected wave be close to the bulk eigenmode with its energy flow along a free boundary. The resonance parameters have been found for a medium with an arbitrary anisotropy. General relations are concretized for monoclinic, rhombic, and hexagonal systems. A criterion is formulated for an optimal selection of crystals in which the resonant reflection is close to the conversion one, when almost all of the energy from the incident beam of the pump wave falls into the near-surface narrow high-intensity reflected beam. Estimates and illustrations are given for such crystals as an example. The intensity of the reflected beam increases with its narrowing, but its diffraction divergence also increases with this narrowing. Nevertheless, the intensity of the beam can be increased by a factor of 5–10 at sufficiently high frequencies while keeping its divergence at an acceptable level. Amplification by two orders of magnitude can be achieved by compressing the beam in two dimensions through its double reflection.     

无粘流中能捕捉较弱的接触间断和激波的反扩散差分格式

在研究弱入射激波遇到对称楔以后的马赫反射现象时,激波管实验不易测出很弱的接触间断,也不易捕捉到马赫反射与正规反射转换的条件。文章一方面研究了可压流体力学欧拉方程的数值方法,首先是用反扩散法改进接触间断的计算;另一方面根据格式粘性的特性和它引出的很微小的熵的变化规律来显示很弱的接触间断和反射激波。这样才易于将对三波点的分析推进一步。文[5,6]曾预言了一种反散波是连续的压缩波的新的激波反射类型。我们设想并根据计算初步确认这新类型反射实际应该是简单马赫反射,反射波虽弱仍是激波。     

Quantum reflection times for attractive potential tails

When a wave packet with a narrow momentum distribution is quantum reflected in a purely attractive potential proportional to -1/r(alpha), alpha>2, it generally experiences a time gain compared to a free particle reflected at r=0; for alpha=3 and very low energies there are large time delays. In quantum reflection of an atomic beam by a surface, such a time gain (delay) represents an apparent plane of reflection which is shifted in front of (behind) the surface. The quantum reflected wave is always delayed with respect to the classical particle accelerated in the attractive potential.     

Reflection of Lamb waves obliquely incident on the free edge of a plate

The reflection of obliquely incident symmetric and anti-symmetric Lamb wave modes at the edge of a plate is studied. Both in-plane and Shear-Horizontal (SH) reflected wave modes are spawned by an obliquely incident in-plane Lamb wave mode. Energy reflection coefficients are calculated for the reflected wave modes as a function of frequency and angle of incidence. This is done by using the method of orthogonal mode decomposition and by enforcing traction free conditions at the plate edge using the method of collocation. A PZT sensor network, affixed to an Aluminum plate, is used to experimentally verify the predictions of the analysis. Experimental results provide support for the analytically determined results.     

电介质膜对受抑全内反射结构中古斯-汉欣位移的增强

受抑全反射结构中,反射光束和透射光束的古斯汉欣(Goos-Hanchen)位移同时存在,对称双棱镜结构的受抑全反射古斯汉欣位移通常只有波长量级,在实验中很难测量。计算了在入射角大于棱镜与空气界面的临界角小于棱镜与薄膜界面临界角时,镀有电介质膜的对称双棱镜的受抑全反射过程中入射光束的反射系数和透射系数的复数表达式。利用稳态相位法计算得出透射光束和全反射光束的古斯汉欣位移。结果表明,反射光束和透射光束古斯汉欣位移量相同,与入射角大小、薄膜厚度以及空气层厚度有关。在入射角小于但接近棱镜与薄膜界面的临界角,薄膜厚度和空气层厚度一定时,古斯汉欣位移量共振增强达到波长的数百倍。     

Reflection of a spherical wave from a plane surface

A simple approach to the reflection of a spherical sound wave from a locally reacting plane surface is developed. The theory is based on a generalization of the method of images, and expresses the reflected wave as a series in terms of the reciprocal of distance from the image source. Although no general proof that this form for the reflected wave satisfies the surface boundary condition to all orders is available, it has been shown that the first five terms of this solution does satisfy the boundary condition, and agrees with the existing exact solutions for this type of reflection.     

Radiation reflection by a plasma with electron temperature anisotropy

The reflection of a test electromagnetic wave normally impinging on a plasma surface is investigated within the formalism of the surface impedance. The plasma is assumed to possess an anisotropic two-temperature bi-Maxwellian electron velocity distribution function. The linearly polarized impinging wave during reflection transforms into an elliptically polarized one, the degree of ellipticity depending on the electron temperature anisotropy. Polarization modifications of the reflected wave are particularly important in the conditions of the anomalous skin-effect, when the influence of the wave magnetic field on the electron kinetics in the skin layer is strong. Relations are reported connecting the reflected wave basic parameters to those of the reflecting plasma surface, making possible, through the experimental determination of the reflected wave characteristics, to find the plasma electron concentration and the two effective temperatures. Received 21 May 2002 / Received in final form 21 August 2002 Published online 6 November 2002 RID="a" ID="a"e-mail: zarcone@unipa.it     

Particle-in-Cell Simulation of the Reflection of a Korteweg-de Vries Solitary Wave and an Envelope Solitary Wave at a Solid Boundary

Reflections of a Korteweg-de Vries(KdV) solitary wave and an envelope solitary wave are studied by using the particle-in-cell simulation method.Defining the phase shift of the reflected solitary wave,we notice that there is a phase shift of the reflected KdV solitary wave,while there is no phase shift for an envelope solitary wave.It is also noted that the reflection of a KdV solitary wave at a solid boundary is equivalent to the head-on collision between two identical amplitude solitary waves.     

Applications of the causality condition to one-dimensional acoustic reflection problems.

The causality condition is examined as a means of determining frequency-domain information about a submerged object from a partial knowledge of its acoustic reflection characteristics. A one-dimensional problem is considered in which an acoustic wave reflects from an object that is described by the impedance it presents to the fluid. Two new applications of the causality condition to the frequency-domain analysis of this problem are investigated and illustrated by numerical examples. In each application, the causality condition is used to find the object's complex impedance from a knowledge of the reflected wave's magnitude. The first application is to experimental studies where one desires a knowledge of an object's complex impedance but practical limitations only allow a measurement of the reflected wave amplitude. Analysis shows that the causality condition may be used to determine the phase of the reflected wave, and hence the object's impedance, if the reflection coefficient is minimum phase. When this is true, examples suggest that the phase of the reflection coefficient may be accurately determined from the causality condition even in the presence of noise and band-limited data. The second application is to design situations, where one wishes to create an object that reflects sound with a specified frequency-dependent magnitude. The causality condition may aid the designer by providing a knowledge of all causal object impedances that produce the same reflection coefficient magnitude. A numerical example is presented in which a variety of causal object impedances produce the same reflection coefficient magnitude over an infinite frequency range.     

Reflection of spin and spin-elastic waves at the interface of a ferromagnetic half-space

This paper considers the reflection of pure spin and spin-elastic (or magneto-elastic) waves at the interface of a ferromagnetic half-space and a vacuum. For pure spin waves two cases are considered, with exchange effects, and without. It is shown that when exchange effects are taken into account, volume spin waves in the ferromagnetic half space incident at the boundary with the vacuum generate a reflected volume spin wave, and an accompanying compound surface wave propagating along the boundary and consisting of two partial inhomogeneous spin waves in the ferromagnetic half-space and a partial magneto-static inhomogeneous surface wave in the vacuum. When exchange effects are neglected the incident wave generates only a reflected volume wave in the ferromagnetic half-space.

Reflection and transmission of spin-elastic (or magneto-elastic) waves has been considered only in the case of the absence of exchange effects. An incident volume wave generates a volume spin-elastic reflected wave and one inhomogeneous magneto-static accompanying surface wave.

Excitations of the magnetic field are not transmitted into the vacuum in both cases when the exchange effect is neglected. In all cases the reflection of a spin wave has the character of a full internal reflection.     

Classification of shock wave reflections from a wedge. Part 1: Boundaries and domains of existence for different types of reflections

Classification of various types of the reflections of a shock wave over straight wedge is supposed. The idea about entire reflection phenomenon as a result of interaction of two processes—the shock wave refection process and the flow deflection process—serves as a basis for the classification. To recognize the types of reflection, changes in the shapes of the reflected wave, Mach stem and contact surface are taken into account. The boundaries and domains of existence for various types of reflection are reported. New names for some types of reflection are proposed.     

凝聚介质中斜激波的反射

结合数值模拟和理论分析,研究凝聚介质中斜激波反射.采用龙格-库塔控制体积间断有限元方法,数值求解"刚性气体"状态方程形式下的欧拉方程组;理论分析凝聚介质中斜激波反射模式;运用激波极曲线理论,给出典型激波强度下正规反射向非正规反射过渡的临界角及波后状态.比较数值模拟结果和激波极曲线理论分析结果,得到典型弱、强斜激波的反射图像.