Coupling of Acoustical Plane p-Wave to a Cased Borehole

Theoretical and numerical study on the coupling acoustic field of the plane p-wave to a cased borehole is carried out. The medium outside the cased borehole is modeled as the porous medium. The scattering field characteristics in the cased borehole are investigated when a plane fast p-wave is incident in tilt to the cased borehole from the porous medium. The scattering fields inside and outside the cased borehole are analyzed and deduced by Biot＇s theory under the boundary conditions on each interface, and they are numerically studied. It is found that the scattering field has strong resonant characteristics and there exists a series of resonant frequencies and peaks. The effects of the frequency, radii of each interface, incident angle, porosity, and other parameters on the resonant acoustic field have been investigated in detail in the fast and slow formations respectively. The resonant characteristics of the scattering field are also analyzed from the physical sense.     

Resonant photoemission study ofK-derived valence-band states in K X C60

The electronic structure of K-doped C₆₀ was investigated by photoemission (PE) and X-ray absorption near-edge structure (XANES) studies at the C-1s and K-2p thresholds. In addition, information on the local K-derived partial density of states in superconducting K₃C₆₀ was obtained by resonant PE at the K-2p _1/2 threshold. The experimental observations support a complete charge transfer from K to C₆₀ and we clearly observe a finite density of states atE _F . From resonant PE, occupied states with K-p, d character could be identified in the binding-energy region from 1.5 to 8 eV below, but not directly at the Fermi level. This partial-density-of-states structure agrees well with the results of our band-structure calculations based on the local-density approximation.     

Femtosecond surface plasmon resonance dynamics and electron-electron interactions in silver nanoparticles

Femtosecond excitation and relaxation of nonequilibrium electrons are investigated in silver clusters using a two color pump-probe technique with resonant excitation of the surface plasmon resonance and off resonant probing. The excitation process is shown to be identical to that in metal films, and permits creation of a strongly athermal single electron excitation in a time scale shorter than the duration of the pulses (25-30 fs), in agreement with the free-electron absorption model. Following the time evolution of the nonequilibrium distribution yields information on the internal energy redistribution dynamics of the conduction electrons and of its modification by confinement in metal clusters. Received 1st December 2000     

Nonlocal effects in the optical response of composite materials with metallic nanoparticles

A phenomenological model for the optical response of composite materials with metallic nanoparticles is presented. This model applies the conventional effective medium theories (EMT) but takes into account the spatial dispersion effects in the dielectric response of the metallic nanoparticles. This leads to an EMT that depends on the size of the particles. Numerical results from a model computation shows that this effect due to the nonlocal optical response of the nanoparticles can increase the resonant absorption frequency of the composite significantly for particles of very small sizes; and can lead to resonant absorption even in the Bruggeman symmetric EMT—a feature which is believed to be absent in the conventional treatment where local response for the metal particles has been assumed.     

Polarization Properties of Quantum Cyclotron Radiation

We present a simplified formulae system describing the polarization properties of the quantum cyclotron radiation by a nonrelativistic thermal electron in a very strong magnetic field. In this system, each of the derived quantities, including the Einstein coefficients, the absorption cross sections, and scattering cross sections, as well as the absorption coefficients by plasma, is divided into the perpendicularly and parallelly polarized components. The results, especially the absorption and scattering cross sections of resonant frequency photons, are potentially important in x-ray and γ-ray astronomy, particularly in the study of gamma-ray bursts and pulsars.     

Tri-component phononic crystals for underwater anechoic coatings

Localized resonance in phononic crystal, composed of three-dimensional arrays of composite units, has been discovered recently. The composite unit is a high-density sphere coated by soft silicon rubber. In this Letter, the absorptive properties induced by the localized resonance are systemically investigated. The mode conversions during the Mie scattering of a single coated lead sphere in unbounded epoxy are analyzed by referring the elements of the scattering matrix. Then the anechoic properties of a slab containing a plane of such composite scatterers are investigated with the multiple-scattering method by accounting the effects of the multiple scattering and the viscous dissipation. The results show that the longitudinal to transverse mode conversion nearby the locally resonant region is an effective way to enhance the anechoic performance of the finite slab of phononic crystal. Then, the influences of the viscoelasticity of the silicon rubber and the coating thickness on the acoustic properties of the finite slab are investigated for anechoic optimization. Finally, we synthetically consider the destructive scattering in the finite slab of phononic crystal and the backing, and design an anechoic slab composed of bi-layer coated spheres. The results show that the most of the incident energy is absorbed at the desired frequency band.     

Microwave solitons in molecular magnets via electromagnetically induced transparency

Using the Schrödinger-Maxwell equations, we investigate the formation of microwave solitons in a crystal of molecular magnets. This system is subjected to one dc magnetic field and two (probe and coupling) ac resonant magnetic fields. The results show that the probe magnetic field can freely propagate in a crystal of molecular magnets due to quantum interference. Furthermore, within certain parameter range, both bright and dark microwave solitons can occur in such a highly resonant medium. We also obtain the analytical expressions for the phase shift and absorption coefficient of the probe magnetic field.     

Nonlinear optical behaviors in a silver nanoparticle array at different wavelengths

The optical nonlinearities of an Ag nanoparticle array are investigated by performing Z-scan measurements at the selected wavelengths (400, 600, 650, and 800 nm). The nonlinear refraction index in the resonant region (around 400 nm) exhibits a significant enhancement by two orders compared with that in the off-resonant region (around 800 nm)), and exhibits an sign alternation of the resonant nonlinear absorption, which results in a negligible nonlinear absorption at a certain excitation intensity. Moreover, a low degree of nonlinear absorption was measured at the edges of the resonant region (600 and 650 nm), which is attributed to the competition of the saturated absorption and the two-photon absorption processes.     

Nonlinear optical behaviours in a silver nanoparticle array at different wavelengths

The optical nonlinearities of an Ag nanoparticle array are investigated by performing the Z-scan measurements at the selected wavelengths (400, 600, 650, and 800 nm). The nonlinear refraction index in the resonant region (around 400 nm) exhibits a significant enhancement by two orders compared with that in the off-resonant region (around 800 nm)), and exhibits an sign alternation of the resonant nonlinear absorption, which results in a negligible nonlinear absorption at a certain excitation intensity. Moreover, a low degree of nonlinear absorption was measured at the edges of the resonant region (600 and 650 nm), which is attributed to the competition of the saturated absorption and the two-photon absorption processes.     

Dynamics of a three-level atom in a resonant light field

The paper is devoted to a consideration of the motion of a three-level atom in two resonant light waves. A kinetic equation of the Fokker-Planck type for the atomic distribution function is derived, which is valid when the recoil energy is small compared to the linewidths of the resonant transitions. The detailed behaviour of the radiation force and the diffusion tensor are studied numerically. The case of exact resonance and the nonresonant case are both considered. It is shown that a detuning from exact resonance results in a drastic decrease of the resonant light pressure force. For the detuning we determine the condition, under which an efficient action of the light pressure on a three-level atom takes place.     

Acoustic anechoic layers with singly periodic array of scatterers: Computational methods,absorption mechanisms,and optimal design

The acoustic properties of anechoic layers with a singly periodic array of cylindrical scatterers are investigated. A method combined plane wave expansion and finite element analysis is extended for out-of-plane incidence. The reflection characteristics of the anechoic layers with cavities and locally resonant scatterers are discussed. The backing is a steel plate followed by an air half space. Under this approximate zero transmission backing condition, the reflection reduction is induced by the absorption enhancement. The absorption mechanism is explained by the scattering/absorption cross section of the isolated scatterer. Three types of resonant modes which can induce efficient absorption are revealed. Due to the fact that the frequencies of the resonant modes are related to the size of the scatterers, anechoic layers with scatterers of mixed size can broaden the absorption band. A genetic optimization algorithm is adopted to design the anechoic layer with scatterers of mixed size at a desired frequency band from 2 kHz to l0 kHz for normal incidence, and the influence of the incident angle is also discussed.     

Optical absorption in asymmetric double quantum wells driven by two intense terahertz fields

Optical absorption is investigated for asymmetric double quantum wells driven by a resonant terahertz field and a varied terahertz field both polarized along the growth direction. Rich nonlinear dynamics of the replica peak and the Autler-Townes splitting of various dressed states are systematically studied in undoped asymmetric double quantum wells by taking account of multiple factors, such as the frequency of the varied terahertz field and the strength of the resonant terahertz field. Each electron subband splits into two dressed states when the resonant terahertz field is applied in the absence of the varied terahertz field, the optical absorption spectrum shows the first order Autler-Townes splitting of the electron subbands. When a varied terahertz field is added into the resonant system, the replica peak and the second order Autler-Townes splitting of the dressed states near the band edge respectively emerge when the varied terahertz field is non-resonant and resonant with these dressed states. When the strength of the resonant terahertz field is increased, the first order Autler-Townes double peaks and the replica peak in the optical absorption spectrum shift with the shifts of the dressed states. The presented results have potential applications in electro-optical devices.     

A Wide Band Strong Acoustic Absorption in a Locally Network Anechoic Coating

Composite materials with interpenetrating network structures usually exhibit unexpected merit due to the cooperative interaction. Locally resonant phononic crystals （LRPC） exhibit excellent sound attenuation performance based on a periodical arrangement of sound wave scatters. Inspired by the interpenetrating network structure and the LRPC concept, we develop a locally network anechoic coating （LNAC） that can achieve a wide band of underwater strong acoustic absorption. The experimental results show that the LNAC possesses an excellent underwater acoustic absorbing capacity in a wide frequency range. Moreover, in order to investigate the impact of the interpenetrating network structure, we fabricate a faultage structure sample and the network is disconnected by hard polyurethane （PU）. The experimental comparison between the LNAC and the fanltage structure sample shows that the interpenetrating network structure of the LNAC plays an important role in achieving a wide band strong acoustic absorption.     

Interaction of a supersonic beam of toluene with a resonant RF electric field

Deflection of a cold supersonic toluene beam seeded in He has been observed when these molecules interact with both a static and a resonant oscillating electric field. The toluene beam splits into two beams each one peaking at a deflection angle of 1 degree towards the positive and negative direction of the Stark field when the employed resonant frequency between the two Stark levels of the toluene molecule is 1411 kHz. This deflection angle is about four orders of magnitude higher than the value one would expect from the toluene dipole moment and the employed RF field gradient. Different hypothesis are suggested to explain the observed strong beam splitting including the possibility of transverse beam interferences induced by both the resonant RF field and the transverse uniform electric field. A theoretical model is presented based on molecular beam interferences induced by the resonant RF field which seems to account satisfactorily for the present observations.     

Dimensional resonances of elastic and magnetoelastic vibrations in two layered structure

Peculiarities of dimensional resonances of elastic and magnetoelastic waves in bi-layered insulator structure: ferromagnetic film–non-magnetic elastic substrate have been investigated. Dependences of resonant frequencies of vibration modes upon thicknesses of magnetic and non-magnetic layers, elastic and magnetoelastic parameters, applied magnetic field have been calculated. The presence of peculiarities of resonant frequencies harmonics behavior with change of magnetic layer thickness have been shown.     

HARMONIC EMSSIONS FROM LASER-PRODUCED PLASMAS

Haimonic veneration as a result of resonant absorption in laser-roduced plasmas is investigated by considering the steepened plasma density gradients in the vicinity of the critical density. We obtained an efficiency of the second harmonic emission which is in better agreement with experiments than previous theories. The present calculation has also been extended to including the efficiency of the third harmonic emission, the results obtained indicate that at sufficiently high laser intensities, the third harmonic emission generated by the resonant absorption can also be observed.     

一种含圆柱形谐振散射体的黏弹材料低频吸声机理研究

利用层多重散射法分析了一种含圆柱形谐振散射体黏弹 材料吸声层在钢背衬条件下的低频吸声特性. 该吸声层由包覆软材料的圆柱空腔周期嵌入橡胶材料中构成, 其中, 散射体轴线与吸声层平行. 结果表明, 20 mm厚吸声层在1000–3000 Hz具有良好的吸声性能. 通过综合分析单个散射体的吸收截面、单层周期散射体的声吸收、 结构内部位移场以及耗散功率密度分布, 揭示了该吸声层的低频吸声机理. 关键词： 水声吸声 吸收截面 耗散功率密度     

A tunable infrared plasmonic polarization filter with asymmetrical cross resonator

A tunable infrared plasmonic polarization filter is proposed and investigated in this paper. The filter is based on the sandwich absorption structure which consists of three layers. The top layer is an array of asymmetrical cross resonator.The middle and bottom layers are dielectric spacer and metal film respectively. By absorbing specific wavelength of the incident light perfectly, the reflection spectrum of the structure shows filter performance. The calculated results show that the absorption wavelength is strongly dependent on the length of branch of the asymmetrical cross resonator which is parallel to the light polarization and independent of the length of the vertical one. Therefore, the asymmetrical cross resonator filter structure opens the way for freely tuning the filtering wavelength for a different light polarization. We can fix a resonant wavelength(absorption wavelength) corresponding to one polarization and change the resonant wavelength for the other polarization by adjusting the corresponding branch length of the asymmetrical cross resonator, or change the two resonant wavelengths of both two polarizations at the same time.     

使用共振多程光声池的CO_2和SF_6的无多普勒饱和吸收光谱

本文使用共振多程池的光声探测法对CO_2和SF_6分子的无多普勒饱和吸收光谱进行了实验研究.测量CO_2的吸收凹陷约为2.5MHz,实验中观察到5个SF_6的吸收凹陷、凹陷深率约为1.9MHz.两种情况的分辨率均大于10~7.     

The electric field effects on intersubband optical absorption of Si δ-doped GaAs layer

The intersubband transitions in Si δ-doped GaAs structures is theoretically investigated for different applied electric fields. For an uniform distribution the electronic structure has been calculated by solving the Schrödinger and Poisson equations self-consistently. From our calculations, it is found that the subband energies and intersubband optical absorption is quite sensitive to the applied electric field. This gives a new degree of freedom in various device applications based on the intersubband transitions of electrons.