Resonance compression of an acoustic beam in a crystal

The resonance excitation of an intense acoustic beam in a crystal is described for a special geometry of pump-wave reflection from the crystal surface. The resonance appears in the vicinity of the total internal reflection angle under the condition that the wave field in a compressed reflected beam propagating almost parallel to the surface is close to the volume eigenmode satisfying the free boundary condition. Criteria for the existence of such modes are considered in detail. Conversion conditions are analyzed under which a “parasitic” reflected wave of the same branch as the incident wave is absent and entire energy from the incident wave falls within a narrow intense acoustic beam of another branch. It is shown that, when the surface is chosen parallel to the crystal symmetry plane, the conversion criterion is reduced to the sole condition on the elastic moduli of the medium. Analysis is performed by analytic and numerical methods for skew cuts of monoclinic, rhombic, trigonal, and hexagonal crystals, when the boundary is the symmetry plane, while the sagittal plane has no symmetry. A number of crystals are found in which resonance excitation is very close to conversion.     

X‐ray third‐order nonlinear plane‐wave Bragg‐case dynamical diffraction effects in a perfect crystal

Two‐wave symmetric Bragg‐case dynamical diffraction of a plane X‐ray wave in a crystal with third‐order nonlinear response to the electric field is considered theoretically. For certain diffraction conditions for a non‐absorbing perfect semi‐infinite crystal in the total reflection region an analytical solution is found. For the width and for the center of the total reflection region expressions on the intensity of the incidence wave are established. It is shown that in the nonlinear case the total reflection region exists below a maximal intensity of the incidence wave. With increasing intensity of the incidence wave the total reflection region's center moves to low angles and the width decreases. Using numerical calculations for an absorbing semi‐infinite crystal, the behavior of the reflected wave as a function of the intensity of the incidence wave and of the deviation parameter from the Bragg condition is analyzed. The results of numerical calculations are compared with the obtained analytical solution.     

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.     

Broadband resonant cavity inside a two-dimensional sonic crystal

A rectangular cavity inside a two-dimensional sonic crystal was theoretically and experimentally characterized by examining its response to a cylindrical source emitting narrow-band filtered noise bursts with central frequencies ranging from 2 to 12 kHz. A broadband intensity resonance was observed for frequencies within the full band-gap region of the sonic crystal (5.5–6.5 kHz). Unlike ordinary resonances, this broadband resonance depends on the reflection properties of the sonic crystal forming the surrounding walls rather than on the geometry of the cavity.     

Resonance excitation of polaritons and plasmons at the interface between a uniaxial crystal and a metal

A resonance is theoretically predicted in which a plane electromagnetic wave in a transparent optically uniaxial crystal, reflected from its metallized surface, creates a wave field at the interface whose intensity is considerably higher than the intensity of the incident wave. In the crystal, a high-intensity (bulk or surface) polariton is excited, whereas, in the metal, a large-amplitude localized plasmon is excited. The structure of the fields created in the crystal is close to that of the fields of bulk polaritons at the boundary with a perfectly conducting surface. The nonideality of the metal is taken into account in the Leontovich impedance approximation. Conditions are found under which the resonance is accompanied by a full transformation of the incident wave into a polariton-plasmon localized near the interface. This coupled wave can be considered as a pumped eigenmode. The intensity of the wave field localized at the boundary can amount to 10–15 times the intensity of the incident wave in the visible range. The resonance half-width with respect to the angles of incidence amounts to a few degrees. In the infrared range, the excitation factor can be an order of magnitude higher, while the resonance half-width sharply decreases down to about 0.1° for a wavelength of 5 μm. Conditions are obtained for the resonance excitation of high-intensity bulk polaritons that arise as reflected modes propagating at a small angle to the boundary. The phenomena investigated are completely attributed to the anisotropy of the crystal.     

Resonance switching in a large sonic crystal cavity

In this work we present an experimental realization of resonance switching inside a large, relative to the lattice constant, sonic crystal (SC) cavity. The resonances arise from a temporary trapping of the acoustical energy between two sonic crystal slabs when the frequency of the wave falls within a partial band-gap region (thus, the propagation inside the sonic crystal is inhibited for normal incidence and a total reflection occurs). We show that by modifying the geometry of the sonic crystal lattice, from square to centered rectangular, it is possible to switch some resonances on and off, for a certain frequency region corresponding to the second band-gap for normal incidence. We also study the generalization of this phenomenon for the same lattice and band-gap region, as a function of the length of the cavity and the filling fraction the lattice.     

Diffraction of X-ray spatially inhomogeneous beam in a crystal with cubic nonlinearity

Diffraction of a spatially inhomogeneous X-ray wave was theoretically studied in a crystal with cubic nonlinear response to the strength of an external electric field. Using numerical calculations in case of two-beam diffraction of a narrow incident beam, the intensity distributions on the output surface of the crystal was investigated depending on the thickness and intensity of the incident beam. The results of numerical calculations of integral (spatial) coefficients of transmission and reflection are given as functions of incident wave intensity for fixed thickness of the crystal.     

X-ray diffraction in AT-cut quartz single crystal at acoustic excitation at the fundamental frequency

X-ray diffraction on different atomic planes of an AT-cut quartz crystal is studied experimentally in the Laue geometry in case of excitation by acoustic waves at the first resonant (fundamental) frequency. Acoustic waves lead to an increase in the integral intensity of the reflection-diffracted beam. The amplification coefficients in reflection are measured in dependence on the amplitude of a.c. voltage applied to the crystal at the resonant frequency. The frontal distributions of the intensity of the beam diffracted in the reflection direction are obtained for different atomic planes.     

固-固无限周期声子晶体中SH波全反射隧穿的谐振理论

利用边界条件推导出SH波在一维固-固声子晶体中的转移矩阵, 得出一维固-固无限周期声子晶体中SH波的色散关系. 通过建立一维固-固无限周期声子晶体的谐振腔模型并利 用共振条件得出SH波的全反射隧穿的波长公式. 利用波长公式研究了SH波全反射隧穿的波长变化规律, 其结果与色散法一致. 解释了一维固-固无限周期声子晶体中SH波的全反射隧穿效应产生的原因. 关键词： 声子晶体 SH波 全反射隧穿 谐振理论     

Neutron backscattering on vibrating silicon crystals-experimental results on the neutron backscattering spectrometer IN10

Neutron backscattering on vibrating silicon crystals has been measured in transmission geometry on the backscattering spectrometer IN10 at the Institute Laue-Langevin (ILL). The crystals were excited into longitudinal mechanical resonance vibrations by LiNbO₃ transducers glued to the crystal surface. The transmitted intensity of the 111 Bragg reflection was measured as a function of the deformation amplitude at vibration frequencies from 2.5 MHz to 30 MHz. By vibrating the initially perfect silicon crystal, an increase in E/E of the diffracted profile was achieved, accompanied by an increase of the backreflected intensity. The width of the reflection and the resolution is a linear function of the ultrasonic strain over a wide range of excitation strengths. E/E can be easily tuned in a reproducible way by varying the voltage on the piezoelectric transducer. For the highest excited ultrasonic strain an increase of the backscattered intensity of about a factor of 50 is observed. Sound wave amplitudes can be determined from the measured width in E/E of the transmission curves. The obtained results are relevant for the design and operation of intensity-to-resolution tunable monochromators for high resolution instruments.     

Theory of light diffraction in thick anisotropic reflection holographic gratings

Diffraction of light in thick anisotropic holographic media is considered. Solutions for the wave amplitude, diffraction efficiency, and angular mismatch are given in the reflection geometry for the case of refractive index modulation. A comparison with Kogelnik’s isotropic theory is made.     

Effect of the Temperature Gradient on the X-ray Diffraction Spectrum of a Quartz Crystal

The spectra of X-ray diffraction from the reflecting atomic plane (1 0 1? 1) of a quartz single crystal are studied in Laue geometry under the action of temperature gradient on a BDER-KI-11K spectrometer with a resolution of 300 eV on the Am241 line of 17.74 keV. The temperature gradient leads to an increase in the intensity of the diffracted beam depending on the heating temperature. It is shown that the intensity of X-ray diffraction in Laue geometry may increase at a temperature gradient of 250°C/cm by two orders of magnitude in comparison with the uniform temperature state of the crystal. The rocking curve of the reflected beam is obtained at a fixed observation angle of 6° and a specified temperature gradient. It is demonstrated that the intensity of the reflected beam increases with increasing temperature gradient (to a certain value), while the spectral width of the reflection line remains constant and is governed by the energy resolution of the spectrometer. A further growth in the temperature gradient leads to an increase in the spectral width of the reflection line with decreasing intensity of the reflected beam.     

Backscattering of light by a finite two-dimensional crystal plate in the region of exciton resonance: Antispecular reflection

It is shown that scattering of a plane monochromatic wave by a finite two-dimensional crystal (quantum well or Langmuir-Blodgett film) in the region of exciton resonance shows, along with the peak of specular reflection, a peak of comparable magnitude corresponding to backward scattering propagating toward the incident beam (antispecular reflection). The effect is related to reflection of the exciton by the boundary of the two-dimensional crystal. The solution of the scattering problem for a simple model system with a Frenkel exciton, demonstrating this phenomenon, is given.     

Rocking Curves of X-Ray Asymmetric Bragg Diffraction with Two-Dimensional Curvature of Wave Front

The X-ray asymmetric Bragg diffraction in a perfect semi-infinite crystal with plane entrance surface is considered taking into account the two-dimensional curvature of the wave front of an incident wave. An expression for reflection coefficient using the Green function is obtained in the approximation of locally plane wave and the rocking curves are investigated as functions of angular departure from the selected exact Bragg direction in the diffraction plane and in the direction perpendicular to the diffraction plane. The shape, position and dimensions of total reflection region of rocking curves are studied depending on the degree of asymmetry of diffraction geometry. The requirements to the spatial and temporal coherence for obtaining the rocking curves are estimated.     

Numerical analysis of the reflection and absorption of electromagnetic wave in nonuniform magnetized plasma slab

In this paper, a model for calculating the reflection and absorption powers of electromagnetic wave (EM wave) in nonuniform magnetized plasma slab is given out based on layer propagation theory. The effects of various plasma parameters and different values of magnetic field intensity on the reflected and absorbed powers are discussed. The results illustrate that the thickness of plasma seldom affects the reflection of radar wave, but it can broaden or reduce the absorption width. Meanwhile, the background magnetic field intensity has an influence upon the results, and it could change the resonance spectrum of magnetized plasma. We also find out that, with appropriate plasma density, collision frequency and magnetic field intensity, more than 90% of radar wave power can be absorbed and the resonant absorption band is about 2 GHz.     

Propagation of fast leaky surface waves in a system of metal electrodes on a lithium tetraborate crystal

Dispersion curves are calculated for fast leaky surface acoustic waves in a periodic system of metal electrodes on a lithium tetraborate crystal. The periodic Green’s function analysis is applied to the piezoelectric halfspace. To allow for the mechanical properties of the electrodes, a perturbation method is developed that is accurate to within the first order of the electrode thickness-to-period ratio, and the finite-element method is used to investigate the higher-order effects. The reflection factor of a system of two open electrodes is shown to have a minimum at a certain electrode thickness. The fast leaky-to-Rayleigh wave conversion factor and the Rayleigh wave reflection factor are studied as functions of the electrode thickness.     

Electrooptic reflection with surface plasmons

Electrooptic reflection with surface plasmons is studied both experimentally and theoretically. An attenuated total reflection (ATR) geometry is used in which a quartz electrooptic crystal onto which a thin Ag film has been evaporated is contacted to the hypotenuse face of a high-index prism. Modulation in the ATR of an He–Ne beam near the angle for surface plasmon excitation is observed when a low-frequency electric field is applied to the quartz crystal. Experiments have been done with both single and multiple boundary surface waves. The angular spectra show phase reversal structure due to the interference of the modulated reflected wave with the ATR wave. The modulated reflectance is calculated directly from a well-known solution to the boundary value problem in nonlinear optics. Good quantitative agreement with the observations is obtained.     

Reflection of elastic waves in a crystal of Heusler alloy Ni<Subscript>2</Subscript>MnGa in the phase transition range

The reflection of longitudinal and transverse acoustic waves from the free surface of the ferromagnetic shape memory alloy Ni₂MnGa that is located in the ranges of the premartensite and martensite phase transformations is considered. The propagation directions and amplitudes of the waves reflected in the (001) plane of the crystal are determined. They acquire the character of substantially quasi-longitudinal and quasi-transverse vibrations rather than being pure modes. The angles of wave reflection and conversion are shown to be effectively controlled by temperature and a magnetic field due to the colossal acoustic anisotropy of the crystal over the wide range of its phase transitions. Beginning from a certain critical angle of incidence of a quasi-transverse wave, the quasi-longitudinal wave having appeared upon reflection becomes an accompanying surface vibration, and it can be emitted into the bulk of the crystal when the phase transition point is approached. Two angles of full conversion of an incident quasi-longitudinal wave into a quasi-transverse wave are established, and their temperature dependences are found. Trivisonno’s experimental data for the ultrasound velocity and absorption in an Ni₂MnGa crystal are used to numerically estimate these acoustic effects.     

Intense terahertz emission from undoped GaAs/n-type GaAs and?InAs/AlSb structures grown on Si substrates in the transmission-geometry excitation

Intense terahertz radiation was generated from femtosecond laser-irradiated InAs and GaAs layers on Si substrates. Results show that InAs/Si and GaAs/Si films can be excited in reflection and transmission geometries. The InAs/Si film exhibited weaker emission for both excitation cases but it will be more feasible as a spectroscopic THz source due to the absence of complex spectral features in its emission spectrum. The GaAs/Si emission is characterized by Fabry?CPerot oscillations but it is 90% of that of p-InAs bulk crystal emission intensity in the reflection geometry. Excitation fluence measurements showed that the InAs/Si film saturates easily due to the laser??s shallow penetration depth in InAs.     

Material property estimates from ultrasound attenuation in fibre suspensions

An investigation of a new method for measuring fibre material properties from ultrasonic attenuation in a dilute suspension of synthetic fibres of uniform geometry is presented. The method is based on inversely solving an ultrasound scattering and absorption model of suspended fibres in water for the material properties of the fibres. Experimental results were obtained from three suspensions of nylon 66 fibres each with different fibre diameters. A forward solution to the model with reference material values is compared to experimental data to verify the model’s behaviour. Estimates of the shear and Young’s modulus, the compressional wave velocity, Poisson’s ratio and loss tangent from nylon 66 fibres are compared to data available from other sources. Experimental data confirms that the model successfully predicts that the resonance features in the frequency response of the attenuation are a function of diameter. Consistent estimated values for the compressional wave velocity and the Poisson’s ratio were found to be difficult to obtain but in combination gave values of shear modulus within previously reported values and with low sensitivity to noise. Young’s modulus was underestimated by 54% but was consistent and had low sensitivity to noise. The underestimation is believed to be caused by the assumption of isotropic material used in the model. Additional tests on isotropic fibre would confirm this. Further analysis of the model sensitivity and the reasons for the resonance features are required.