Spatial modulation of diffracted X-ray beam by bulk acoustic waves

Laue-geometry X-ray diffraction has been studied experimentally in quartz crystals excited by bulk acoustic waves. These waves deform reflecting atomic planes, which leads to spatial modulation of diffracted beams. We show that in the beam section perpendicular to the diffraction vector the intensity stripes appear whose number depends on the order of harmonic of the excited acoustic field. The intensity of appearing fringes depends on the amplitude of alternative voltage applied to the crystal.     

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.     

X-ray Bragg diffraction in asymmetric backscattering geometry

We observe three effects in the Bragg diffraction of x rays in backscattering geometry from asymmetrically cut crystals. First, exact Bragg backscattering takes place not at normal incidence to the reflecting atomic planes. Second, a well-collimated (approximately 1 microrad) beam is transformed after the Bragg reflection into a strongly divergent beam (230 microrad) with reflection angle dependent on x-ray wavelength--an effect of angular dispersion. The asymmetrically cut crystal thus behaves like an optical prism, dispersing an incident collimated polychromatic beam. The dispersion rate is approximately 8.5 mrad/eV. Third, parasitic Bragg reflections accompanying Bragg backreflection are suppressed. These effects offer a radically new means for monochromatization of x rays not limited by the intrinsic width of the Bragg reflection.     

Angle-dependent infrared absorption spectroscopy at electrode/electrolyte interfaces

Angle-dependent internal reflection spectroscopy is performed in the attenuated total reflection setup for an electrochemical cell with a Fourier transform infrared spectrometer. The working electrode is a thin Pt film evaporated onto a hemispherical Si prism. The refractive index of the Pt film obtained from the experiment is found to differ from the value for bulk material. The difference is ascribed to the surface corrugation of the Pt surface and the film thickness in the nanometer range. The function of reflection intensity versus angle of incidence changes significantly when a resonant absorption occurs in the electrolyte medium. The angle-dependent absorption band intensity of CO adsorbed on the Pt film under potential control reveals changes in magnitude and an inversion of the band for different angles of incidence. This behaviour is explained by the excitation of resonant surface plasmon waves at the Pt/electrolyte interface and by multiple reflections occurring at the interfaces. A simulation for the three-layer system Si/Pt/electrolyte agrees with the experimental results.     

Study of X-ray diffraction processes in ADP crystals under the influence of temperature gradient

The influence of temperature gradient on the intensity of X-ray beam reflected from atomic planes of antiferroelectric crystal of ammonium dihydro-phosphate (ADP) was studied. It was found that under the influence of temperature gradient on ADP crystal, the intensity of diffracted X-ray beam initially decreases at small values of gradient and then monotonously increases with increasing gradient. It is assumed that in the initial period of the action of temperature gradient the crystal domains have no time to orient in the same direction, and the intensity of beam decreases owing to the scattering of X-rays in different directions when reflected from the walls of boundaries of domains. After the alignment of domains, their arrangement in the same direction, separate areas of unidirectional domains are formed under the action of temperature gradient and the intensity of diffracted X-ray beam increases, as confirm the experimental data. The specified mechanism is supposed to occur also in other crystals having the domain structure.     

First observation of multiple volume reflection by different planes in one bent silicon crystal for high-energy protons

Multiple volume reflection by different planes in a bent silicon crystal with its 〈111〉 axis orientation close to the beam direction was observed for the first time for 400 GeV/c protons at the CERN SPS. The proton beam was deflected to the side opposite to the crystal bend by an angle of about 67 μrad, which is five times larger than in a single volume reflection by the (110) bent planes. The registered efficiency of one side deflection was about 84%. It was shown that multiple volume reflection transforms to a single volume reflection when the orientation angle of the 〈111〉 axis relative to the beam direction is increased.     

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.     

Spatial modulation of X-rays in the presence of external acoustic field

The two-crystal X-ray diffraction from (111?1) reflecting atomic planes of quartz single crystal with AT-cut was experimentally considered in the Laue geometry, when both the crystals are simultaneously in the field of bulk acoustic waves. It is shown that as a result of spatial modulation of reflected beam an odd number of parallel X-ray beams is obtained.     

Volume reflection of a proton beam in a bent crystal

Volume reflection predicted in the mid-1980s by Taratin and Vorobiev has been observed for the first time in the interactions of a 70 GeV proton beam with a short bent crystal. Incident protons deviate from convex atomic planes in the bulk of the crystal as a result of coherent interaction with bent lattice around the tangency point of particle trajectory with a curved atomic plane. The deflection angle 2theta(R) was found to be (39.5+/-2.0) microrad, or (1.65+/-0.08)theta(c) in terms of the critical angle for channeling. The process has a large probability with respect to channeling and takes place in the angular range equal to the bend angle of atomic planes. It could possibly open new fields of application of crystals in high-energy particle beam optics.     

均匀磁化等离子体与雷达波相互作用的数值分析

采用平板几何对不同磁场强度下等离子体对电磁波的吸收、反射和透射特性进行了数值分析.结果表明,不同磁场可以显著改变等离子体对不同频率的雷达波的吸收和反射特性.当雷达波频率接近等离子体高混杂频率时,磁化等离子体将对该波产生强的共振吸收,带宽在2GHz左右,吸收比可达90%以上.因此,通过适当调整磁场强度、等离子体密度和碰撞频率,可实现较宽雷达波段的等离子体隐身 关键词： 电磁波 磁化等离子体 共振吸收 等离子体隐身     

Phase conjugation under four-wave mixing on resonant and thermal nonlinearities at relatively high reflection coefficients

The dependences of the reflection (transmission) coefficients and the half-width of the spatial frequency band on the pump-wave intensity are obtained for an object (signal) wave in degenerate and quasi-degenerate four-wave mixers on resonant and thermal nonlinearities. The effect of the difference of the pump-wave frequencies on the phase-conjugation quality is studied at relatively high reflection coefficients.     

Volume reflection of 1-GeV protons by a bent silicon crystal

The volume reflection of 1-GeV protons by a bent crystal has been observed. The crystal is made of single crystal silicon. The (111) atomic planes are bent owing to the elastic quasimosaicity effect, which makes it possible to reduce the crystal length for a beam to 30 μm. It is found that the probability of the reflection effect is higher than the probability of the channeling effect (0.71 vs. 0.63), and the deflection angle of the protons reflected inside the crystal is equal to 1.39 ± 0.04 in terms of the critical angle for channeling under the conditions of the experiment (170 μrad). The width of the reflected peak is equal to 1.76 ± 0.04 in the same units. The protons that are not involved in channeling at the angular position of maximum channeling undergo volume reflection and are deflected in the direction opposite to the channeled beam by the angle 1.01 ± 0.05 in terms of the critical angle for channeling. The width of the reflected peak is equal to 1.94 ± 0.08 in the same units. Original Russian Text ? Yu.M. Ivanov, N.F. Bondar’, Yu.A. Gavrikov, A.S. Denisov, A.V. Zhelamkov, V.G. Ivochkin, S.V. Kos’yanenko, L.P. Lapina, A.A. Petrunin, V.V. Skorobogatov, V.M. Suvorov, A.I. Shchetkovsky, A.M. Taratin, W. Scandale, 2006, published in Pis’ma v Zhurnal éksperimental’noĭ i Teoreticheskoĭ Fiziki, 2006, Vol. 84, No. 7, pp. 445–450.     

Controlling the intensity and fluctuations of light upon its selective reflection

Selective reflection of a semiconductor laser beam by a resonant gas is studied. The experiments were performed under conditions where the laser frequency is determined by the laser cavity mode structure. In this case, characteristic steps—intensity quasi-levels—arise on the reflectivity resonances. It is shown that the height of the quasi-levels depends on the degree of saturation of the resonance reflection by the laser field. Low-noise reflectivity regimes with different sensitivities to external signals are found on these quasi-levels.     

X-ray diffraction in KDP crystal in the presence of temperature gradient

We investigate the effect of temperature gradient on the intensity of X-ray radiation reflected from different atomic planes of a KDP crystal, depending on the crystal thickness. We have studied the phenomenon of transfer of the X-ray beam intensity for x- and z-cuts of the KDP crystal in the presence and in the absence of temperature gradient. It is shown that the intensities of reflected radiations do not saturate because in conditions of the experiment the further increase in temperature gradient leads to destruction of the sample. It is revealed that in some cases at small values of temperature gradient the intensity of reflected radiation decreases initially by more than 10%, then increases monotonically with the increase in temperature gradient.     

Directive metamaterial-based subwavelength resonant cavity antennas – Applications for beam steering

This article presents the use of composite resonant metamaterials for the design of highly directive subwavelength cavity antennas. These metamaterials, composed of planar metallic patterns periodically organized on dielectric substrates, exhibit frequency dispersive phase characteristics. Different models of metamaterial-based surfaces (metasurfaces), introducing a zero degree reflection phase shift to incident waves, are firstly studied where the bandwidth and operation frequency are predicted. These surfaces are then applied in a resonant Fabry–Perot type cavity and a ray optics analysis is used to design different models of ultra-compact high-gain microstrip printed antennas. Another surface presenting a variable reflection phase by the use of a non-periodic metamaterial-based metallic strips array is designed for a passive low-profile steering beam antenna application. Finally, the incorporation of active electronic components on the metasurfaces, allowing an electronic control of the phase responses, is applied to an operation frequency reconfigurable cavity and a beam steering cavity. All these cavity antennas operate on subwavelength modes, the smallest cavity thickness being of the order of λ/60. To cite this article: A. Ourir et al., C. R. Physique 10 (2009).     

附加球面相位引致Airy光束在单轴晶体传输时的两次镜像演化

利用单轴晶体光束传输理论,求得了具有附加球面相位Airy光束在单轴晶体中的传输公式.数值模拟计算结果表明,线偏振附加球面相位Airy光束在晶体中传输时仍为线偏振,但不是传输不变的.粗略地讲,具有附加球面相位的Airy光束在晶体中传输时,近场是传输不变的;而在由晶体寻常与非寻常折射率和球面半径共同确定的两个特定传输距离处,传输光束转换成了取向不同的Gaussian-Airy光束,且高斯依赖的束宽度敏感地与截断因子相关;而当光束依次穿过此两位置时光斑花样先后相对于两横向轴平面做镜像演化,且镜像演化顺序也与晶体寻常和非寻常折射率相对大小密切相关,其总的效果是远场强度花样能恢复原样但花样取向产生了关于对过横平面二、四象限平分平面的镜像演化.这些结果表明,通过恰当选择晶体材料(即折射率)和附加球面相位的半径R,可以调控光束花样的形状、取向及表征各向异性材料的相关性质.     

Emission of Photons at the Interaction of a High-Energy Electron Beam with a Sequence of Bent Single Crystals

A radiation source based on the emission of electrons and positrons moving in a short bent crystal has been recently discovered. The emission of particles is due to oscillations of their trajectories near the point of reflections, where trajectories approach a tangent to bent atomic planes. In the experiment performed with the secondary electron beam of the U70 accelerator, it has been shown that the emission intensity can be increased by using a sequence of oriented bent crystals. Passing through six 2.5-mm-long silicon crystals, 7-GeV electrons lose on average 2.0 GeV on emission. This value is several times larger than that in an amorphous medium. Thus, an intense source of radiation has been demonstrated with prospects of application at accelerators.     

The cumulative resonance in atomic ground state induced by modulated light field

The interaction of an atomic system with a light beam, intensity-modulated at the ground-state Zeeman frequency is analysed by the dressed-atom method. When the light field is resonant, this interaction produces a new type of degereracy of dressed-atom energy levels which is not of level crossing type. This gives rise to the cumulative resonance with a width depending on the light intensity.     

Control of absorption spectrum of a one-dimensional resonant photonic crystal

The crystal under consideration is a layered structure consisting of alternating layers of two materials, one of which is a resonantly absorbing gas. It is shown that the combination of the dispersion of an atomic gas with the dispersion of a photonic-bandgap structures allows one to efficiently control the transmission spectra of s- and p-polarized modes in these combined systems. It is found that the spectrum is highly sensitive to the position of the gas resonance frequency with respect to the bandgap edge and to a change in the gas pressure. The transmission, reflection, and absorption spectra of the resonant photonic crystal are studied at an angle of incidence equal to the Brewster angle of a seed photonic crystal. Possible applications of the found particular features of the dispersion of resonant photonic crystals are discussed.     

Travelling Wave Window with Matching Gratings

At millimeter waves, travelling wave dielectric windows are advantageous over commonly used resonant ones due to the higher power capability. An attractive method to compensate the wave reflections from vacuum-dielectric and dielectric-vacuum boundaries is using anti-mirror reflection gratings (Fig. 1) [1]. In this paper, the power loss due to the finite cross-section of the wave beam and the frequency mismatch relative to the resonant value are calulated for the latter type of window.