X‐ray focusing by the system of refractive lens(es) placed inside asymmetric channel‐cut crystals

An X‐ray one‐dimensionally focusing system, a refracting–diffracting lens (RDL), composed of Bragg double‐asymmetric‐reflecting two‐crystal plane parallel plates and a double‐concave cylindrical parabolic lens placed in the gap between the plates is described. It is shown that the focal length of the RDL is equal to the focal distance of the separate lens multiplied by the square of the asymmetry factor. One can obtain RDLs with different focal lengths for certain applications. Using the point‐source function of dynamic diffraction, as well as the Green function in a vacuum with parabolic approximation, an expression for the double‐diffracted beam amplitude for an arbitrary incident wave is presented. Focusing of the plane incident wave and imaging of a point source are studied. The cases of non‐absorptive and absorptive lenses are discussed. The intensity distribution in the focusing plane and on the focusing line, and its dependence on wavelength, deviation from the Bragg angle and magnification is studied. Geometrical optical considerations are also given. RDLs can be applied to focus radiation from both laboratory and synchrotron X‐ray sources, for X‐ray imaging of objects, and for obtaining high‐intensity beams. RDLs can also be applied in X‐ray astronomy.     

狭窄波束型高频引力辐射与电磁场的作用效应

从弱引力场的Einstein-Maxwell方程出发,讨论了晶体空间阵列的狭窄波束型高频引力辐射与电磁场的作用效应,并给出了扰动解。计算表明,在TT(Transvese Traceless)坐标系中,最优辐射方向的引力波束是纯十型极化的,并可使同频的电磁波产生倍频的扰动效应,使静态电磁场产生与时间成线性关系的累积扰动效应。对于任意方向上的引力辐射波束,在垂直于引力波矢的平面内仍然是纯十型极化的。在高频辐射和实验室典型尺度条件下,TT坐标系与Fermi坐标系的差异是可以忽略的。 关键词：     

Computer simulation of the zernike phase contrast in hard x-ray radiation using refractive lenses and zone plates

Computer simulation of an experiment on imaging transparent (phase) microobjects using the Zernike phase contrast method under hard X-ray radiation has been performed. The beam parameters typical for synchrotron radiation sources of the third generation were used in calculations. Both a refracting lens and a zone plate have been considered as a focusing element. The phase shifting quarter-wave plate is located at a spot of the point source image. The results of calculations have shown that the method can be successfully used for objects with the sizes greater than 0.1 μm along and 1 μm across the beam. It has been shown that the contrast is caused not only by increasing the intensity within the shadow of the objects, but also by decreasing the intensity in the area beyond the objects, which is necessary to retain the integral intensity.     

Joint moments of the wave beam amplitude and inverse power in an absorbing random medium with lens properties

This paper presents a derivation of a system of closed equations for joint moments of the amplitude and inverse power of a wave beam propagating in a regularly inhomogeneous dissipative random medium. The radiation transfer in the medium is characterized by non-conservation of the total radiation energy flux and by the existence of power fluctuations. The statistics of the wave beam power fluctuations have been studied. Information on the power statistical characteristics is applied to close the system of equations for joint moments. For task parameters which are not very strict (an effective radius of the wave beam should be considerably less than the outer scale of the turbulence) a system of independent equations for arbitrary joint moments has been obtained. The equations for the first two lower joint moments of the beam intensity and inverse power have been solved analytically. With the solutions obtained the effective wave beam parameters were calculated, i.e. the beam mean displacement, effective broadening and tremble variance (the beam wandering variance) for the propagation of radiation in the refractive channel of an absorbing turbulent medium. Radically new characteristics of the behaviour of the effective parameters in random absorbing and transparent media have been revealed.     

Joint moments of the wave beam amplitude and inverse power in an absorbing random medium with lens properties

Abstract

This paper presents a derivation of a system of closed equations for joint moments of the amplitude and inverse power of a wave beam propagating in a regularly inhomogeneous dissipative random medium. The radiation transfer in the medium is characterized by non-conservation of the total radiation energy flux and by the existence of power fluctuations. The statistics of the wave beam power fluctuations have been studied. Information on the power statistical characteristics is applied to close the system of equations for joint moments. For task parameters which are not very strict (an effective radius of the wave beam should be considerably less than the outer scale of the turbulence) a system of independent equations for arbitrary joint moments has been obtained. The equations for the first two lower joint moments of the beam intensity and inverse power have been solved analytically. With the solutions obtained the effective wave beam parameters were calculated, i.e. the beam mean displacement, effective broadening and tremble variance (the beam wandering variance) for the propagation of radiation in the refractive channel of an absorbing turbulent medium. Radically new characteristics of the behaviour of the effective parameters in random absorbing and transparent media have been revealed.     

The scanning acoustic microprobe: II. Application to the measurement and characterization of a piston reflector

In a pulsed ultrasound beam, echoes detected from a flat, circular piston of arbitrary size depend on the time-space characteristics of the entire pulse-echo measurement system, being a function of as many parameters as it takes to accurately define the system. In the limiting case of a target that is small relative to the spatial extent of an interrogating plane wave, an echo pattern is known to be a relatively simple function of the dimensionless product k0b, where k0 is the wave number and b is the radius of the target. In a companion paper preceding this one [F. E. Barber, J. Acoust. Soc. Am. 90, 8-17 (1991)], the author has described the scanning acoustic microprobe, a pulse-echo system in which the time-space properties of the interrogating waves are specified completely by k0 and a single additional parameter s0, which is the characteristic radius of a spherically symmetric, Gaussian-distributed scattering volume. In this system, the reflection pattern of a flat, circular piston of any arbitrary size is thus a function of two dimensionless parameters, namely k0b and b/s0. In this paper, this functional relationship is derived, a physical system is described, and analytical and experimental results are reported. It is shown that the diameter, orientation, and impedance mismatch properties of this simple target can be measured unambiguously over a range of target sizes from about a wavelength (2 pi/k0) to a beam diameter (about 3s0). For a typical ultrasound system, this is about a 5-1 range; i.e., a range extending to target sizes about five times smaller than can be detected in a simple B-mode imaging system.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of a Quasi-Stationary Field in Contact Radiometry

We analyze theoretically the thermal radiation of an absorbing half-space, received by an antenna of an arbitrary size located at an arbitrary height above the half-space surface. It is shown that the measured power consists of quasi-stationary (near-field) and wave components. The relative contribution of each component depends on the antenna size and height. Conditions under which the near-field component is dominant are found. It is shown that the total power of the radiation received from a uniformly heated medium is entirely determined by the medium temperature and is independent of the antenna parameters. If the influence of the quasi-stationary field is strong, then the received radiation is formed in a layer whose effective depth is less than the skin-layer depth. This effect can be used for detection of the near-field component of thermal radiation.     

Solar radiation concentrators paired with multijunction photoelectric converters in ground-based solar power plants (part I)

We have developed a method for determining parameters of radiation concentrator in solar power plants. To estimate the efficiency of concentrators in the form of Fresnel lenses in setups with three-junction photoelectric converters, the concept of the efficiency of the concentrator–photoelectric converter pair has been introduced. We have proposed a method for calculating the refracting profile of concentrators taking into account the dispersion relation for the refractive index and its variations with temperature for the material of the refracting profile of the concentrator (Wacker RT604 silicone compound). The results of calculation make it possible to achieve the maximal efficiency of the concentrator–photoelectric converter pair in the presence of chromatic aberrations in the optical system of solar radiation concentration.     

离子通道中相对论电子注的切连科夫辐射

对入射等离子体的相对论电子注(REB)在离子通道中可能产生切连科夫(Cherenkov)辐射的问题进行了论证与研究.利用线性理论分析了离子背景下的注-波互作用关系，导出了系统的色散方程与同步辐射条件.结果表明，系统的电磁不稳定性是由离子通道中TM模与电子注模通过电子注耦合所致，其微观机理是离子对电子注的聚焦.对处于运动等离子体状态下的离子-注系统进行了严格地理论分析，获得了通道内辐射波的频偏与波增长率公式，并通过数值模拟计算讨论了系统有关参数对它们的影响. 关键词： 离子通道 等离子体 切连科夫辐射     

On the theory of the relativistic cross-sections for stimulated bremsstrahlung on an arbitrary electrostatic potential in the strong electromagnetic field

On the base of relativistic generalized eikonal approximation wave function the multiphoton cross-sections of a Dirac particle bremsstrahlung on an arbitrary electrostatic potential and strong laser radiation field are presented. In the limit of the Born approximation the ultimate analytical formulas for arbitrary polarization of electromagnetic wave have been obtained and numerically analyzed. Received 5 April 2001 / Received in final form 18 March 2002 Published online 28 June 2002     

Focusing femtosecond X‐ray free‐electron laser pulses by refractive lenses

The possibility of using a parabolic refractive lens with initial X‐ray free‐electron laser (XFEL) pulses, i.e. without a monochromator, is analysed. It is assumed that the measurement time is longer than 0.3 fs, which is the time duration of a coherent pulse (spike). In this case one has to calculate the propagation of a monochromatic wave and then perform an integration of the intensity over the radiation spectrum. Here a general algorithm for calculating the propagation of time‐dependent radiation in free space and through various objects is presented. Analytical formulae are derived describing the properties of the monochromatic beam focused by a system of one and two lenses. Computer simulations show that the European XFEL pulses can be focused with maximal efficiency, i.e. as for a monochromatic wave. This occurs even for nanofocusing lenses.     

Influence of the forward wave on the efficiency of generating microwave radiation in a relativistic backward wave tube

An analysis is made of the influence of the forward wave on the operation of a relativistic uniform backward wave tube. The conditions are determined under which it is necessary to take account of asynchronous waves in the processes of energy exchange between the electromagnetic field and the electron beam. It is shown that the forward wave can strongly influence the nature of the beam modulation. This causes a change in the conditions for its interaction with the synchronous wave. Methods are proposed for optimizing the oscillator which enable the efficiency of the backward wave tube to be increased by a factor of 1.5–2. In the experiments an oscillator is realized which has a uniform electrodynamic system, an efficiency of 25%, and a microwave radiation power of 650 MW at 3 cm wavelength. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 49–55, December, 1996.     

Diffractive Multifocal Focusing of Gaussian Beams

The problem of diffraction of a spherical wave with Gaussian amplitude distribution on two infinitesimally thin and ideally reflecting screens with apertures on an optical axis is solved within the quasi-optical approximation. It is shown that when a Gaussian beam illuminates a bicomponent diffraction system with small Fresnel numbers in a near zone of the second screen, the effect of diffractive multifocal focusing of radiation is observed. In this case, the diffraction picture from the second screen in the focal planes represents the circular nonlocal bands of the Fresnel zones with a bright narrow peak at the center, whose intensity can exceed by six times the value of the incident wave intensity. The energy efficiency of diffractive focusing of Gaussian beams by the bicomponent diffraction system can be as high as 70%. The proposed diffractive method allows the focusing of the wide-aperture beams without using classical refraction elements such as lenses and prisms, and it is applicable to both low-intensive and high-power radiation.     

Self-Diffraction of Bessel Light Beams in a Nonlinear Medium

The phenomenon of self-diffraction of Bessel light beams (BLB) in a nonlinear liquid medium has been studied experimentally and theoretically for the first time. Diffraction maxima which do not correspond to integer orders for an induced periodic structure have been registered. It has been shown that the appearance of these maxima is due to the initial BLB modulation, which can be caused by the departure of the axicon refracting surface from the ideal conical surface, as well as by the imperfection of the form of the Gaussian beam incident on the axicon.     

Diffractive Multifocal Focusing of Gaussian Beams

The problem of diffraction of a spherical wave with Gaussian amplitude distribution on two infinitesimally thin and ideally reflecting screens with apertures on an optical axis is solved within the quasi-optical approximation. It is shown that when a Gaussian beam illuminates a bicomponent diffraction system with small Fresnel numbers in a near zone of the second screen, the effect of diffractive multifocal focusing of radiation is observed. In this case, the diffraction picture from the second screen in the focal planes represents the circular nonlocal bands of the Fresnel zones with a bright narrow peak at the center, whose intensity can exceed by six times the value of the incident wave intensity. The energy efficiency of diffractive focusing of Gaussian beams by the bicomponent diffraction system can be as high as 70%. The proposed diffractive method allows the focusing of the wide-aperture beams without using classical refraction elements such as lenses and prisms, and it is applicable to both low-intensive and high-power radiation.     

An eigenvalue-based approach for the design of reflector systems and prisms with a specified image orientation change

The image orientation change (IOC) of an object following its reflection by a system comprising an arbitrary number of flat boundary surfaces can be described using a merit function (Γ) expressed in the form of a 3×3 matrix. The present study proposes a design methodology for stable-IOC reflector and prism systems in which the merit function is solved using an eigenvalue-based approach. It is shown that a reflector system remains IOC-stable following its rotation about the eigenvector of the IOC merit function, provided that the image can still physically enter the system’s aperture. Furthermore, it is shown that an IOC-stable prism can be obtained by adding two refracting flat boundary surfaces at the entrance and exit positions of the light ray in an optical system comprising multiple reflectors provided that the condition n _n =Γn ₁ is maintained. Illustrative examples are provided to demonstrate the validity of the proposed design approach.     

Diffractive Focusing of a Gaussian Beam

The problem of diffraction of a spherical wave with Gaussian amplitude distribution on two infinitesimally thin and ideally reflecting screens with apertures on an optical axis is solved within the framework of the quasi-optical approximation. It is shown theoretically and experimentally that when a Gaussian beam illuminates such a type of bicomponent diffraction system with small Fresnel numbers in a near zone of the second screen, the effect of diffractive multifocal focusing of radiation is observed. In this case, the diffraction picture from the second screen in the focal planes represents the circular nonlocal bands of the Fresnel zones with a bright narrow peak at the center, whose intensity can exceed by six times the value of the incident wave intensity. The energy efficiency of diffractive focusing of Gaussian beams by the bicomponent diffraction system can be as high as 70%. The diffractive method proposed allows the focusing of wide-aperture beams without using classical refraction elements such as lenses and prisms, and it is applicable to both low-intensity and high-power radiation.     

On the formation of X-ray microbeams utilizing a short-focus refractive lens and a laboratory radiation source

The possibility of focusing an X-ray beam from a laboratory radiation source using a short-focus refractive composite lens is shown. The lens consists of 161 spherical biconcave epoxy lenses, each with a curvature radius of 50 μm. A Metal Jet (Excillium^TM) microfocus X-ray tube, with a focal-spot size of 20 μm and containing a liquid helium anode, is used as a radiation source. The size of the focal spot in the image plane is 2.4 μm, which corresponds to the theoretical estimate. The possibility of using the composite refractive lens to form a parallel polychromatic X-ray beam is demonstrated. The results obtained allow discussion of the possibility of applying short-focus refractive X-ray lenses for X-ray microanalysis using laboratory sources; such microanalysis is currently a prerogative of synchrotron radiation sources only.     

Dynamic optical transfer function of an acoustooptic modulator with a damped sound wave

Based on a spatially dependent dynamic optical transfer function of an acoustooptic modulator, a dynamic model of light modulation by a sound signal under conditions of acoustic damping is developed. A system of equations describing the dynamics of the acoustooptic interaction is given. Solutions of this equation for arbitrary power level and spatial-time structure of a sound signal are found. It is shown that acoustooptic damping has the strongest effect in a nonlinear modulation regime. Here, the dissipation of an acoustic signal suppresses the higher harmonic of the dynamic optical transfer function. An analytical model of the dynamic optical transfer function for a low level of acoustooptic coupling is given. It is shown that when the pump beam aperture is much greater than the spatial size of an acoustic signal, the time response of the acoustooptic modulator response is identical within a phase factor to the amplitude profile of a pump beam apodized by the exponential dependence of the amplitude of a damped sound wave. Otherwise, the sound damping produces almost no distortions of a plane top of the acoustooptic modulator response to a pulsed signal and shows itself only under phase mismatch conditions. Here, the asymmetry of overshoots at the edges of the acoustooptic modulator response is observed. Calculated plots are presented, which illustrate the aforementioned specific features of the transient process under conditions of acoustic damping in the acoustooptic interaction in paratellurite.     

Nonreciprocal waveguides and lenses based on the Fresnel-Fizeau effect

The propagation of a radiation beam in a medium whose velocity varies in the direction transverse to the axis of the beam is considered. It is shown that, under these conditions, the Fresnel-Fizeau effect of partial entrainment of light leads to focusing of the beam and to the possibility of the waveguide propagation of light. The corresponding waveguides and lenses have the property of nonreciprocity; i.e., their characteristics change with the reversal of the direction of propagation. The estimates obtained show the feasibility of observing the effect for optical radiation.