Grazing-angle scattering of electromagnetic waves in periodic Bragg arrays

A new powerful approximate approach for the theoretical analysis of Bragg scattering in oblique strip-like periodic arrays with the scattered wave propagating almost parallel to the array boundaries – grazing-angle scattering (GAS) – is introduced and justified. This approach is based on allowance for the diffractional divergence of the scattered wave by means of the parabolic equation of diffraction and Fourier analysis. The divergence is demonstrated to be an intrinsic physical cause of GAS. Detailed theoretical analysis of steady-state GAS is carried out for bulk and guided optical modes. It is demonstrated that the most interesting feature of GAS in arrays of width that is greater than a critical width is a unique combination of two strong simultaneous resonances with respect to frequency and angle of scattering. In such wide arrays, GAS is demonstrated to be not only unusually sensitive to angle of scattering, but also to small variations of array width and grating amplitude. Entire concentration of the resonantly strong scattered wave inside the array is shown to be possible. A relationship between GAS, conventional Bragg scattering, and extremely asymmetrical scattering (i.e. where the scattered wave propagates parallel to the array boundaries) is analysed. Applicability conditions for the used approximations and obtained results are derived and discussed.     

Nonlinear interaction of strong laser fields in vacuum

Quantum electrodynamics (QED) predicts that electromagnetic fields interact with each other in vacuum. We study the possibility of revealing this interaction experimentally with intensities on the order of 10²⁴–10²⁶ W/cm², which may be available in the next generation of laser systems. In particular, we investigate high-order harmonic generation in vacuum via the collision of two ultrastrong counterpropagating laser pulses. The experimental feasibility of the related process of stimulated light-by-light scattering is also examined. Finally, the importance of including diffractive effects to describe the nonlinear interaction between an x-ray probe and a strong, focused optical standing wave is pointed out.     

Extremely asymmetrical scattering of slab modes in periodic Bragg arrays

The steady-state extremely asymmetrical scattering of electromagnetic modes in a slab with a periodically corrugated boundary is analyzed theoretically. A new approach, based on allowance for the diffractional divergence of a scattered wave, is used with the approximation of slowly varying amplitudes and a Fourier analysis. The structure of the incident and scattered waves inside and outside the array is determined. The amplitudes of the scattered waves are found to be much larger than the amplitude of the incident wave. The typical time of relaxation to steady-state scattering is found to depend on the distance from the array boundary through which the incident wave enters the array. Conditions of applicability of the results obtained are also presented.     

Measuring magnetic fields in plasmas by means of light scattering

The theory of light scattering in plasmas containing a magnetic field yields the special case of modulated scattering spectra. The modulation frequency is governed by the field in the plasma and is equal to the electron cyclotron frequency. In this investigation magnetic fields in a plasma were determined by a laser scattering experiment. The experimental data were: electron densityn _e=10¹⁶cm^?3, electron temperatureT _e=3.2 eV, scattering angle θ=90 °, scattering parameter α=0.6, and a maximum field in the plasma of 125 kG. The spectrum measured at the maximum magnetic field was modulated with 3.6 × 10¹¹ Hz. In scattering experiments with a field reduced by about 20% the observed modulation frequency was 2.8 × 10¹¹ Hz. A thermal spectrum with a smooth profile was found when no field was present in the plasma. Applying the theory of cyclotron modulated spectra one obtains from the scattering experiment magnetic fields of 128, 100, and 0 kG. Within the experimental accuracy these values agree well with the fields determined by means of magnetic probes. Other possible interpretations of the measured deviations from thermal spectra (modulation with the plasma frequency or additional cold electron components in the plasma) are discussed, but they afford no explanation. This experiment has domonstrated that magnetic fields in plasmas can be measured locally and almost without disturbance by means of light scattering.     

强场下真空中粒子对产生的研究进展

随着激光技术的飞快发展,激光强度不断提高,超强外场下真空中正负电子对产生的过程,即能量向质量转化过程,已经成为一个研究热点。主要综述了近几年量子Vlasov方程方法和计算量子场论(数值求解Dirac方程)方法在研究强场下真空中正负电子对产生方面的进展,分别介绍了空间均匀场和空间不均匀场下的粒子对产生的情况。第一种情况主要介绍双脉冲结构振荡电场中电子-正电子对的产生、强双频振荡电场中非微扰电子-正电子对的产生、频率调制的激光场中电子-正电子对的产生和Dirac真空对啁啾外场的快速分辨。第二种情况主要介绍优化空间局域电场提高粒子对的产生率、多个势阱-垒结构的振荡场对粒子对产生的增强、振荡 Sauter 电势中正负电子对产生的问题、操纵Dirac真空以控制其在场诱导下的衰变、作为信息传输介质的Dirac真空还有正负电子对产生中的相干和非相干啁啾机制的转变。     

内混合强吸收气溶胶粒子光散射的等效性

以黑碳和水两种成分组成的内混合单分散气溶胶粒子为例,根据其各消光效率因子、吸收效率因子和散射相函数,分析了用等效折射率来描述含有不同成分的内混合气溶胶系统的适用性。结果表明：在瑞利散射区和几何光学区内,内核（碳粒）体积比为0.01,0.1,0.5,0.9时,消光效率在大多数尺度参数下等效性都很好,但在米散射区内相对较差;当体积比大于0.3时,其吸收效率、消光效率等效性较好;除瑞利散射区外,散射相函数在各体积比下的等效性都很差。当考虑内混合气溶胶粒子系统的散射和吸收特性时,一般不难找到等效折射率,但在光散射技术中,应用相函数反演等效折射率的可靠性还有待商榷。     

结构光周期图案的统一解码方法

针对结构光解码问题,在传统相位测量轮廓术（PMP）理论的基础上,提出一种结构光周期图案的统一解码方法。通过把经典PMP的相位计算公式运用到任何周期波图案中,比如三角波、锯齿波、方波图案等,通过多频时间相位展开算法实现了对包含高频率的图案的相位展开。实验结果表明,提出的方法达到了相对高的精度,验证了此方法的有效性。     

结构光周期图案的统一解码方法

针对结构光解码问题,在传统相位测量轮廓术（PMP）理论的基础上,提出一种结构光周期图案的统一解码方法。通过把经典PMP的相位计算公式运用到任何周期波图案中,比如三角波、锯齿波、方波图案等,通过多频时间相位展开算法实现了对包含高频率的图案的相位展开。实验结果表明,提出的方法达到了相对高的精度,验证了此方法的有效性。     

内混合强吸收气溶胶粒子光散射的等效性

 以黑碳和水两种成分组成的内混合单分散气溶胶粒子为例,根据其各消光效率因子、吸收效率因子和散射相函数,分析了用等效折射率来描述含有不同成分的内混合气溶胶系统的适用性。结果表明：在瑞利散射区和几何光学区内,内核（碳粒）体积比为0.01,0.1,0.5,0.9时,消光效率在大多数尺度参数下等效性都很好,但在米散射区内相对较差;当体积比大于0.3时,其吸收效率、消光效率等效性较好;除瑞利散射区外,散射相函数在各体积比下的等效性都很差。当考虑内混合气溶胶粒子系统的散射和吸收特性时,一般不难找到等效折射率,但在光散射技术中,应用相函数反演等效折射率的可靠性还有待商榷。     

Coherent X-rays excited by a relativistic electron crossing a periodic stratified structure in Bragg scattering geometry

The dynamic theory of coherent X-rays excited by a relativistic electron crossing an artificial periodic stratified structure in Bragg scattering geometry has been developed for the general case of asymmetric reflection. Expressions describing the spectral-angular characteristics of radiation have been derived and investigated.     

Intersubband transport in quantum wells in strong magnetic fields mediated by single- and two-electron scattering

We show theoretically that in quantum wells subjected to a strong magnetic field the intersubband current peaks at magnetic field values, which reveal the underlying specific intersubband scattering mechanism. We have designed and grown a superlattice structure in which such current oscillations are clearly visible, and in which the transition from the purely single-electron to the mixed single- and two-electron scattering regimes can be observed by tuning the applied voltage bias. The measurements were conducted in ultrahigh magnetic fields (up to 45 T) to obtain the full spectrum of the current oscillations.     

Self-modulation and nonlinear diffraction of strong light fields in dispersive media

The problem of space-time self-modulation of strong laser fields is solved by the method of successive approximations. The nonlinear diffractional and nonlinear dispersive transformations of beam width and pulse duration are studied with emphasis on the reciprocal effects of nonlinear diffraction and dispersion. The effect of incomplete initial time and space coherence on the space-time parameters of a laser field and the conversion of its coherent properties is examined.Moscow State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 35, No. 6/7, pp. 525–532, June–July, 1992.     

Bragg gratings in air-silica structured fibers

We report on grating writing in air-silica structured optical fibers with pure silica cores by use of two-photon absorption at 193 nm. A decrease in propagation loss with irradiation was observed. The characteristic growth curves were obtained.     

Bragg scattering of light on the vortex lattice in the rotating helium II

It is shown that the light scattering on the vortex lattice in the rotating pure and impure He II is considerably increased by the generation of the second sound.     

Dynamically measuring the holo-information of light fields in three-dimensional space using a periodic polarization-structured light

Spatially structured light field has attracted great attention due to its novel properties and application potential in numerous fields. Among them, the most striking one is the polarization-structured light, known as the vector beam. Here, using a periodic polarization-structured light, we propose a method to dynamically measure the holo-information of light fields, including the amplitude, phase, and polarization distributions, in three-dimensional(3D) space. The measurement system is composed of a Mach-Zender interferometer involving a liquid crystal polarized grating in the reference arm, which is simple, stable, and easy to operate. Featuring the single-shot measurement, this method supports observing the dynamic variation of object light fields. The accuracy, 3D polarimetry, and dynamic observation of this method are validated by measuring a calibrated quarter-wave plate, a vector vortex beam, a Poincaré beam, and a stressed polymethyl methacrylate sample.     

Coherent X-ray radiation generated by a relativistic electron beam in a periodic layered medium in the Bragg scattering geometry

A dynamic theory is developed for coherent X-ray radiation generated when a diverging beam of relativistic electrons crosses a periodic layered target. Expressions describing the spectral–angular characteristics of coherent X-ray radiation are obtained and analyzed in the Bragg scattering geometry.     

A multiphoton treatment of near resonant light scattering in intense fields

The scattering by a two-state atom of monochromatic light of arbitrary intensity and detuning, is treated by solving Schrödinger's equation directly for the state of the scattered field. The calculation is performed in a basis of dressed atom states, which include to all orders the coupling of the incident field to the atom. Amplitudes describing sequences of scattering events are found by a nonperturbative method. A scattered intensity spectrum, directly related to the rate of change of energy in any given scattered field mode, is constructed in terms of bilinear products of the amplitudes representing all possible sequences of photon emission, and in the steady state limit the spectrum is found to be identical to the widely quoted results of Mollow. A discussion is devoted to understanding in physical terms the various quantities appearing in the expression for the intensity spectrum.