Diffraction characteristics of planar corrugated waveguides

The diffraction efficiency curves are calculated as a function of the angle of incidence for planar waveguides with corrugation on the waveguide-air boundary or on the waveguide-substrate boundary, respectively. It is shown that the reflectance of the system is increased up to 100% because of the excitation of the waveguide mode. A detailed phenomenological study is carried out taking into account the influence of the waveguide thickness and the corrugation depth. Possible applications of these waveguides as a narrow-band reflection filter and selective mirror are discussed.     

Diffraction characteristics of planar absorption gratings

Planar (co)sinusoidal conductivity (absorption) transmission gratings are analyzed using rigorous coupled-wave theory. The first-order and higher-order diffraction efficiencies are determined over the entire range of possible conductivities and Bragg angles of incidence (or equivalently, grating periods) for H-mode polarization incident plane waves. The maximum possible first diffracted order efficiency is found to be 5.26%. Rigorous results are compared to approximate results from the Raman-Nath theory and the two-wave first-order coupled-wave (Kogelnik) theory. A regime parameter, , is defined which delineates the regions of Raman-Nath diffraction behavior ( < 1) and the region of two-wave first-order diffraction theory behavior ( > 1). Likewise, the angular selectivity characteristics of conductivity gratings are determined from rigorous theory and are compared with corresponding results from approximate theory.     

Diffraction selection of modes in planar backward-wave oscillators

We study the possibility of obtaining spatially coherent radiation in a planar backward-wave oscillator fed with a wide sheet electron beam by using natural diffraction divergence of the radiation. This method is efficient for Fresnel parameters being approximately equal to unity. To solve the two-dimensional boundary problem, the spatio-temporal approach is used, which allows one to determine both the self-excitation conditions and the parameters of the stationary generation regime and the boundaries of its stability. The developed theory is used to evaluate the possibility of achieving diffraction synchronization of the radiation in a high-power planar backward-wave oscillator operated in the 8-mm wavelength band and based on a sheet relativistic electron beam produced by the U-3 accelerator of the Institute of Nuclear Physics of the Siberian Branch of the Russian Academy of Sciences, Novosibirsk, Russia.     

Diffraction effects in low-energy electron emission

The origin of diffraction peaks in the energy distribution of intensity of low-energy (< 1000 eV) electron emission from crystals is discussed from the standpoint of the dynamical theory of diffraction. The emitted electrons are considered to originate at relatively incoherent point sources in the crystal. The two-beam approximation of dynamical theory is used. The theory accounts for the chief regularities of diffraction peaks: temperature-dependence of peak intensities like that for low-energy electron diffraction (LEED) peaks, correlation of peak energies with X-ray absorption fine structure, and correlation of peak energies with the energies of normal-incidence LEED peaks in specular reflection. It is shown that the conditions for diffraction peaks coincide with the conditions for emergence of Kikuchi lines. It is predicted that for energies just above those of diffraction peaks, such emergences should be observable in the angular distribution of emission as intensity minima for emission along low-index crystal axes. Theory of Kikuchi band profiles is developed in an Appendix.     

Electron-phonon interaction in cylindrical and planar quantum wires

Within the framework of the Feynman variational method the bipolaron binding energy and the bipolaron effective mass in cylindrical and planar quantum wires with ‘parabolic’ confinement are calculated. An analogy is found between the effects due to strong confinement and those due to the application of a strong magnetic field. Strengthening the confinement leads to an enlargement of the bipolaron stability region.     

Diffraction effects on soliton propagation

An averaged-Lagrangian method is used to analyze diffraction effects on propagation of solitons of various types in homogeneous media. It is shown that diffraction can counteract the self-focusing of dark and gray envelope solitons described by the nonlinear Schrödinger equation and solitons described by the Korteweg-de Vries equation when the soliton intensities do not exceed certain values. Conversely, diffraction enhances the self-focusing of dark and gray envelope solitons described by the modified Korteweg-de Vries equation, kinks described by the sine-Gordon equation, and domain walls in the u 4 model, which is explained by mutual correlation between transverse and longitudinal soliton dynamics. Critical parameters that determine soliton stability with respect to self-focusing are found for several models.     

Diffraction mode selection in planar Bragg resonators of optical and microwave wavelength ranges

Using the coupled waves approach complemented by the time-domain quasi-optical approximation, we solve a 2D diffraction problem which allows to evaluate the eigenfrequencies, quality factors and spatial structures of eigenmodes in planar Bragg resonators with a finite length and width of the corrugated area. We find the values of the Fresnel parameter determined by the geometrical dimensions of the system which allows for efficient transverse mode discrimination due to the larger diffraction losses of the modes with higher transverse numbers.     

Diffraction effects in guided photonic band gap structure

The concept of the photonic band gap (PBG) structures stems from ideas of Yablonovitch. The idea is to design components so that they affect the properties of photons, in much the same way that ordinary semiconductor crystals affect the properties of electrons. In fact, the PBG structures forbid propagation of photons for a particular range of energy. They can be used to realise optical filters with large stop band and sharp transmission resonance. In the guided PBG structures, the existence of diffractive effects in the vertical dimension could limit the quality factor of such filters. In this paper, we have investigated the origin of diffraction losses in one-dimensional guided PGB structures using 2D and 3D numerical tools. We propose an analytical approach based on Bragg diffraction relation to explain these losses phenomenon. From this approach, the influence of some design parameters on the electromagnetic behaviour and the spectral response of PBG resonators will be explained.     

Polarization properties of the acousto-optical interaction in a planar layer

Polarization features of the Bragg diffraction of light on ultrasound in a planar layer are studied. It is shown that the rotation of the polarization plane of the diffracted wave is determined by Fresnel reflection of the s and p components of the incident light and by anisotropy of the photoelastic effect in the crystalline layer perturbed by the ultrasound. It is established that the inclusion of an analyzer into the scheme of the acousto-optical interaction allows one to transform the rotation of the polarization plane of the diffracted wave transmitted through and reflected from the layer into the amplitude modulation of light.     

Diffraction of light in intermediate regime of acoustooptical interaction at oblique incidence

The diffraction of light by ultrasound in an isotropic medium at arbitrarily small angles of incidence onto the acoustic layer is calculated. The intermediate regime of diffraction is considered, for which the conditions of observation of the Raman-Nath and Bragg diffraction are not satisfied in terms of the wave parameter. The effect of the acoustooptical interaction length and angle between the incident light beam and the acoustic wave on the light intensity distribution over the diffraction peaks is studied as a function of the acoustic power. Specific features of the transition from the Raman-Nath to the Bragg diffraction regime through the intermediate regime of diffraction are analyzed.     

Diffraction effects in backscattering and Auger production near crystal surfaces

Computations have been made for 1 keV electrons of the dependence of the backscattering on the direction of the incident beam using the many-beam dynamical theory of electron diffraction. Values of the relevant absorption and backscattering parameters are given for electrons in this energy range. For the zero-loss component and the first bulk plasma loss component of backscattering the results of the computations are in moderately good agreement with experiment. The computed angular structure is considerably more pronounced than that observed but is rather sharp and may not be fully resolved in practice. The extension of the method to investigate the related angular effects observed in the Auger signal is discussed. It is suggested that the effect may be used to locate the position of Auger emitting atoms in the unit cell as well as their depth distribution near the surface of the crystal.     

Weak interaction effects in positronium

Within the context of the Weinberg-Salam (standard) model we study possible effects of weak interactions in positronium (Ps), such as parity mixing and weak decays of Ps states. As expected, weak interaction amplitudes in Ps turn out to be extremely small, their magnitude being characterized byG·m _e ² ?3·10^?12 whereG is Fermi's constant andm _e the electron mass. We show that the standard model forbids parity-violating correlations in a large class of Ps reactions and decays due to CP conservation in the lepton sector. We then consider situations in which parity-odd effects in Ps will occur in the standard model and may even be large enough to be observable. Beyond the context of the standard model we discuss the decay of orthopositronium into a photon and the hypothetical axion under the assumption that the mass of the axion is smaller than twice the mass of the electron.     

Electron-electron interaction effects in bismuth

Measurements of the far infrared magneto-optical transmission spectra of bismuth at 311 and 337 μ and in fields up to 100 kG at 4°K are described. The sensitivity of the spectra of transmitted magneto plasma waves to the electron scattering in bismuth is discussed. Particular attention is drawn to excitonic effects associated with weakly allowed hole cyclotron resonance absorption lines observed in the spectra. A simple Cooper pair like argument is given to illustrate the nature of these inter subband excitons. The features entering into a more complete theory of the excitons are also briefly discussed.     

Electron-electron interaction effects in bismuth

Measurements of the far infrared magneto-optical transmission spectra of bismuth at 311 and 337 and in fields up to 100 kG at 4°K are described. The sensitivity of the spectra of transmitted magneto plasma waves to the electron scattering in bismuth is discussed. Particular attention is drawn to excitonic effects associated with weakly allowed hole cyclotron resonance absorption lines observed in the spectra. A simple Cooper pair like argument is given to illustrate the nature of these inter subband excitons. The features entering into a more complete theory of the excitons are also briefly discussed.Work supported by the National Science Foundation under Grant #GH40968     

Broken time reversal of light interaction with planar chiral nanostructures

We report unambiguous experimental evidence of broken time-reversal symmetry for the interaction of light with an artificial nonmagnetic material. Polarized color images of planar chiral gold-on-silicon nanostructures consisting of arrays of gammadions show intriguing and unusual symmetry: structures, which are geometrically mirror images, lose their mirror symmetry in polarized light. The symmetry of images can be described only in terms of antisymmetry (black-and-white symmetry) appropriate to a time-odd process. The effect results from a transverse chiral nonlocal electromagnetic response of the structure and has some striking resemblance with the expected features of light scattering on anyon matter.