Theory of the radiation of dipoles placed within a multilayer system

A rigorous theory of radiation from dipoles embedded inside an arbitrary multilayer system is presented. In particular, we derive explicit expressions for the angular distribution of the electromagnetic field and the intensity radiated by the dipole into the surrounding media. Under the assumptions of mutual incoherence of the dipole radiation the calculations are extended to a layer of radiating dipoles. Special configurations corresponding to (i) a single dipole near a dielectric interface, (ii) a dipole layer surrounded by semi-infinite dielectric media, and (iii) a dipole layer placed on top of a waveguide layer are discussed in detail. This theoretical analysis has important consequences for the optimization of optical chemical sensors and biosensors that are based on fluorescence emission.     

Transition radiation as a practical X-ray source

Transition radiation is generated when an energetic electron crosses the boundary between two different media. Theoretical calculations predict soft X-ray spectra and angular distributions that agree with experimental results. Further, recent advances in microstructure technology create the possibility of designing transition radiation sources for several practical applications. For example, calculations indicate that a stack of 60 1-μm thick beryllium foils exposed to a beam of 100 MeV electrons will produce about 120 J of 1–2 keV X-rays per coulomb of incident charge. The highest intensity sources appear to be bright enough for application to microlithography.     

Theory of X-ray diffraction in nanoporous crystals with lateral quasi-periodicity

A theory of X-ray scattering from nanoporous crystals in the presence of a short-range structural order of the scattering elements (crystallites) has been developed within the framework of a statistical approach. It is shown that the angular distribution of the scattering intensity is determined by the coherence of the lateral quasi-periodicity of the porous material. Structural characteristics of a porous indium phosphide are determined using by fitting the results of theoretical calculations to the available experimental data.     

Coupled-wave Analysis of the Bragg Effect Waveguide Coupler

Abstract

A coupled-wave analysis is presented for the Bragg effect waveguide coupler. This device consists of a slanted volume phase grating which is integrated inside the lightguiding layer of an optical waveguide. The wave propagation is described by a set of leaky wave modes. Self-consistency relations are derived to determine the angular mode spectrum and the corresponding radiation loss coefficients. A maximum input coupling efficiency of 80% is predicted for an incident Gaussian beam. The power of a waveguide mode should be emitted into a single output beam reaching an efficiency of almost 100%.     

The effect of fluctuations in the lateral period of a multilayer grating on the scattering of hard synchrotron radiation

The effect of fluctuations in the lateral period of a nonideal multilayer grating on the scattering of hard synchrotron radiation (X-ray frequency range) was studied. Maps of the angular distribution of the coherent and diffuse scattering intensity in the reciprocal space are constructed. Theoretical diffraction curves calculated for a Ni/C multilayer grating are compared to the experimental data obtained by high-resolution triple-axis diffractometry. It is shown that good agreement between theory and experiment is observed for a fluctuation dispersion within 3% of the average lateral period of the grating.     

Electronic structure of p-type ultraviolet-transparent conducting CuScO2 films

We investigate the electronic structure of CuScO₂ thin films grown on sapphire and mica substrates by pulsed laser deposition. X-ray diffraction and microanalysis confirm that the films have the expected delafossite crystal structure and stoichiometric proportions. The electronic structure is investigated by means of X-ray and ultraviolet photoelectron spectroscopy. Electronic states in the range 0-1350 eV are identified, making reference to theoretical density-of-states calculations up to 80 eV. Photoelectron spectra near the Fermi energy confirm the p-character of the films. Optical absorption spectroscopy shows that the films are transparent up to 3.7 eV and exhibit an intense excitonic peak, with a direct gap energy of 4.24 ± 0.05 eV at room temperature. Ab initio band structure calculations confirm the direct character of CuScO₂ and allow for an assignment of the direct gap to an electronic transition at the L point of the rhombohedral Brillouin zone.     

Energy-dispersive analog of the Zernicke-Prins method for determining the atomic radial distribution function

A modification of the Zernicke-Prins method is proposed for determining the radial distribution of atoms in a solid. The modification consists in measuring the spectra of scattered broadband X-ray radiation instead of the angular distribution of a quasi-monochromatic radiation scattered from the probed sample. It is shown that, according to the proposed approach, it is possible to eliminate the principal difficulty of the traditional Zernicke-Prins method, which is related to a limited region of variation of the argument in the measured angular distribution of scattered radiation.     

硅太赫兹波导配置及性能分析

针对现有太赫兹辐射源在输出频率可调性及输出功率方面的局限性。本文从非线性介质的Maxwell方程入手对非线性聚合物的光学性质进行理论分析,建立硅波导配置的数学模型。利用硅材料的高折射率,设计了硅太赫兹波导。分析了太赫兹波导模式图,讨论了硅太赫兹波导模式的有效折射率、波导损耗、连续波输出功率与输出频率的关系,实验结果表明:硅太赫兹波导产生的太赫兹波连续可调,输出太赫兹波频率范围宽至0.1THz到15THz,输出功率可达到微瓦级。     

Experimental evaluation of a hollow glass fiber

Very high interest in making a low-loss fiber for the infrared has been stimulated by important applications in optical communication, surgery, cutting, welding, and heat treatment. The leaky waveguide is one of the most promising types of future fiber in the infrared region where low-loss materials are not available or not suitable for making fibers (i.e., CO2 laser light lambda = 10.6 microm). In this paper a comparative model of a He-Ne laser beam and an oxide glass leaky hollow fiber for a CO2 laser light beam and a chalcogenide glass leaky hollow fiber are studied. Measurements of attenuation, dependence of output power on diameter and angle, and the angular dependence of output angle vs input angle were made. The experimental data were compared with theoretical calculations, and the critical value of the wall thickness for minimum attenuation is given.     

X-ray diffraction investigation of cation distribution in CdχCu1-χFe2O4 ferrite system

Assuming the distribution of copper ions on tetrahedral (A) and octahedral (B) sites in the polycrystalline Cd_χCu_1-χFe₂O₄ (χ=0.2, 0.4, 0.6, 0.8 and 1.0) ferrite system, the theoretical intensities are calculated. Structural refinement of the diffraction pattern converges the R factor to between 0.0103 and 0.0315. Theoretical intensity ratios of (220) to (222) planes are considered, as these reflections are sensitive to cations on the A and B sites. Close agreement of the theoretical intensity ratio with the intensity ratio observed from X-ray diffraction pattern supports the occupancy of cadmium on the tetrahedral site. The cation distribution is predicted employing a new method wherein a graph of theoretical intensity is plotted against the inversion parameter for the (220) plane. The experimentally observed intensity of the same plane is then used to get an exact inversion parameter with the help of this plot. The cation distribution estimated by this method supports the cation distribution predicted by the magnetization method.     

Thermal efficiency of pulsed laser radiation increased by methods of multibeam acousto-optic diffraction

Bragg multibeam acousto-optic diffraction of pulsed laser radiation (with 150-ns pulse duration) has been studied under conditions of significant overlap between the neighboring beams. It is established that the initial beam with a Gaussian angular distribution of intensity can be transformed into diffracted radiation with a nearly rectangular profile of intensity provided that the laser pulse duration is much shorter than the period of interference beats in the overlapping regions. Using this circumstance, it is possible to increase the efficiency of high-power lasers in material processing (laser cutting, welding, engraving, etc.) with a threshold character of the radiation effect. This possibility was experimentally verified for a fiber laser operating at a 1.07 μm wavelength.     

Intermode light diffusion in multimode optical waveguides with rough surfaces

A theoretical analysis of incoherent intermode light power diffusion in multimode dielectric waveguides with rough (corrugated) surfaces is presented. The correlation length a of the surface-profile variations is assumed to be sufficiently large (a less less than lambda/2pi) to permit light scattering into the outer space only from the modes close to the critical angles of propagation and yet sufficiently small (a less less than d, where d is the average width of the waveguide) to permit direct interaction between a given mode and a large number of neighboring ones. The cases of a one-dimensional (1D) slab waveguide and a two-dimensional cylindrical waveguide (optical fiber) are analyzed, and we find that in both cases the partial differential equations that govern the evolution of the angular light power profile propagating along the waveguide are 1D and of the diffusion type. However, whereas in the former case the effective conductivity coefficient proves to be linearly dependent on the transverse-mode wave number, in the latter one the linear dependence is for the effective diffusion coefficient. The theoretical predictions are in reasonable agreement with experimental results for the intermode power diffusion in multimode (700 x 700) optical fibers with etched surfaces. The characteristic length of dispersion of a narrow angular power profile evaluated from the correlation length and standard deviation of heights of the surface profile proved to be in good agreement with the experimentally observed changes in the output angular power profiles.     

K Correlations and Facet Models in Diffuse Scattering

The angular intensity distribution of radiation scattered by a wide range of random media can be accounted for by assuming effective source amplitude correlations involving modified Bessel functions K _v. We investigate how such correlations can be derived from physical models of stochastic scattering systems, namely facet models with suitable slope distribution, and consider the cases of constant and varying r.m.s. slope or facet size. We study the pertinent photon statistics of the radiation scattered by these model systems and look for a link between K correlations and K distributions. As an application we revisit the concepts and laws of diffuse reflectance and discuss the existence of lambertian scatterers.     

Decrease in the incoherent near-edge X-ray scattering in narrow waveguides

The role of incoherent scattering in the attenuation of transmitted radiation has been studied using the statistical theory of X-ray beam scattering in a narrow rough collimator. Based on the obtained approximation expressions for the coefficients of waveguide mode damping, it is predicted that the level of scattering near the absorption edge must exhibit a significant decrease due to dispersion, which is also manifested by decreasing intensity of the backscattered radiation.     

Theory of diffuse X-ray scattering from an epitaxial layer with quantum rings

A theory of diffuse X-ray scattering from a crystalline layer containing quantum rings (QRs) randomly distributed in lateral directions has been developed. The QR is modeled by a truncated cone with a crater that has the shape of an embedded inverted truncated cone. Reciprocal-space maps of the scattering intensity distribution are obtained by numerical calculations for epitaxial layers containing QRs of various shapes. It is established that the angular distribution of the diffuse scattering significantly depends on the slope of the external cone generatrix, as well as on the crater shape and dimensions.     

Fabrication, characterization, and theoretical analysis of controlled disorder in the core of optical fibers

We present results of experimental and theoretical studies of polarization-resolved light transmission through optical fiber with disorder generated in its germanium-doped core via UV radiation transmitted through a diffuser. In samples longer than a certain characteristic length, the power transmitted with preserved polarization is observed to be distributed over all forward-propagating modes, as evidenced by the Rayleigh negative exponential distribution of the near-field intensity at the output surface of the fiber. Furthermore, the transmitted power becomes also equally distributed over both polarizations. To describe the optical properties of the fibers with the experimentally induced disorder, a theoretical model based on coupled-mode theory is developed. The obtained analytical expression for the correlation function describing spatial properties of the disorder shows that it is highly anisotropic. Our calculations demonstrate that this experimentally controllable anisotropy can lead to suppression of the radiative leakage of the propagating modes, so that intermode coupling becomes the dominant scattering process. The obtained theoretical expressions for the polarization-resolved transmission fit very well with the experimental data, and the information extracted from the fit shows that radiative leakage is indeed small. The reported technique provides an easy way to fabricate different configurations of controlled disorder in optical fibers suitable for such applications as random fiber lasers.     

Optical fiber design and the trapping of Cerenkov radiation

Cerenkov radiation is generated in optical fibers immersed in radiation fields and can interfere with signal transmission. We develop a theory for predicting the intensity of Cerenkov radiation generated within the core of a multimode optical fiber by using a ray optic approach and use it to make predictions of the intensity of radiation transmitted down the fiber in propagating modes. The intensity transmitted down the fiber is found to be dominated by bound rays with a contribution from tunneling rays. It is confirmed that for relativistic particles the intensity of the radiation that is transmitted along the fiber is a function of the angle between the particle beam and the fiber axis. The angle of peak intensity is found to be a function of the fiber refractive index difference as well as the core refractive index, with larger refractive index differences shifting the peak significantly toward lower angles. The angular range of the distribution is also significantly increased in both directions by increasing the fiber refractive index difference. The intensity of the radiation is found to be proportional to the cube of the fiber core radius in addition to its dependence on refractive index difference. As the particle energy is reduced into the nonrelativistic range the entire distribution is shifted toward lower angles. Recommendations on minimizing the quantity of Cerenkov light transmitted in the fiber optic system in a radiation field are given.     

Near- and far-field properties of annular CO(2) waveguide lasers

The theoretical and experimental investigation of field properties of annular waveguide lasers with a Fabry-Perot resonator is presented. Oscillation with high azimuthal mode order leads to an annular intensity distribution in the far field with almost linear dependence of the annular diameter on the mode order. Low-order fields form a distinct focal spot in the far field. A laser device with a discharge area of 6-cm diameter and 53-cm length yields an output power of 600 W.     

Effect of the Angular Distribution of Cathode Ions on the Current Flow in a High-Current Vacuum Arc

The effect of the angular distribution of cathode ions on the parameters of a vacuum arc is investigated. It is shown that, at current densities characteristic of a high-current arc, collisions of flows of fast ions from different cathode spots may result in a high value of the ion temperature at the cathode plasma boundary. The boundary in current density of the region of a stable current flow as a function of the width of angular distribution of cathode ions is determined. It is found that, in the region of supersonic flow of ions, the permissible value of the current density drops rapidly as the angular distribution width increases, and in the subsonic region, it rises rapidly. An expression is obtained for the maximum possible ion sound velocity and for the equivalent critical ion velocity. Two- and three-fluid hydrodynamic models are used to perform calculations that illustrate characteristic distributions of parameters in the discharge gap. Analysis of the calculation results makes it possible to discuss the characteristic features of current flow in a vacuum discharge.     

Modeling the detector of charge states of relativistic multicharged ions

A way to identify charge distributions of relativistic multicharged ions by recording the angular distribution of the Cherenkov radiation of ions is analyzed; preliminarily, ions with different charges are separated by ion velocities in an external target with a large charge number. As a result, when an ion beam enters the Cherenkov radiator, different charges radiate at different angles to the direction of the ion motion and the radiation intensity is proportional to the fraction of ions with a given charge in the beam.