Near-field effect in thermal radio-frequency radiation

The near-field effect was experimentally observed in the thermal radiation of an absorbing medium in the rf range. The radiation from a temperature-stratified aqueous medium was measured at a wavelength of 31 cm using specially developed electrically small antennas. The effect manifests itself as a decrease in the effective thickness of a layer in which the received radiation is formed and in the dependence of this thickness on the receiving antenna’s size and its height above the medium surface.     

X-ray synchrotron radiation in medium

Energy and angular distributions of X-ray synchrotron radiation produced by an ultra relativistic electron moving in a medium are discussed. Calculations show that the medium suppresses strongly the yield of the radiation for the electron Lorentz factor smaller than some cut-off value depending on the medium electron density and magnetic field applied.     

Bend radiation in optical fibers

A new, relatively simple way of calculating bend radiation is developed. As is often done in propagation over gently curved surfaces, the fiber curvature is accounted for by placing a straight fiber in a fictitious medium that is inhomogeneous in the plane of the bend. For a gently curved fiber, the medium is slowly varying and the WKB method is used to approximate the radial field dependence. This solution is related to the unperturbed straight fiber solution through an ansatz consistent with the WKB solution. The propagating modes in this approximation remain orthogonal, allowing an immediate generalization to the multimoded case. The radiation loss per unit length is calculated two ways and is consistent with results in the literature.     

Non-equilibrium statistical mechanics of synchrotron-Cerenkov radiation

The synergic synchrotron-Cerenkov (SC) radiation emitted by a relativistic charged particle under the combined effect of the constant external magnetic field and the collective interactions in the ambient plasma (medium) is given in the framework of the non-equilibrium statistical mechanics developed by Prigogine and his co-workers. Starting from the formal solution of the Liouville equation, the one-particle distribution function is calculated. Restricting the motion of the test particle to a circular orbit in the plane normal to the magnetic field, we use the above distribution function to calculate the power emitted per unit solid angle by the test particle as a function of time. We have thus obtained the time evolution of the synergic SC radiation which in the asymptotic limit reproduces the results of Schwinger and his co-workers. It is also shown that the collective interactions within the system produces a shift in the frequency of the outcoming radiation.     

Bend radiation in optical fibers

Abstract

A new, relatively simple way of calculating bend radiation is developed. As is often done in propagation over gently curved surfaces, the fiber curvature is accounted for by placing a straight fiber in a fictitious medium that is inhomogeneous in the plane of the bend. For a gently curved fiber, the medium is slowly varying and the WKB method is used to approximate the radial field dependence. This solution is related to the unperturbed straight fiber solution through an ansatz consistent with the WKB solution. The propagating modes in this approximation remain orthogonal, allowing an immediate generalization to the multimoded case. The radiation loss per unit length is calculated two ways and is consistent with results in the literature.     

Incoherent properties of induced radiation

The evolution of a quantized electromagnetic field in a thermally excited dispersion medium is determined by two scattering channels. The coherent channel is formed exclusively by the elastic scattering of quanta. The incoherent channel, along with elastic scattering processes, necessarily contains inelastic scattering processes, including induced radiation. Interference between the channels is absent because of the orthogonality of the wave functions of the medium in its final states, which correspond to different scattering channels. Therefore, in an excited medium, interference processes that are not described by its refractive index may arise. An interference pattern of this kind can be formed, in particular, as a result of the superposition of the resonance radiation incident on an excited medium and the radiation reflected from this medium. In this case, the conventional perturbation theory proves to be inadequate.     

On reflection of radiation from moving objects

An analytical expression for the field reflected from a nonuniformity of the velocity of a moving medium is obtained. Specific features of the reflected radiation depending on the velocity and sizes of the non-uniformity domain and on the angle of incidence of radiation relative to the direction of motion are studied. The feasibility of experimental detection of the reflected radiation is demonstrated.     

Transmission of electric dipole radiation through an interface

We consider the transmission of electric dipole radiation through an interface between two dielectrics, for the case of a vertical dipole. Energy flows along the field lines of the Poynting vector, and in the optical near field these field lines are curves (as opposed to optical rays). When the radiation passes through the interface into a thicker medium, the field lines bend to the normal (as rays do), but the transmission angle is not related to the angle of incidence. The redirection of the radiation at the interface is determined by the angle dependence of the transmission coefficient. This near-field redistribution is responsible for the far-field angular power pattern. When the transmission medium is thinner than the embedding medium of the dipole, some energy flows back and forth through the interface in an oscillating fashion. In each area where field lines dip below the interface, an optical vortex appears just above the interface. The centers of these vortices are concentric singular circles around the dipole axis.     

Self-focusing of laser radiation in cluster plasma

The self-focusing of laser radiation in plasma with ionized gaseous clusters is studied both analytically and numerically. An electrodynamic model is proposed for cluster plasma in a field of ultrashort laser pulse. The radiation self-action dynamics are studied using the equation for wave-field envelope with allowance for the electronic nonlinearity of the expanded plasma bunches and the group-velocity dispersion in a nanodispersive medium. It is shown that, for a laser power exceeding the self-focusing critical power, the wave-field self-compression occurs in a medium with dispersion of any type (normal, anomalous, or combined). Due to the strong dependence of the characteristic nonlinear field on the size of ionized cluster, the corresponding processes develop faster than in a homogeneous medium and give rise to the ultrashort pulses.     

Concept of formation length in radiation theory

The features of electromagnetic processes are considered which connected with finite size of space region in which final particles (photon, electron–positron pair) are formed. The longitudinal dimension of the region is known as the formation length. If some external agent is acting on an electron while traveling this distance the emission process can be disrupted. There are different agents: multiple scattering of projectile, polarization of a medium, action of external fields, etc. The theory of radiation under influence of the multiple scattering, the Landau–Pomeranchuk–Migdal (LPM) effect, is presented. The probability of radiation is calculated with an accuracy up to “next to leading logarithm” and with the Coulomb corrections taken into account. The integral characteristics of bremsstrahlung are given, it is shown that the effective radiation length increases due to the LPM effect at high energy. The LPM effect for pair creation is also presented. The multiple scattering influences also on radiative corrections in a medium (and an external field too) including the anomalous magnetic moment of an electron and the polarization tensor as well as coherent scattering of a photon in a Coulomb field. The polarization of a medium alters the radiation probability in soft part of spectrum. Specific features of radiation from a target of finite thickness include: the boundary photon emission, interference effects for thin target, multi-photon radiation. The theory predictions are compared with experimental data obtained at SLAC and CERN SPS. For electron–positron colliding beams following items are discussed: the separation of coherent and incoherent mechanisms of radiation, the beam-size effect in bremsstrahlung, coherent radiation and mechanisms of electron–positron creation.     

Simulation of interaction of radiation with nanoparticles

Enhancement of microwave radiation at wavelength λ ∼ 10 cm in a cavity is simulated using the system of constituent equations derived earlier. It is shown that a radiation energy density of W ∼ 1000 J/m³ can be attained. Pumping of the medium containing conducting nanoparticles is carried out with a stationary electric field. The required mass concentration of nanoparticles and pumping field are estimated. A method of obtaining active medium using a statitionary electric field for enhancement of microwave radiation in a wavelength range of λ ∼ 10 cm is proposed. In this method, extended conducting nanoparticles should be sputtered.     

Quantization of the radiation field in an anisotropic dielectric medium

There are both loss and dispersion characteristics for most dielectric media. In quantum theory the loss in medium is generally described by Langevin force in the Langevin noise (LN) scheme by which the quantization of the radiation field in various homogeneous absorbing dielectrics can be successfully actualized. However, it is invalid for the anisotropic dispersion medium. This paper extends the LN theory to an anisotropic dispersion medium and presented the quantization of the radiation field as well as the transformation relation between the homogeneous and anisotropic dispersion media.     

Effects of inversionless lasing in two-level media from the point of view of specificities of the spatiotemporal propagation dynamics of radiation

We report the results of computer simulation of the emission of radiation by an extended two-level medium in a ring cavity. The cases of using strong external monochromatic, quasi-monochromatic, and biharmonic radiation for pumping the two-level medium are analyzed. It is shown that the emission of radiation with spectral content different from that of the pump radiation, which is interpreted as the inversionless oscillation, is the result of the spatiotemporal dynamics of light propagation in an extended two-level medium imbedded in a cavity. The appearance of this radiation is not related to known resonances of amplification of a weak probe field in a thin layer of the two-level system (the effect of inversionless oscillation) induced by strong resonance monochromatic or biharmonic field, as was thought before.     

Quantum approach to the description of amplification of radiation from an array of nanotubes

The mechanism of generating terahertz radiation by zigzag-type nanotubes is considered using the two-point unit phase cell. The nanotubes are oriented along the radiation field. The radiation gain factor is estimated and the mechanism of excitation of a nonlinear medium based on a nanotube array is discussed.     

Nonlocal theory of radiation propagation in a random-density medium in the correlation approximation

General relations are derived for the variation of a linear functional of the radiation field in a medium having a homogeneous isotropic random density. The influence of the density fluctuations of the medium on radiation fluxes with different scattering orders is estimated.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 96–101, September, 1972.     

Transition radiation of seismic waves at the atmosphere-earth interface

The method of contour integration is used for solving the problem of transition radiation of elastic waves by a mass source, which travels uniformly in a gas normal to the interface between homogeneous gaseous and elastic halfspaces and disappears at the time it touches the solid surface. We have obtained asymptotic formulas for the field of transition radiation, which hold true near the fronts of longitudinal and transverse spherical waves and a conical wave. An exact analytical expression for the field of transition radiation has been obtained for the observation points located on the source trajectory extension in a solid. The influence of interaction between longitudinal and transverse waves, which occurs on the surface of elastic medium, on the space distribution of the field of transition radiation is analyzed. Radiophysical Research Institute, Nizhny Novgorod, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 40, No. 10, pp. 1210–1223, October, 1997.     

Cerenkov radiation in anisotropic media by the methods of quantum electrodynamics

We examine the Cerenkov effect in a transparent anisotropic medium by the methods of quantum electrodynamics. We first show that in such a medium the electric field intensity is not transverse. By resolving the field we distinguish the contributions to the intensity of the radiation corresponding to the transverse and longitudinal components of the field. Focusing also on the role of the spin, we show that its effect is significant since the intensity can increase or decrease compared with that of a spinless particle.     

Anharmonic pulses due to transition radiation

The possibility of using the transition radiation of an electron bunch to generate nonstationary anharmonic pulses in free space and in a dispersive medium is demonstrated. It is shown that the transition radiation of the bunch gives rise to an infinite train of nonstationary anharmonic pulses described by solutions to the Klein-Gordon equation. The polarization of the resulting field at its leading edge appears to be different from the field polarization at the interface. The leading edge becomes steeper with time. It is shown that the bunch density longitudinal distribution and the parameters of the dispersive medium can be determined from the values of the fields and their derivatives near the leading edge of the resulting signal.     

Similarity effects in multiple scattering of coherent radiation: Phenomenology and experiments

Based on phenomenological concepts of statistics of effective optical paths for multiple scattering of coherent radiation, an analysis is carried out of similarity effects observed for the dependences of statistical moments of the scattered field on the relaxation parameters with a dimension of reciprocal length. Within the framework of the diffusion approximation, expressions are obtained that describe the autocorrelation function of fluctuations of the scattered-field amplitude, the degree of polarization, and the normalized intensity of scattered light for media with a finite absorption length in the case of forward scattering of coherent radiation in a plane layer of an isotropic scattering medium. The results of the analysis show the similarity of the dependences of these quantities on the corresponding spatial scales. Experiments with model scattering media (aqueous suspensions of polystyrene spherical particles) supported the existence of similarity effects in multiple scattering. An experimental study was made of the relation between the characteristic depolarization length and the transport length for multiple scattering of coherent radiation in a plane layer. The effective value of the radiation diffusion coefficient providing the best agreement between the experimental and the calculated values of parameters of the scattered field is shown to be independent of the absorption coefficient of a medium.     

Speckle polarization diagnostics of scattering media using partially coherent radiation

The paper discusses a method for probing a randomly inhomogeneous medium using partially coherent radiation and polarization filtering of the radiation scattered by the medium. The method is based on the analysis of the contrast of speckle-modulated images of the object under study as a function of the coherence length of the probe radiation. A theoretical justification of the method is given as applied to systems of discrete scatterers with the subsequent modification of the results obtained for continuously distributed scattering systems. The results of experimental testing of the developed method for scattering media characterized by nondiffusion conditions of propagation of the probe radiation are presented and compared with the results of the theoretical analysis.