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L. Sh. Grigoryan

Publications and source records attributed to L. Sh. Grigoryan.

At least 19 recordsLinked to original sources
arXiv2026

Radiation processes in dielectric cylindrical waveguides

Dielectric cylindrical waveguides are widely used for confining and guiding of electromagnetic waves in relatively wide range of frequencies. They have found numerous technological and scientific applications in telecommunications, medicine, material science, photonics and quantum optics. The electromagnetic field Green function is the central object in investigations of different types of radiation processes in those structures. In this paper, we review and further develop the recurrence procedure for evaluating the electromagnetic field Green function in a medium made of any number of homogeneous cylindrical layers. The general results are specified for a cylindrical waveguide immersed in a homogeneous medium. Expressions are provided for all the components of the Green tensor in both regions inside and outside the cylinder. As an application of the results for the Green function, we consider the radiation of a charged particle rotating around a dielectric cylinder. The intensities for all types of radiation processes are discussed. They include the synchrotron-Cherenkov radiation at large distances from the cylinder and the radiation on guided and surface polaritonic modes confined inside or near the surface of the cylinder. The paper provides explicit formulas for the electromagnetic fields and the spectral-angular densities of those radiations. It also includes a numerical and comparative analysis.

physics.optics↗
arXiv2026

Fermionic Casimir densities for a uniformly accelerating mirror in the Fulling-Rindler vacuum

We investigate the local characteristics of the Fulling-Rindler vacuum for a massive Dirac field induced by a planar boundary moving with constant proper acceleration in $(D+1)$-dimensional flat spacetime. On the boundary, the field operator obeys the bag boundary condition. The boundary divides the right Rindler wedge into two separate regions, called RL and RR regions. In both these regions, the fermion condensate and the vacuum expectation value (VEV) of the energy-momentum tensor are decomposed into two contributions. The first one presents the VEVs in the Fulling-Rindler vacuum when the boundary is absent and the second one is the boundary-induced contribution. For points away from the boundary, the renormalization is reduced to the one for the boundary-free geometry. The total VEVs are dominated by the boundary-free parts near the Rindler horizon and by the boundary-induced parts in the region near the boundary. For a massive field the boundary-free contributions in the fermion condensate and the vacuum energy density and effective pressures are negative everywhere. The boundary-induced contributions in the fermion condensate and the energy density are positive in the RL region and negative in the RR region. For a massless field the fermion condensate vanishes in spatial dimensions $D\geq 2$, while the VEV of the energy-momentum tensor is different from zero. This behavior contrasts with that of the VEVs in the Minkowski vacuum for the geometry of a boundary at rest relative to an inertial observer. In the latter case, the fermion condensate for a massless field is nonzero, while the VEV of the energy-momentum tensor becomes zero. The obtained results are used to investigate the VEV of the fermionic energy-momentum tensor in weak gravitational fields and background geometries that are conformally related to Rindler spacetime.

hep-th↗
arXiv2025

Radiation of surface polaritons by an annular beam coaxially enclosing a cylindrical waveguide

We investigate the radiation of surface polaritons by an annular beam that coaxially encloses a cylindrical waveguide surrounded by a homogeneous medium. By using the Green dyadic, the electromagnetic potentials and the electric and magnetic fields are found inside and outside the waveguide. The expression for the energy losses is derived for the general case of the dispersion for dielectric permittivities inside and outside the cylinder. A comprehensive analysis is presented in the spectral range corresponding to the radiation of surface polaritons. The highest peaks in the spectral distribution are obtained for intermediate values of the beam velocity. In the limit of transparent medium the spectrum of radiated surface polaritons is discrete and the corresponding frequencies are determined by the eigenvalue equation for the cylindrical waveguide. Numerical examples are presented for the Drude model of dispersion.

physics.optics↗
arXiv2024

Quasidiscrete spectrum Cherenkov radiation by a charge moving inside a dielectric waveguide

We investigate the Cherenkov radiation by a charge uniformly moving inside a dielectric cylindrical channel in a homogeneous medium. The expressions for the Fourier components of the electric and magnetic fields are derived by using the electromagnetic field Green tensor. The spectral distribution of the Cherenkov radiation intensity in the exterior medium is studied for the general case of frequency dispersion of the interior and exterior dielectric functions. It is shown that, under certain conditions on the dielectric permittivities, strong narrow peaks appear in the spectral distribution. The spectral locations of those peaks are specified and their heights and widths are estimated analytically on the base of the dispersion equation for the electromagnetic eigenmodes of the cylinder.

physics.optics↗
arXiv2023

Surface polariton excitation and energy losses by a charged particle in cylindrical waveguides

We investigate the emission of surface polaritons (SPs) by a charged particle moving inside a dielectric cylinder parallel to its axis. It is assumed that the cylinder is immersed into a homogeneous medium with negative dielectric permittivity in the spectral range under consideration (active medium). The expressions for scalar and vector potentials and for electromagnetic field strengths are provided inside and outside the cylinder. Those fields are expressed in terms of the SP eigenmodes of the waveguide and we give detailed analysis for their properties. The SP energy fluxes through the plane perpendicular to the cylinder axis are evaluated in the interior and exterior media. The energy flux is directed towards the charge motion inside the cylinder and towards the opposite direction in the exterior region. The relativistic effects may essentially increase the radiated energy. Important features of relativistic effects include the possibility of essential increase of the radiated energy, the narrowing the confinement region of the SP fields near the cylinder surface in the exterior region, enlarging the frequency range for radiated SPs, and the decrease of the cutoff factor for radiation at small wavelengths compared with the waveguide radius. The general results are specified for the Drude dispersion in the exterior medium. By using the Green tensor we also evaluate the total energy losses of the charged particle for general case of the interior and exterior dielectric functions. The corresponding results are compared with those previously discussed in the literature. The numerical data are presented in terms of scale invariant quantities that allows to clarify the features of the SP radiation for different values of the waveguide radius.

physics.optics↗
arXiv2023

Coherent radiation from a chain of charged particles on a circular orbit around a dielectric ball

We investigate the electromagnetic radiation from a chain of relativistic charged particles uniformly rotating along equatorial orbit around a dielectric ball. For weak absorption of radiation in the ball material, at certain rotation frequency the radiation intensity becomes significantly higher than the corresponding value for a chain of charges rotating in free space or in a transparent medium with the same dielectric constant as that for the ball material. In the case of an equidistant distribution of charged particles along the orbit we determine the values of parameters of the problem for which the charges in the chain emit coherently. It is shown that the radiation intensity on a given harmonic increases in proportion to the square of the number of emitting charges. We also show that the relative shifts in the particles locations up to 10\% do not destroy the coherence properties of the radiation. It is demonstrated that the coherence effects may also dominate in the radiation intensity for chains with non-equidistant distributions of particles. The numerical results obtained for different dielectric balls have revealed the emitted radiation to be in the GHz/THz frequency ranges. The high-power radiation of the chain propagates in the angular range with respect to the rotation plane determined by the Cherenkov condition for the velocity of the chain projection onto the ball surface.

physics.acc-ph↗
arXiv2020

Cherenkov radiation and emission of surface polaritons from charges moving paraxially outside a dielectric cylindrical waveguide

We investigate the radiation from a charged particle moving outside a dielectric cylinder parallel to its axis. It is assumed that the cylinder is immersed into a homogeneous medium. The expressions are given for the vector potential and for the electric and magnetic fields. The spectral distributions are studied for three types of the radiations: (i) Cherenkov radiation (CR) in the exterior medium, (ii) radiation on the guided modes of the dielectric cylinder, and (iii) emission of surface polaritons. Unlike the first two types of radiations, there is no velocity threshold for the generation of surface polaritons. The corresponding radiation is present in the spectral range where the dielectric permittivities of the cylinder and surrounding medium have opposite signs. The spectral range of the emitted surface polaritons becomes narrower with decreasing energy of the particle. The general results are illustrated for a special case of the Drude model for dispersion of the dielectric permittivity of the cylinder. We show that the presence of the cylinder may lead to the appearance of strong narrow peaks in the spectral distribution of the CR in the exterior medium. The conditions are specified for the appearance of those peaks and the corresponding heights and widths are analytically estimated. The collective effects of particles in bunches are discussed.

physics.acc-ph↗
arXiv2020

Self-amplification of radiation from an electron bunch inside a waveguide filled with periodic medium

We investigate the radiation from a bunch of relativistic electrons moving along the cylindrical waveguide axis, assuming that the waveguide is partially loaded by a medium with periodic dielectric permittivity and magnetic permeability. The spectral distribution of the radiation energy flux through the cross section of the waveguide is studied at large distances from the medium. The analysis is based on the corresponding exact solution of Maxwell equations for the case of a single electron moving along the waveguide axis. The results of numerical calculations are presented in the special case of layered medium consisting of a finite number of dielectric plates separated by vacuum gaps. We show that under certain conditions on the problem parameters the quasi-coherent Cherenkov radiation generated by the electron bunch inside the plates is self-amplified at certain waveguide modes. A visual explanation of this phenomenon is provided that reproduces the main features to rather good accuracy.

physics.acc-ph↗
arXiv2020

Angular distribution of high power radiation from a charge rotating around a dielectric ball

We study the angular distribution of the radiation from a relativistic charged particle uniformly rotating along an equatorial orbit around a dielectric ball. Earlier it was shown that for some values of the problem parameters and in the case of weak absorption in the ball material, the radiation intensity on a given harmonic can be essentially larger than that for the same charge rotating in the vacuum or in a homogeneous transparent medium having the same real part of dielectric permittivity as the ball material. The generation of such high power radiation is a consequence of the constructive superposition of electromagnetic oscillations of Cherenkov radiation induced near the trajectory of the particle and partially locked inside the ball. The angular distribution of the number of the emitted quanta is investigated for such high power radiation. It is shown that the radiation is mainly located in the angular range near the rotation plane determined by the Cherenkov condition for the velocity of the charge image on the ball surface. The numerical analysis is given for balls made of strontium titanate, melted quartz and teflon in the gigahertz and terahertz frequency ranges.

physics.acc-ph↗
arXiv2019

Synchrotron radiation from a charge circulating around a cylinder with negative permittivity

We investigate the radiation from a charged particle rotating around a dielectric cylinder with a negative real part of dielectric permittivity. For the general case of frequency dispersion in dielectric permittivity, expressions are derived for the electric and magnetic fields and for the angular density of the radiation intensity on a given harmonic. Compared with the case of a cylinder with positive real part of the permittivity, new interesting features arise in the nonrelativistic limit and for the radiation at small angles with respect to the cylinder axis. Another feature is the appearance of sharp narrow peaks in the angular density of the radiation intensity for large harmonics. We analytically estimate the height, width and the location of these peaks. The influence of the imaginary part of dielectric permittivity on the characteristics of the peaks is discussed. The analytical results are illustrated by numerical examples. We show that the radiation intensity on a given harmonic, integrated over the angles, can be essentially amplified by the presence of the cylinder.

physics.optics↗
arXiv2012

Stimulated excitation of resonant Cherenkov radiation at a large number of neighbouring waveguide modes

The resonance Cherenkov radiation generated from a train of equally-spaced unidimensional electron bunches travelling along the axis of a hollow channel inside an infinite cylindrical waveguide filled with (weakly dispersing) transparent dielectric has been investigated. It was shown that its excitation might be stimulated at a large number of neighboring modes of the waveguide. A visual explanation of this effect is given and the possibility of its observation in the range of terahertz radiation is discussed.

physics.acc-ph↗
arXiv2012

Self-amplified Cherenkov radiation from a relativistic electron in a waveguide partially filled with a laminated material

The radiation from a relativistic electron uniformly moving along the axis of cylindrical waveguide filled with laminated material of finite length is investigated. Expressions for the spectral distribution of radiation passing throw the transverse section of waveguide at large distances from the laminated material are derived with no limitations on the amplitude and variation profile of the layered medium permittivity and permeability. Numerical results for layered material consisting of dielectric plates alternated with vacuum gaps are given. It is shown that at a special choice of problem parameters, Cherenkov radiation generated by the relativistic electron inside the plates is self-amplified. The visual explanation of this effect is given and a possible application is discussed.

physics.acc-ph↗
arXiv2009

Radiation from a charged particle-in-flight from a laminated medium to vacuum

The radiation from a charged particle-in-flight from a semi-infinite laminated medium to vacuum and back,- from vacuum to the laminated medium, has been investigated. Expressions for the spectral-angular distribution of radiation energy in vacuum (at large distances from the boundary of laminated medium) were obtained for both the cases with no limitations on the amplitude and variation profile of the laminated medium permittivity. The results of appropriate numerical calculations are presented and possible applications of the obtained results are discussed.

cond-mat.other↗
arXiv2009

Some features of electromagnetic field of charged particle revolving about dielectric ball

A relativistic electron uniformly rotating along an equatorial orbit around a dielectric ball may generate Cherenkov radiation tens of times more intense as that in case of revolution of a particle in a continuous, infinite and transparent medium. The root-mean-square values of electric and magnetic field strengths of particle are practically not localized in the central part of the equatorial plane of ball and close to the poles of ball.

physics.class-ph↗
arXiv2007

On features of the radiation from an electron moving along a helix inside a cylindrical hole in a homogeneous dielectric

The radiation from a charge moving along a helical trajectory inside a cylindrical hole in homogeneous dielectric medium is investigated. Prompted by availability of materials with large dielectric permittivity $ε$ and small absorption, we discuss the features of this type of radiation for media with $ε\gg 1$. It is shown that there are high peaks in the angular distribution of radiation intensity at well-defined harmonics. The conditions are specified for the cavity-to-helix radii ratio, $ρ_{1}/ρ_{0}$, under which the angle-integrated radiation intensity on some harmonics exceeds that in the empty space. Though the amplification of radiation intensity increases with increasing $ε$, the corresponding "resonant" values of $ρ_{1}/ρ_{0}$ ratio are practically independent of the dielectric permittivity of surrounding medium. It is shown that an analogous amplification of radiation takes place essentially for the same values of $ρ_{1}/ρ_{0}$ also for the radiation in a cylindrical waveguide with conducting walls. An explanation of this phenomenon is given.

hep-th↗