The effect of chemical composition of a crystallization medium on the cBN stoichiometry

Abstract

The effect of chemical composition of the crystallization medium in synthesis on and some properties of cubic boron nitride (cBN) has been studied. Cubic BN crystals with the lowest free boron content and a boron-nitrogen ratio close to the stoichiometric one are grown in a multicomponent crystallization medium in a Li₃N-BN system. These crystals are characterized by a higher strength and thermal stability.     

Torque equations for spin and orbital angular momenta of radiation fields and Faraday effect

The spin angular momentum S of light has never been linked to the Faraday rotation of light traveling in an optically active medium possessing a rotational invariance of a crystal, because there was no helicity term associated with the phase shift in the previous torque equation for S. In order to relate the change in S with time to the Faraday rotation, therefore, we derived an exact torque equation for S. As a result, a magnetic helicity term appeared in a new torque equation for S, so that one-half of the phase shift derived from the helicity term was equivalent to the Faraday rotation angle. However, the orbital angular momentum L had no relation to the Faraday rotation. It was thus clarified that the change in S with time is related to the Faraday rotation angle of light traveling in an optically active medium, owing to the appearance of the helicity term without a rotational invariance around the optical axis. It was also demonstrated theoretically that the Faraday rotation is accompanied by a torque acting on the crystal so that the total angular momentum of light and matter is conserved.     

Optics of moving media

Light experiences a moving medium as an effective gravitational field. In the limit of low medium velocities the medium flow plays the role of a magnetic vector potential. We review the background of our theory [U. Leonhardt and P. Piwnicki, Phys. Rev. A 60, 4301 (1999); Phys. Rev. Lett. 84, 822 (2000)], including our proposal of making optical black holes. Received: 4 July 2000 / Revised version: 17 October 2000 / Published online: 6 December 2000     

Efficient nonlinear-optical frequency conversion in periodic media in the presence of diffraction of the pump and harmonic fields

It has been predicted by Shelton and Shen [Phys. Rev. A 5, 1867 (1972)] and observed by Kajikawa et al. [Jpn. J. Appl. Phys. Lett. 31, L679 (1992)] and Yamada et al. [Appl. Phys. B 60, 485 (1995)] that the efficiency of nonlinear-optical frequency conversion increases significantly in a nonlinear periodic medium and, accordingly, the intensity of the generated harmonic increases as the fourth power of the sample thickness, as opposed to the square law observed in homogeneous media. In this paper it is shown that the same enhancement of the efficiency of nonlinear-optical frequency conversion in a nonlinear periodic medium can be achieved using an ordinary pump wave in the form of a plane wave when both the pump wave and the harmonics are diffracted by the periodic structure of the nonlinear medium. The phenomenon is analyzed quantitatively in the example of second-harmonic generation. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 12, 793–799 (25 December 1999)     

Homogenization of a set of parallel fibres

Abstract

Rigorous results concerning the possibility of homogenizing a set of parallel fibres are given from the viewpoint of electromagnetic scattering. We deal with the classical time-harmonic Maxwell problem and distinguish the two cases of polarization, i.e. electric field parallel to the fibres (E∥) and magnetic field parallel to the fibres (H∥). Assuming a low density of fibres, we obtain, in the E∥ case, an effective medium with a possibly negative permittivity, whereas in the H∥ case the fibres disappear completely. On the other hand, for a high density of fibres, the E∥ case leads to a perfectly conducting medium and the H∥ case to a dielectric medium with a surface current on its boundary.     

Mean value of an effective refractive index for stratified random media

Abstract

We introduce an effective refractive index of a one-dimensional random medium, which describes a wavefield propagating in this medium. The probability density of the effective index and its mean value are found due to the diffusion approximation.     

Casimir-Polder interaction of excited media

The potential of long-range interaction between two dissimilar atoms, one of which is excited, drops as 1/R ₂ with the distance for the Casimir-Polder limit of large distances in comparison with the wave-length of atom transitions (E.A. Power and T. Thirunamachandran, Phys. Rev. A 51, 3660 (1995)). It is shown that such a dependence, obtained with the help of perturbation technique, results in a divergence for the interaction potential between an excited atom and a medium of dilute gas. We develop a nonperturbative method based upon quantum Green’s functions (Yu. Sherkunov, Phys. Rev. A 72, 052703 (2005)) to calculate the interaction potential for an excited atom and a ground-state atom embedded in a dielectric medium, taking into account the absorption of photons in the dielectric medium. The exponential suppression of the interaction between the atoms is demonstrated. The force acting on an excited atom near the interface of dilute gas medium is calculated. The result is no more divergent. The force between gas media in Casimir-Polder regime is calculated as well. The text was submitted by the author in English.     

The random analogue of Epstein's transition layer

Abstract

For a quasi-homogeneous, random medium with variance, varying along the same direction as tanh, the mean Green function is obtained as an exact solution of Dyson's equation in a bilocal approximation. The coherent part of the field of a plane wave, falling on a bounded, randomly fluctuating medium with a non-sharp boundary, is studied in detail. In the case of small-scale fluctuations, a medium of this kind is shown to be a random analogue of a transient Epstein layer.     

Role of stationary photon statistics in a high-Q cavity mode while it is exciting the active laser medium

It has been shown by Yu. M. Golubev, M. I. Kolobov, and I. V. Sokolov, Zh. éksp. Teor. Fiz. 111, 1579 (1997) [JETP 84, 864 (1997)], that when an optical cavity is excited by external radiation from a sub-Poisson laser the cavity mode may be in either a sub-Poisson or a Poisson stationary state. This is not important for a resonant medium which is excited into the upper laser level while interacting with this mode inside the cavity. The degree of regularity of the excitation will be identical to that of the initial light flux incident on the cavity, and this ultimately ensures the same sub-Poisson lasing as for strictly regular pumping of the resonant medium. Zh. éksp. Teor. Fiz. 113, 1223–1234 (April 1998)     

Quasi elasto-electromagnetic surface waves on a piezo-plasma-like layer

Considering a piezo-plasma-like layer with finite thickness and hexagonal symmetry whose main symmetry axis is parallel to the z axis and approximating it by an isotropic medium, we study the coupling of the elastic wave with plasma properties of the medium with and without spatial dispersion and collisions. In this case we investigate the coupled surface quasi elasto-electromagnetic wave propagating on the interface of piezoelectric layer with vacuum. Furthermore, the coupling of elasticity and ion-acoustic waves is investigated.     

Phase fluctuations and localization length in layered randomly inhomogeneous media

Abstract

We consider the problem of the inclined incidence of a plane wave on the halfspace of a randomly inhomogeneous layered medium without absorption. The probability distribution of the reflection coefficient phase with respect to the incidence angle is calculated. With the help of this distribution we analyse the quantities connected with localisation effects—the length localization and moments of the intensity inside the medium.     

On the influence of two coexisting species of susceptibility-producing structures on the R21 relaxation rate

PurposeTissue microstructure can influence quantitative magnetic resonance imaging such as relaxation rate measurements. Consequently, relaxation rate mapping can provide useful information on tissue microstructure. In this work, the theory on relaxation mechanisms of the change of the relaxation rate ∆R₂^⁎ in the presence of spherical susceptibility sources in a spin bearing medium is validated in simulations and phantom experiments for the coexistence of two species of susceptibility sources.MethodsThe influence of coexisting spherical perturbers with magnetic susceptibilitys of different signs was evaluated in Monte Carlo simulations including diffusion effects in the surrounding medium. Simulations were compared with relaxometry measurements at 1.5 Tesla and at 3 Tesla. The phantoms used to validate the simulations were built from agarose gel containing calcium carbonate and tungsten carbide particles of different size and concentration.ResultsThe Monte Carlo simulations showed, that the change in relaxation rate only depends on the overall amount of susceptibility producing structures in the simulation volume and no difference was found, if mixtures of positive and negative particles were simulated. Phantom measurements within the static dephasing regime showed linear additivity of the effects from positive and negative susceptibility sources that were present within the same voxel.ConclusionsIn summary, both the simulations and the phantom measurements showed that changes in the relaxation rate ΔR₂^⁎ add up linearly for spherical particles with different susceptibilities within the same voxel if the conditions for the static dephasing regime are fulfilled. If particles with different susceptibilities have both different sizes and violate the conditions of the static dephasing regime, effects on relaxation rates might no longer be linear.     

Plane waves in a layered weakly dissipative randomly inhomogeneous medium

Abstract

In the framework of the embedding method the authors consider the stationary and non-stationary problem of a plane-wave incident on a randomly inhomogeneous medium. For the stationary problem there are three regions of sufficiently different behaviour of the wavefield intensity moments inside a weakly dissipative medium. For the non-stationary problem they succeeded in calculating the average intensity at t→+∞ by means of analytical prolongation of the stationary problem solution with respect to the absorption parameter. The time asymptotic of the averaged intensity on the boundary slab is also obtained for a finite-thickness slab.     

Wave diffraction by rough interfaces in an arbitrary plane-layered medium

Abstract

The problem of electromagnetic wave scattering by a slightly rough interface in an arbitrarily layered medium is solved by a small-perturbation method. The bistatic amplitude of scattering as well as the scattering cross sections for statistically rough surfaces are calculated for linear polarized waves. Along with scattering into up-going waves in a homogeneous medium and scattering cross sections in down-going waves into a layered medium, scattering amplitudes from a rough interface in the arbitrarily layered medium are obtained.     

Propagation of a beam of gamma rays in the field of a monochromatic laser wave

The head-on propagation of a beam of γ grays through the field of a laser wave is investigated. The optical properties of the laser wave (as a medium) are described by the dielectric tensor. The refractive indices are determined, and the polarization characteristics of electromagnetic normal modes capable of propagating in such a medium are investigated. Relations are derived to describe the variation of the initial polarization and intensity of a γ-ray beam as it propagates in a laser field. The influence of laser intensity on the investigated process is discussed. Zh. éksp. Teor. Fiz. 112, 2016–2029 (December 1997)     

Propagation of a monochromatic electromagnetic plane wave in a medium with nonsimple motion

An exact analytical solution is obtained for the trajectory of the wave vector of a monochromatic electromagnetic plane wave in a medium with nonsimple motion. It is shown that the spatial dragging of the electromagnetic wave by the moving medium can be described correctly in the general case only if relativistic terms of order β ² are taken into account. Zh. Tekh. Fiz. 69, 97–100 (May 1999)     

Correlation characteristics of waves in lossy media with smooth fluctuations of the refractive index with oblique incidence on boundary

Abstract

The features of coherence function (and angular spectrum) and also the fluctuation of amplitude and phase of wavefield in a random strongly absorptive medium are investigated in the case of arbitrary angle of incidence at the surface.

In this paper it is elucidated that with oblique incidence a dissipation in the random medium can accelerate the accumulation of wave fluctuations and its incoherence. This effect strongly depends on the rate of decrease of the ‘wings’ of the scattering indicatrix. An analytical theory (Rytov's approximation and modified method of parabolic equation) has been modelled by Monte Carlo simulation of wave propagation, and also by numerical solution of the model transfer equation. It is revealed that the width of an angle spectrum can nonmonotonically change with the immersion to the absorptive medium.     

High-Pressure X-ray Studies of Zn at Room and Low Temperatures with a He-Pressure Medium

High-pressure powder X-ray diffraction experiments have been done on Zn with a He-pressure medium at room and low temperatures. No anomaly in the c / a axial ratio has been found even at low temperatures. This implies that the c / a anomaly, which is predicted to appear associated with the electronic topological transitions, would be extremely small.     

Statistics of amplified light in periodically correlated layered systems with randomness

Abstract

We study the statistics of reflection and transmission coefficients of light in randomly layered amplifying media that are periodic on average. We are interested in one-dimensional universal scaling behaviours in such systems. Our study shows that while a homogeneous medium boundary condition is capable of reproducing universal scaling at low frequencies, a periodic medium boundary condition is necessary for high frequencies. Although the statistics depends on the boundary condition, the saturation length, where the reflection coefficient reaches a stationary distribution, and the localization length do not. Implications of these results are discussed.     

Statistical inversion from reflections of spherical waves by a randomly layered medium

Abstract

We show that the asymptotic theory developed for the analysis of reflected wave pulses from randomly layered media can be used to solve statistical inverse problems. In particular, we recover the large scale behavior of medium properties from a single realization of reflected wave pulses observed at different points on the surface.