Excitation Of Medium Research Articles

Technique for reducing surface wave at an air/left‐handed medium (LHM) interface or excitation of the forward wave in an LHM

AbstractIn this article, we analyze the illumination by an electromagnetic wave of an air/left‐handed medium (LHM) interface. It is shown that by placing a periodic structure of resistors near the LHM it is possible to suppress or to reduce the surface wave that is present at the interface. For our demonstration, we consider a semi‐infinite lossless left‐handed medium having the same constitutive parameters as those of air but with opposite signs. Numerical results, obtained by using a full‐wave time domain technique, are presented to validate our approach and the proposed scheme. This work can extend the potential applications of left‐handed materials, because it shows that it is possible to use LHM slabs having large width. In addition, this work shows that it is possible to excite the forward wave of an LHM instead of the backward wave. © 2008 Wiley Periodicals, Inc. Microwave Opt Technol Lett 51: 280–284, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24036

Generalized radiation law for excited media in a nonequilibrium steady state

We consider the interacting system of light and matter and present quantum-kinetically exact relations for the propagation of light through arbitrarily absorbing and dispersive steadily excited media in a specified geometry. We arrive at an energy flow law which may be regarded as a generalization of the Kirchhoff and Planck radiation laws to nonequilibrium. The field fluctuations are shown to generally split up into medium- and vacuum-induced contributions, of which only the latter governs the energy transport between the medium and the environment. A thorough derivation of the law is given and it is discussed together with interesting details in the underlying relations and physics. Especially, in the context of excited semiconductors, quasiequilibrium emission and lasing, as well as spectral features in the presence of quantum condensation are addressed.

Analytical decoupling of poroelasticity equations for acoustic-wave propagation and attenuation in a porous medium containing two immiscible fluids

Poroelasticity theory has become an effective and accurate approach to analyzing the intricate mechanical behavior of a porous medium containing two immiscible fluids, a system encountered in many subsurface engineering applications. However, the resulting partial differential equations in the theory intrinsically take on a coupled form in the terms pertinent to inertial drag, viscous damping, and applied stress, making it difficult to obtain closed-form, steady-state analytical solutions to boundary-value problems except in special cases. In the present paper, we demonstrate that, for dilatational wave excitations, these partial differential equations can be decoupled analytically into three Helmholtz equations featuring complex-valued, frequency-dependent normal coordinates that correspond physically to three independent modes of dilatational wave motion. The normal coordinates in turn can be expressed in the frequency domain as three different linear combinations of the solid dilatation and the linearized increment of fluid content for each pore fluid, or equivalently, as three different linear combinations of total dilatational stress and two pore fluid pressures. These representations are applicable to strain-controlled and stress-prescribed boundary conditions, respectively. Numerical calculations confirm that the phase speed and attenuation coefficient of the three dilatational waves represented by the Helmholtz equations are exactly identical to those obtained previously by numerical solution of the dispersion relations for dilatational wave excitation of a porous medium containing two immiscible fluids. Thus, dilatational wave motions in unsaturated porous media subject to suitable boundary conditions can now be accurately modeled analytically.

Fano Profiles Induced by Near-Field Coupling in Heterogeneous Dimers of Gold and Silver Nanoparticles

The near-field coupling between a gold and a silver spherical nanoparticle is investigated theoretically. Fano profiles are observed in the absorption cross section of the gold nanoparticle due to the coupling between the spectrally localized surface plasmon resonance of the silver nanoparticle and the continuum of interband transitions of the gold one. The effect of dimer internal characteristics (particle sizes and distance), surrounding medium (through the refractive index), and external excitation (polarization and propagation directions) are addressed. In particular, it is shown that the near-field coupling can be tuned from the weak to the strong regime by rotating the polarization direction, and that the Fano profiles are sensitive to the shadowing effect even for small particle sizes.

Light efficiency of cathodoluminescent screens excited by a runaway electron beam

A technique employing electron beams generated by an open gas discharge is proposed for measuring the light efficiency of phosphor coatings of cathodoluminescent screens. The total light efficiencies of various phosphor coatings in the medium excitation energy range (ɛ < 7 keV) are estimated with allowance for both the direct radiation flux outgoing from the phosphor screen and the backward radiation flux propagating along the exciting electron beam. The possibility is demonstrated of creating a high-luminance (∼20000 cd/m2) cathodoluminescent source with a light efficiency of ∼60 lm/W.

Frequency shifting of sub-100 fs laser pulses by stimulated Raman scattering in a capillary filled with pressurized gas

The technique of Raman conversion of sub-100 fs laser pulses based on excitation of active medium by two orthogonally polarized pulses has been developed for Raman lasers with a glass capillary. 52 fs Stokes pulse at the wavelength of 1200 nm has been generated by stimulated Raman scattering of 48 fs Ti:sapphire laser pulse at the wavelength of 800 nm in hydrogen. 13% energy conversion efficiency has been achieved at pulse repetition rate up to 2 kHz.

Quantum theory of microchip lasers with intracavity SRS-conversion

Quantum theory of diode-pumped passively Q-switched microchip lasers with intracavity transient and quasi-stationary stimulated Raman scattering (SRS) conversion has been developed, where dynamics of Raman medium excitation, collective vibration dephasing, and intracavity multiwave mixing of the laser, Stokes and anti-Stokes waves are taken into account. By means of canonical transformation we construct the Hamiltonian describing the intracavity electromagnetic field, the Raman medium and their interaction. A branching set of coupled Heisenberg–Langevin dynamic equations has been obtained for the amplitudes of the generated laser, Stokes and anti-Stokes waves, the collective vibration amplitudes and the population of the excited states of the Raman medium. It is shown that the ordinary phenomenological and quasi-stationary semi-classical models for this class of lasers are the special cases of the developed here quantum description. The simulations based on the presented theory match well experimental data on the dependence of the duration and energy of the Stokes pulses on the pump power.

Kinematics of rigidly rotating spiral waves

Spiral waves rigidly rotating in excitable media are studied by use of a free-boundary approach. This study reveals the selection principle which determines the shape and the rotation frequency of spiral waves in an unbounded medium with a given excitability. It is shown that a rigidly rotating spiral in a medium with a strongly reduced refractoriness is supported within a range of the medium excitability restricted by two universal limits. At the low excitability limit the spiral core radius diverges, while at the high excitability limit it vanishes. The simulations performed for the medium excitability higher than the high excitability limit reveal nonstationary rotating waves, which considerably differ from well-studied meandering spiral waves. It is shown how the proposed free-boundary approach can be extended to the case of an arbitrary refractoriness. The predictions of the free-boundary approach are in good agreement with the results from numerical simulations of the underlying reaction-diffusion model and with asymptotics derived earlier for highly and weakly excitable media.

One- and two-dimensional standing pressure waves and one-dimensional travelling pulses using the US-NRC nuclear systems analysis code TRACE

A variety of nuclear system transients can lead to rapid and large local pressure changes that propagate along the hydraulic system at the speed of sound, both in single phase and in two-phase fluids. Because of the relevance for safety issues, nuclear system codes like TRACE need to be assessed with respect to their capabilities to predict pressure wave behaviour. Therefore, we have analyzed the propagation of pressure waves in one-dimensional and two-dimensional configurations, i.e. a pipe and a slab, filled with liquid water. The pressure waves are driven by one-sided pressure boundary conditions, in the one-dimensional case of harmonic or Gaussian shape and in the two-dimensional case also of harmonic shape. The selected harmonic pressure boundary conditions lead to standing pressure waves, while using the Gaussian shape boundary conditions one-dimensional pressure pulses are injected and propagate through the pipe. The agreement of the TRACE results with the analytical solutions are, in general very good to good for the one-dimensional cases with respect to the pressure maxima and a small difference is only obtained in the wave speed. At the resonance frequencies of the one-dimensional standing waves, the code is tested to the extreme and shows that enforcing small time step sizes is crucial for the performance of the code. Non-linear effects are observed in the code results for the large amplitudes encountered at the closest neighborhood of the resonances, where the analytical linear standing wave solution diverges and the linear approximation is outside of its validity range. Also for these non-linear standing waves TRACE yields qualitatively physically correct behaviour as the pressure amplitudes are limited and a plateau is reached. For the one-dimensional pressure pulse of Gaussian shape the change of pulse amplitude and shape was analyzed in a longer system. The maximum amplitude of the pulse is slightly reduced as the pulse travels along the pipe. The effect of numerical diffusion on leading and trailing fronts is slightly asymmetric due to the donor-cell approach used in the numerical integration scheme of TRACE. The accuracy of the code is not negatively influenced by the reflections of the pulse at the boundaries of the pipe. As for the standing waves, the accuracy of the travelling pulse solution calculated by TRACE is negatively affected when the time steps are too large, while the effects of the spatial discretization are rather minor. For the case of two-dimensional standing waves in a slab, a lowest spatial harmonic generated with one wave node in the direction parallel to the pressure driving boundary is considered. TRACE results show an overall good agreement with the linear analytical solution. This good agreement includes for low to medium excitation frequencies the damping properties of the skin effect perpendicular to the pressure boundary, which does not exist in one-dimensional pressure wave propagation, the transition to a harmonic shape of the wave also perpendicular to the pressure boundary and the frequency dependence of the resonance spectrum for further increased frequencies with the rapid changes of the wavelengths encountered. Effects of the model set-up and code limitations with respect to the two-dimensional TRACE model set-up using a TRACE VESSEL component in connection with pressure boundary conditions are discussed, in particular with respect to the underestimation of the damping in the skin effect frequency range and the numerical damping for higher frequencies. All in all, the TRACE code is able to calculate one- and two-dimensional pressure wave propagation in liquid water, when an appropriate spatio-temporal numerical discretization is chosen.

Kinetics of the active medium of a He—Sr+ recombination laser: 2. Achievable energy characteristics

The mechanisms limiting the improvement of the energy characteristics of He—Sr+ recombination lasers with increasing pressure, active volume, and pulse repetition rate are analysed. It is found that the existence of optimal pressure is related to the cooling rate limitation for electrons at high pressures arising from the heating action of the trailing edge of a current pulse and that the average power saturation with increasing discharge tube diameter and pulse repetition rate occurs due to the formation of radial nonuniformity as well as due to the growth of the electron temperature in the afterglow. Calculations involving a mathematical model showed that the attainable maximum of the linear power is equal to ≈6.2 W m-1 for self-heating active elements of BeO ceramics and to ≈7.8 W m-1 with its surface blackened, to ≈7.7 W m-1 for an independent introduction of the vapour and to ≈9.4 W m-1 in combination with blackening, to ≈17 W m-1 for intense forced cooling of the active elements of cylindrical geometry and to ≈29 W m-1 with active elements of rectangular cross section for a wall dimension ratio of 1:3. We demonstrate the feasibility of raising the output power by ≈21% in the excitation of the active medium by pulse bursts with a short interpulse separation.

High-Order Harmonic Transient Grating Spectroscopy in a Molecular Jet

We study high-order harmonic generation in excited media using a four-wave-mixing-like configuration. We analyze the spatial profile of high harmonics emitted by a grating of rotationally excited molecules as a function of the pump-probe delay. We demonstrate a dramatic improvement in the contrast of the diffracted signal relative to the total high harmonic signal. This allows us to observe subtle effects in the rotational wave packet excitation such as the pump-intensity dependence of the wave packet dynamics. High harmonic transient grating spectroscopy can be extended to all forms of molecular excitation and to weak resonant excitation.

Kinematics of wave segments moving through a weakly excitable medium

Traveling wave segments represent typical spatio-temporal patterns in various active media. In particular they appear by a clustering of interacting active Brownian particles under a global constrain. Wave segments are intrinsically unstable and define a separatrix between spiral wave behavior and contracting wave segments. A kinematical model is developed to describe evolution of a wave segment shape during its stabilization by a global feedback to the medium excitability. It is also applied to specify the relationships between the medium excitability and the asymptotic shape of the stabilized wave segment. The model predictions are compared with results from numerical simulations of the underlying reaction-diffusion system.

Investigation of Optimum Field Amplitude for Stress Dependence of Magnetic Barkhausen Noise

An investigation of the sensitivity of magnetic Barkhausen noise (MBN) to changes in uniaxial tensile stress applied to mild steel under conditions of low, medium, and high excitation field amplitudes is presented. Field excitation in the lowest range that was tested produced the most sensitive response to change in stress for the squared-voltage-signal output integrated with respect to time (MBNenergy). The linear response extended to the highest applied stress of 220 MPa, which was 80% of yield strength. In this field range, the angle-dependent MBNenergy and distribution of pulse heights under applied stress conditions was observed to be consistent with previous measurements performed in the medium- to high-field amplitude ranges. However, the number of Barkhausen events was less, and increased linearly with applied stress up to 120 MPa. In contrast, at higher field amplitudes, the initial number of events was greater, but then decreased with increasing applied stress. The results at low field amplitudes are attributed to the additional 180deg domain wall activity, accompanied by increased magnetization level, which could be induced by tensile stress. The reduction in number of events at higher stresses and higher excitation field amplitudes, which corresponds to an increase in MBNenergy and broadened pulse-height distribution, is associated with increased collective Barkhausen behavior. These results demonstrate that improved sensitivity of Barkhausen signals to changes in stress can be achieved in mild steel if optimized field excitation is used.

Cleaning properties of atomic oxygen excited to metastable state 2 s 2 2 p 4 ( S 1 )

A possibility to treat surfaces of various materials by bombardment with neutral atomic oxygen excited to the metastable state 2s22p4(S10), (4.1891 V from the ground) is described. The oxygen dissociation and excitation was developed in plasmas generated in reactors of capacitively coupled dielectric barrier discharge configurations comprised of a quartz or ceramic tube passing throughout two annular electrodes and containing a gas mixture of ∼98% Ar with ∼2% O2 ionized at the atmospheric pressure by the radio frequency discharge at 13.56 MHz and flowing at about 6 m/s. As shown in the experiments, the transition 2s22p4(S10)−2s22p4(D12) with a high degree of probability is responsible for the yellow color of afterglow products, and the lifetime of the metastable state can be as long as 5.4 ms in certain cases, allowing one to separate the excited atomic medium from plasma that it produced. It has been shown that the cleaning process included significant Van der Waals bonds liberation to the depth of several hundred angstroms of surfaces treated, which drastically reduced the percentage of carbon-containing contaminants and changed, to some degree, the chemical structure of surfaces containing chemically bonded oxygen.

Intrathecal Administration of Proteinase-Activated Receptor-2 Agonists Produces Hyperalgesia by Exciting the Cell Bodies of Primary Sensory Neurons

Proteinase-activated receptors (PARs) are a family of G-protein-coupled receptors that are activated by endogenous serine proteinases that cleave the N-terminal domain of the receptor unmasking a "tethered ligand" sequence. Trypsin and other agonists at PAR(2) act on peripheral nerves to augment the transfer of nociceptive information. We tested whether PAR(2) agonists also exert a spinal pronociceptive effect by i.t. administering the selective ligand, Ser-Leu-Ile-Gly-Arg-Leu-NH(2) (SLI-GRL). This produced thermal and mechanical hyperalgesia in rats and mice and augmented mechanical and thermal hyperalgesia seen in the formalin inflammatory pain test. Effects of SLIGRL were abrogated in PAR(2)-deficient mice and were not seen with the inactive control peptide, Leu-Arg-Gly-Ile-Leu-Ser-NH(2). Surprisingly, electrophysiological studies, using whole-cell recording from rat substantia gelatinosa neurons, failed to demonstrate an increase in excitatory transmission or neuronal excitability following treatment with SLIGRL or trypsin. In fact, the actions of trypsin were consistent with a decrease in dorsal horn excitability. SLIGRL and trypsin did, however, depolarize and increase the excitability of large, medium and small primary afferent, dorsal root ganglion neurons. The effects were associated with an increase in conductance at hyperpolarized potentials and a decrease in conductance at depolarized potentials. PAR(2)-like immunoreactivity was found in DRG but not in spinal dorsal horn. These results suggest that activation of DRG neuron cell bodies may account for the pronociceptive actions of i.t. applied PAR(2) agonists. They also imply that pathophysiological release of PAR(2)-activating proteases in the vicinity of DRG neurons may produce profound effects on nociceptive processing in vivo.

Casimir-Polder interaction of excited media

The potential of long-range interaction between two dissimilar atoms, one of which is excited, drops as 1/R2 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.

Effect of inductive-plasma UV preionization on the parameters of XeCl laser radiation

A XeCl laser with a double plasma electrode providing automated inductive-plasma UV preionization has been experimentally studied. Ways to increase the laser efficiency by selecting a proper regime of excitation of the active medium based on various buffer gases (helium, argon, and neon) are considered, depending on the parameters of a two-stage pumping source. It is shown that the scheme of pumping with an inductive-plasma UV preionization source is effective only with a XeCl laser using neon as a buffer gas excited in a fast regime, which can provide a significant increase in the energy parameters of output radiation and, accordingly, in the laser output efficiency.

Selection mechanism for rotating patterns in weakly excitable media

Two types of patterns rigidly rotating within a disk of a weakly excitable medium are studied using the free-boundary approach. The patterns are spots moving along the boundary of the disk and spiral waves rotating around the disk center. The study reveals a selection mechanism that uniquely determines the shape and the angular velocity of these patterns as a function of the medium excitability and the disk radius. These two types of patterns coexist below some critical parameter value, coincide at a bifurcation point, and do not exist above it. The same selection mechanism is applied to describe a limiting case of a spiral wave rotating in an unbounded medium.

Dynamics of chemical wave segments with free ends

A quite simple but useful approach is performed for the analysis of chemical wave segments with free ends. By integrating a reaction–diffusion system we can obtain an analytical expression to understand the dynamics of the wave segments. This integration can yield qualitative information regarding wave development under an external forcing having feedback or noise effects. We conclude that this wave development is influenced not only by medium excitability but also by wave size.

Suppression of Winfree turbulence under weak spatiotemporal perturbation

Winfree turbulence is a chaotic wave pattern developing through negative-tension instability of scroll wave filaments in three-dimensional weak excitable media. Here, we investigate the response of Winfree turbulence to a spatiotemporal forcing in the form of a traveling-wave modulation of the medium excitability. It is shown that turbulent waves can be suppressed much more rapidly by this method, in comparison with the space-uniform modulation of the medium excitability. Since the occurrence of Winfree turbulence is currently regarded as one of the possible mechanisms underlying cardiac fibrillation, this method turns out to be suggestive of a possible low-amplitude defibrillation approach.