Instabilities Propagated Along the External Magnetic Field in Magnetized Active Plasmas

The conditions for spontaneous onset of instabilities propagated along the external (stationary or quasi-stationary) magnetic field in cold and magnetized active plasmas are considered in collisionless approximation. Attention is given to non-resonant cases, i.e. to situations in which the modal frequencies of the wave significantly differ from the frequency of coherent electromagnetic radiation arising due to the active particles. It is shown that instabilities are also possible in these non-resonant cases and that, under certain specified conditions, they develop only in a very narrow wavelength band. The pertaining amplitude increments are evaluated.     

On the excitation of ion cyclotron and electron cyclotron waves by beams of charged particles

The instabilities of the electrostatic waves of a hot plasma in a magnetic field, excited by beams of charged particles, have been studied in some special cases within the framework of the linear theory. We are interested in short-wavelength oscillations with a transverse wavelength compareable with the Larmore radius of ions or electrons, with frequencies in the vicinity of the harmonic cyclotron frequency of ions or electrons. Expressions have been derived for the increments of these instabilities in the case of a beam with an isotropic distribution function in the velocity space, as well as with an anisotropic distribution function. The influence of the beam temperature on the character of the instabilities and the conditions of formation of the instabilities are discussed, and the order of magnitude of the increments is estimated.The authors thanks J. Václavík and V. Kopecký for valuable discussions and comments.     

Phonon-polariton waves on the surface of SiC crystal

The excitation and propagation of traveling phonon-polariton waves on the surface of silicon carbide (SiC) excited by light at a frequency close to the lattice resonance have been investigated. These waves are excited in the presence of the boundary of a metal mask deposited on the crystal surface. The use of the Green’s function has been shown to provide good qualitative agreement with the observed distribution of the amplitude and phase of the field on the surface under these experimental conditions. It has been shown that only consideration of the mask boundary as an extended source of traveling surface waves, which removes the inhibition of the generation of waves in the noncoincidence of the wave vector, cannot quantitatively describe the phenomenon. The spatial resolution of the used scanning near-field optical microscope is no worse than 150 nm at a wavelength of 10 μm.     

The dissipative instability of a relativistic electron beam

The theory of the wall instabilities of surface waves excited by an electron beam is developed. Their classification, linear, quasilinear and nonlinear theory is given. It is shown that dissipative instabilities convert practically all the energy lost by the beam into the heating of the lossy walls confining the beam.     

Optical Trapping with Focused Surface Waves

Near-field optical trapping can be realized with focused evanescent waves that are excited at the water–glass interface due to the total internal reflection, or with focused plasmonic waves excited on the water–gold interface. Herein, the performance of these two kinds of near-field optical trapping techniques is compared using the same optical microscope configuration. Experimental results show that only a single-micron polystyrene bead can be trapped by the focused evanescent waves, whereas many beads are simultaneously attracted to the center of the excited region by focused plasmonic waves. This difference in trapping behavior is analyzed from the electric field intensity distributions of these two kinds of focused surface waves and the difference in trapping behavior is attributed to photothermal effects due to the light absorption by the gold film.     

Active coated nanoparticles: impact of plasmonic material choice

The near- and far-field properties of a number of active coated spherical nanoparticles excited by an electric Hertzian dipole at optical frequencies are investigated. Their enhanced, as well as reduced, radiation effects are demonstrated and compared.     

Wavelength and beam launching effects on silica optical fiber in local area networks

In this paper, we report the modal dispersion of silica graded-index optical fibers as a function of the input mode parameters and lunching conditions in local area network (LAN) context. In that, we examine the mode-depending parameters, namely, modal delay, modal attenuation and mode-coupling effects as a function of wavelength. We show that the number of excited mode groups depends strongly on the spot beam radius when the fiber is excited with an axial Gaussian beam where we find an optimal axial diameter exciting only two mode groups. We present a comparison of the number of excited mode groups, the optimal spot radius beam, the signal penalty and the 3-dB baseband bandwidth enhancement for the optimal axial launching compared with full mode excitation, offset launching and mode-field matched axial launching.     

三平行光子晶体单模波导的耦合特性及其应用

将三光子晶体单模波导的相互耦合看成一个多模干涉系统.本征模的色散曲线相交或近于相交并出现简并模，简并模之间存在强烈耦合并导致模式的分布方式发生转变.多模干涉系统中，不同波长的光波能量在传输过程中由于相干而具有不同的空间输出行为，在近简并点处多模之间的相干解除，能流限制在原输入方向，不发生转移.三光子晶体单模光波导的这种特性可用来制作波分复用或解复用器件. 关键词： 光子晶体波导 简并模 多模干涉 波分复用或解复用     

Three-dimensional negative index of refraction at optical frequencies by coupling plasmonic waveguides

We identify a route towards achieving a negative index of refraction at optical frequencies based on coupling between plasmonic waveguides that support backwards waves. We show how modal symmetry can be exploited in metal-dielectric waveguide pairs to achieve negative refraction of both phase and energy. Control of waveguide coupling yields a metamaterial consisting of a one-dimensional multilayer stack that exhibits an isotropic index of -1 at a free-space wavelength of 400?nm. The concepts developed here may inspire new low-loss metamaterial designs operating close to the metal plasma frequency.     

Excitation of surface electromagnetic waves at an anisotropically conducting vacuum-metamaterial interface by the attenuated total internal reflection method

The excitation of well-localized oblique surface waves above the surface of a dielectric with a one-dimensional array of perfectly conducting wires is studied theoretically using the attenuated total internal reflection method. It is assumed that the distance between the wires and their diameter are much smaller than the surface wavelength. The frequencies of excited surface waves are much lower than the plasma frequency of the metal, and their electric field is orthogonal to the wires. It is shown that such surface waves can be excited with the help of a homogeneous TM wave as well as with the help of a homogeneous wave with an electric field polarized perpendicularly to the wires. It is found that in the course of excitation of oblique waves, the incident TM wave is partly polarized into a wave of the TE type.     

含周期性空腔结构吸声机理的研究

为了研究水下含周期性空腔结构的吸声机理,建立并验证了含轴对称空腔周期性结构吸声特性计算的简化有限元仿真方法,用简化的有限元模型结合遗传算法对水下环境含周期性圆柱空腔结构的吸声性能进行了优化设计.从能量耗散、变形和模态的角度分析了含周期性空腔结构的吸声机理.空腔结构谐振包括表层的弯曲振动和空腔附近的径向变形,且径向运动也...     

Molecular forces caused by the confinement of thermal noise

We have investigated spontaneous surface instabilities of very thin polymer films. Film stability and the wavelength of the dominating unstable mode were found to depend sensitively on the media adjacent to the film. Our experimental results cannot be explained by van der Waals interactions alone. To account for the presence of an additional destabilizing force, we propose that the geometrical confinement of thermally excited acoustic waves gives rise to a force that is strong enough to destabilize thin films. This thermoacoustic effect is of similar magnitude as van der Waals forces.     

Resonant absorption of shear instabilities in flow systems with energy dissipation

It is shown that hydrodynamic and electrohydrodynamic waves excited in two-layer dissipative systems due to shear instabilities experience resonant absorption of two types: at the rotational frequency of particles of the medium and the frequency of their collision with the lattice.     

Solitons in surface stabilized ferroelectric liquid crystals and the determination of the twist elastic constant

Excitation of solitary waves and their propagation in surface-stabilized ferroelectric liquid crystal cells under alternating external electric field is investigated both theoretically and experimentally. The effect of solitary waves on electro-optic response spectra is analyzed for different amplitudes of applied fields, temperatures, and sample thicknesses. It is shown that solitons can only be excited within narrow ranges of frequencies of the sufficiently strong electric fields. The minimal frequency, at which soliton waves appear in ferroelectric smectic liquid crystals, is found to be related to the material constants of these systems. It is proved that measuring this threshold frequency gives the possibility to determine one of the material parameters, if the others are known. In this way, the intra-smectic-layer elastic constant is found for systems with the chevron geometry.     

Phonon-polariton and band structure of electro-magneto-acoustic SH wave propagation oblique to the periodic layered piezoelectric structures

Electro-magneto-acoustic SH waves propagating oblique to the periodic layered piezoelectric structures are studied under the coupling of the acoustic wave and the electromagnetic wave. Band structures of the so-called piezoelectric superlattice and phononic/photonic crystal are given both at acoustic frequencies and at optical frequencies. For the periodic layered piezoelectric structures, phonon-polaritons (the coupling modes of the phonons and photons) are found not only happening near the center of the Brillouin zone (in the long-wavelength limit) at acoustic frequencies, but also being able to appear in the whole Brillouin zone at optical frequencies. Appearing of these phonon-polaritons may provide a way to design a new type of acousto-optic devices.     

Evanescent Plane Waves in Multilayer Structures

We have considered evanescent plane waves in structures with a layer of a substance with ε, μ < 0 and with a layer of a well-reflecting metal, ε < 0, μ ≥ 1. Waves with increased amplitude as compared with the initial wave have been found to occur, due to which evanescent waves with wave number as in the initial wave but with increased amplitude arise behind these layers. A composite material with ε, μ < 0 at optical frequencies are proposed. Surface waves on a metal layer are considered in detail. It is shown that surface waves with a sufficiently arbitrary wave number can be excited. It is also shown that, on very thin layers, surface waves with wave number exceeding ten times that of a homogeneous plane wave in vacuum can be excited. Propagation losses are calculated. For a silver layer, the wave path can be from 30 up to 100 wavelengths. Practical use in developing techniques for optical transformations of short-wave surface waves in 2D space, similar to those in 3D space, are pointed out.     

Frequency-domain modal delay measurement for higher-order mode fiber based on stretched pulse interference

We propose and demonstrate a novel modal delay measurement technique for a higher-order mode fiber (HOF) based on optical frequency-domain reflectometry (OFDR) using an extremely simple, entirely passive, and ultrafast wavelength sweeping mechanism, namely, dispersion-induced optical pulse stretching. We obtained a high temporal resolution of approximately 1.12 ps, which was sufficient for discerning the four excited modes in an HOF with a length of only approximately 5 m. The results from our measurements were very consistent with those obtained by using a traditional time-domain measurement method and a conventional OFDR measurement based on a tunable CW laser. Our proposed technique can be also easily adapted to perform conventional time-domain modal delay measurements for very long HOFs.     

Revisiting the wire medium: an ideal resonant metalens

This article is the first one in a series of two dealing with the concept of a ‘resonant metalens’ we introduced recently. Here, we focus on the physics of a medium with finite dimensions consisting of a square lattice of parallel conducting wires arranged on a sub-wavelength scale. This medium supports electromagnetic fields that vary much faster than the operating wavelength. We show that such modes are dispersive due to the finiteness of the medium. Their dispersion relation is established in a simple way, a link with designer plasmons is made, and the canalization phenomenon is reinterpreted in the light of our model. We explain how to take advantage of this dispersion in order to code sub-wavelength wavefields in time. Finally, we show that the resonant nature of the medium ensures an efficient coupling of these modes with free space propagating waves and, thanks to the Purcell effect, with a source placed in the near field of the medium.     

Parametric magnon–phonon instability in the structure of YIG films

The effects of parametric magnon–phonon instability and the self-modulation of magnetostatic and fast magnetoelastic waves are revealed. It is found that instabilities are caused by the decay of the initial waves stimulated by high-Q acoustic resonances. Decays of the first and the second order (three- and four-magnon–phonon decays) are observed. Their characteristics and existing conditions are determined. Magnetostatic wave decays have both upper and lower thresholds. It is shown that magnetoelastic waves become stable above the upper thresholds. The existence of the upper threshold is due to several competing decay scenarios.     

Modal interference and dynamical instability in a solid-state slice laser with asymmetric end-pumping

We observed complicated emission patterns consisting of different transverse modes and associated intensity pulsations at beat frequencies between pairs of transverse eigenmodes in a solid-state thin-slice Fabry-Perot laser with asymmetric end-pumping. The dependence of transverse patterns and pulsation frequencies on pump power has been demonstrated. The interference among nonorthogonal transverse eigenmodes, which are formed in a deformed Fabry-Perot microcavity possessing an asymmetric, gradient refractive-index potential for optical waves, is proposed for explaining observed instabilities. Intensity modulations have been remarkably reproduced by numerical simulations of model equations.