Bare Modes Research Articles

Magnetostatic modes hybridization in left-handed cylindrical shells

In this paper, we study theoretical formalism of the surface magnetostatic modes of a left-handed cylindrical shell (i.e. a cylindrical shell with both negative dielectric permittivity and negative magnetic permeability) with a dielectric core and provide an explicit form of the dispersion relation for surface magnetostatic modes, in terms of interaction between the bare modes of the individual surfaces of the cylindrical shell. In the case of a metallic core, we found only one magnetostatic mode for each angular momentum.

Plasmon hybridization in metallic nanotubes

We study theoretical formalism of the plasmon hybridization in a metallic nanotube and find an explicit form of the dispersion relation for surface plasmons, in terms of interaction between the bare plasmon modes of the individual surfaces of the nanotubes. In the special case when the longitudinal wave vector is zero ( q = 0 ) , the plasmon hybridization of a nanotube has a behavior similar to the spherical nanoshell.

Plasma wave propagation in a pair of carbon nanotubes

Plasma waves which propagate in a pair of parallel metallic carbon nanotubes are studied within the framework of the classical electrodynamics. Electronic excitations over the each nanotube surface are modeled by an infinitesimally thin layer of free-electron gas which is described by means of the linearized hydrodynamic theory. An explicit form of plasmons dispersion in terms of interaction between the bare plasmon modes of the individual surfaces of the nanotubes is presented in this Letter.

Plasmon hybridization in spherical nanoparticles.

We show that the plasmon resonances in single metallic nanoshells and multiple concentric metallic shell particles can be understood in terms of interaction between the bare plasmon modes of the individual surfaces of the metallic shells. The interaction of these elementary plasmons results in hybridized plasmons whose energy can be tuned over a wide range of optical and infrared wavelengths. The approach can easily be generalized to more complex systems, such as dimers and small nanoparticle aggregates.

The screening of polar optical modes in a semiconductor quantum well

The screening of the electrostatic potential of the first two long wave polar optical modes in a semiconductor quantum well is studied for various situations. The potential of the bare modes was obtained in a model accounting for the full coupling between electrical and mechanical vibrations. The dielectric response function of the screening electron gas is described in a quasi 2D multisubband version of the random phase approximation and Hubbard (that explicitly includes exchange) approximations, using in the calculation in all cases the occupied and some of the empty subbands. The purpose of this work is to determine the relative importance of the design parameters.

Optical properties of a laterally confined semiconductor microcavity containing a single quantum well

The semiconductor microcavity with a thin oxide-aperture layer is fabricated, and linear optical transmission spectrum measured for various aperture diameters. First, the observation of bare cavity modes is demonstrated in this microstructure which is capable of confining light field three-dimensionally. Several transverse modes are observed as transmission peaks, which manifests the lateral field confinement achieved well down to 2 μm aperture diameter. And the transmission spectrum of cavity modes coupled to the excitonic resonance is measured for the same microcavity system containing a single quantum well. The result shows that each transverse mode couples to an exciton independently as it approaches the excitonic resonance frequency, giving rise to an anti-crossing behavior between coupled modes.

Screening ineffectiveness and THz emission at bare LO phonon frequencies

Within a hydrodynamic approach we investigate the dynamics of an inhomogeneous electron-hole gas coupled to phonons in Te and the corresponding THz emission. We find that the longitudinal inhomogeneity of the photogenerated electron-hole gas---due to short absorption lengths in Te---gives rise to screening ineffectiveness for nonzero wave vector modes. This allows for THz dynamics and emission at the bare LO phonon frequency ${\ensuremath{\nu}}_{\mathrm{LO}}$ even at high carrier densities. Screening ineffectiveness thus provides an appealingly simple explanation for the existence of bare modes at ${\ensuremath{\nu}}_{\mathrm{LO}}$ in longitudinally inhomogeneous systems such as Te; no lateral inhomogeneity of the excitation spot is needed here.

Resonant scattering and mode coupling in two-dimensional textured planar waveguides.

A heuristic formalism is developed for efficiently determining the specular reflectivity spectrum of two-dimensionally textured planar waveguides. The formalism is based on a Green's function approach wherein the electric fields are assumed to vary little over the thickness of the textured part of the waveguide. Its accuracy, when the thickness of the textured region is much smaller than the wavelength of relevant radiation, is verified by comparison with a much less efficient, exact finite difference solution of Maxwell's equations. In addition to its numerical efficiency, the formalism provides an intuitive explanation of Fano-like features evident in the specular reflectivity spectrum when the incident radiation is phase matched to excite leaky electromagnetic modes attached to the waveguide. By associating various Fourier components of the scattered field with bare slab modes, the dispersion, unique polarization properties, and lifetimes of these Fano-like features are explained in terms of photonic eigenmodes that reveal the renormalization of the slab modes due to interaction with the two-dimensional grating. An application of the formalism, in the analysis of polarization-insensitive notch filters, is also discussed.

INDIRECT THERMAL DRYING OF LIGNITE: DESIGN ASPECTS OF A ROTARY DRYER

ABSTRACT An investigation of the thermal drying of lignite has been carried out, by using an indirect heat pilot rotary drum. The process aims at the production of dry lignite and clean steam as part of a gasification procedure. Both flighted and bare drum modes have been employed. Temperature profiles along the dryer length, the amount of evaporation (moisture conversion) and the solids residence time distribution (RTD) were measured. A non-isothermal model was tested under three different regimes of solids flow. Model integration, by taking account of experimental amount of evaporation at dryer exit and temperature profiles along the dryer length, has been utilized in the validation of drying kinetics and heat transfer correlations. Model predictions compare satisfactorily with the operating data of an indirect heat industrial lignite dryer. Overall heat transfer coefficients of the pilot rotary dryer were found to agree well with those reported for direct heat dryers.

Low-frequency vibrations in a model glass

Previous simulation studies of localized low-frequency vibrational modes in a soft sphere glass are extended to larger samples. We find a boson peak in the spectral density and a maximum in the specific heat divided by T similar to experiment. These maxima are caused by ~quasi!localized vibrations. For finite frequencies these interact strongly with each other and with the extended phonons. This leads to an overdamping of the phonons around the boson peak and to a delocalization of the localized modes. A procedure to split the resulting modes into their bare constituents is presented. The bare localized modes are found to have a low-dimensional structure. The description in terms of the soft potential model is essentially verified.

Quasiharmonic periodic traveling-wave solutions in anharmonic potentials.

Exact solutions of a nonlinear diatomic shell model are investigated in the regime where quasiharmonic and pseudoperiodic traveling waves exist with phonon-type character. The existence regimes of these solutions are determined by the boundary conditions and the model parameters for which especially a strong time and mass dependence is observed. It is found that slowly propagating waves mostly show large displacement responses which can be associated with large dipole moments, while rapidly traveling waves carry a much smaller dipole moment, but still this is appreciably larger than that induced by bare optic-phonon modes. The case of large anharmonicity shows the opposite effect. Here high-frequency responses carry a large dipole moment. The large dipole moments can be associated with effective charges which induce high oscillator strengths in the corresponding phonon modes, incompatible with results deduced from harmonic lattice dynamics. The origin of the large dipole moments in the shell model is investigated by solving for core and shell displacements separately, where ``acoustic-type'' periodic in-phase displacements of core and shell with different amplitudes are observed, as well as pseudoperiodic out-of-phase ``optic-type'' displacements resulting from large anharmonicity, and are also found in the static limit. Besides the displacement frequency spectrum, the effective potentials are calculated which are distinctly different from ${\mathrm{\ensuremath{\varphi}}}_{4}$-type potentials: The potential height is finite with finite width which, in certain cases becomes very small, thus admitting for tunneling through the barrier. \textcopyright{} 1996 The American Physical Society.

Leaky axisymmetric modes in infinite clad rods. I

A detailed computational study is presented for the radial–axial modes— both leaky and nonleaky—in an infinitely clad isotropic rods. The complex phase velocities of leaky modes are located using an application of the argument principle. Particle orbits are determined, and leaky modes are shown to have an asymptotic leakage angle away from the interface. By using the homotopic methods of varying densities and elastic constants, clad-rod modes are compared with those in a bare rod. The topology of the clad-rod mode dispersion diagram differs qualitatively from that of a bare rod, even when the cladding has negligible density, with no velocity cutoffs and with wave mode knitting. Comparison is also given with modes occurring in a cladding without a rod present (a tunnel) and for a planar interface. Most leaky modes can be correlated with rod modes; only a limited number of tunnel modes exist. Energy flow contours within modes are also calculated. The local energy velocity, which generalizes group velocity, can vary considerably in the radial direction for bare rod modes. For leaky modes the contours are quite complex due to the cylindrical geometry, giving rise to apparent shift in wave position across the interface.

Numerical solution of the exact cavity equations of motion for an unstable optical resonator

We solve numerically, we believe for the first time, the exact cavity equations of motion for a realistic unstable resonator with a simple gain saturation model. The cavity equations of motion, first formulated by Siegman ["Exact Cavity Equations for Lasers with Large Output Coupling," Appl. Phys. Lett. 36, 412-414 (1980)], and which we term the dynamic coupled modes (DCM) method of solution, solve for the full 3-D time dependent electric field inside the optical cavity by expanding the field in terms of the actual diffractive transverse eigenmodes of the bare (gain free) cavity with time varying coefficients. The spatially varying gain serves to couple the bare cavity transverse modes and to scatter power from mode to mode. We show that the DCM method numerically converges with respect to the number of eigenmodes in the basis set. The intracavity intensity in the numerical example shown reaches a steady state, and this steady state distribution is compared with that computed from the traditional Fox and Li approach using a fast Fourier transform propagation algorithm. The output wavefronts from both methods are quite similar, and the computed output powers agree to within 10%. The usefulness and advantages of using this method for predicting the output of a laser, especially pulsed lasers used for coherent detection, are discussed.

Central-peak—soft-mode coupling in ferroelectricGd2Gd2(MoO4)3

Transmission measurements on ${\mathrm{Gd}}_{2}$ ${(\mathrm{M}\mathrm{o}{\mathrm{O}}_{4})}_{3}$ in the (5-50)-${\mathrm{cm}}^{\ensuremath{-}1}$ region were performed with use of tunable backward-wave oscillator sources (5-30 ${\mathrm{cm}}^{\ensuremath{-}1}$) and a Fourier spectrometer (30-50 ${\mathrm{cm}}^{\ensuremath{-}1}$). The resulting dielectric spectra show an additional low-frequency dispersion which was fitted with a standard central-peak model. Its characteristic relaxation frequency is \ensuremath{\sim}20 ${\mathrm{cm}}^{\ensuremath{-}1}$ and the coupling between the soft mode and central mode increases near the transition temperature. This model also accounts very well for the weak anomaly in the clamped permittivity ${\ensuremath{\epsilon}}_{c}$ measured at 63 MHz. The same central mode was used to fit earlier Raman soft-mode spectra. All of these data were fitted with a three-coupled-mode model which revealed that the soft-mode spectrum consists of two strongly coupled bare modes: a higher-frequency mode which softens and carries the entire Raman strength and a lower-frequency mode which is hard (59 ${\mathrm{cm}}^{\ensuremath{-}1}$) and Raman inactive. Both of these modes are also coupled to the central mode and this coupling increases sharply near the transition. The relatively large width of the central mode indicates its intrinsic nature and suggests partial disorder near the transition.