TM-like and TE-like Modes Coupling in a Two-Dimensional Photonic Crystal Slab Composed of Truncated Cone Silicon Rods

We numerically investigate the coupling of TE-like modes and TM-like modes in a two-dimensional （2D） photonic crystal （PC） slab composed of truncated cone silicon rods. In such structures, the classification of TE-like modes and TM-like modes is generally impossible and the coupling occurs due to vertical structural asymmetries. The frequency and wavevector dependences of the mode coupling are discussed by investigating the photonic band structures, and the coupling efficiency is studied by examining the transmittance. The results show that the efficiency of mode conversion is strengthened by the vertical asymmetry and weakened by the clear sinall gap. These structures could be used as polarization conversion devices in integrated optics.     

Effects of nonlinear mode coupling on the chaotic dynamics of multimode lasers

The dynamics of individual modes of a cw multimode dye laser in a 2-mirror configuration has been studied theoretically and experimentally. Numerical solutions of the laser rate equations including nonlinear mode coupling exhibit chaotic and stochastic behavior in the regime of high and low power, respectively. Chaotic behavior due to mode coupling has been observed in the operating regime well above threshold. The mode dynamics in this case is characterized by a chaotic attractor with low dimension between 2 and 3. With decreasing laser power the dimension increases, suggesting stochastic behavior due to quantum noise in the limit of the laser threshold.On leave at 1. Institut für Experimentalphysik, Universität Hamburg, Fed. Rep. Germany     

Nonadiabatic dynamics of two strongly coupled nanomechanical resonator modes

The Landau-Zener transition is a fundamental concept for dynamical quantum systems and has been studied in numerous fields of physics. Here, we present a classical mechanical model system exhibiting analogous behavior using two inversely tunable, strongly coupled modes of the same nanomechanical beam resonator. In the adiabatic limit, the anticrossing between the two modes is observed and the coupling strength extracted. Sweeping an initialized mode across the coupling region allows mapping of the progression from diabatic to adiabatic transitions as a function of the sweep rate.     

Exciton-phonon coupling strength in single wall carbon nanotubes

The coupling strength of the exciton and different phonons, including the radial breathing mode (RBM), longitudinal (LO) and transverse (TO) optical phonons, are calculated for different diameter single wall carbon nanotubes (SWNTs) in the framework of tight-binding model. It is found that the exciton-phonon coupling strength with the LO mode or RBM shows a clear (2n+m)-family behavior due to the trigonal warping effect, but it with the TO mode remains to be zero. In the same SWNT, the E₂₂ exciton-phonon coupling strength is found to be slightly smaller than that of E₁₁ exciton. Finally, the exciton-RBM-phonon coupling strengths for several SWNTs are found to be in good agreement with the recent experimental data [Phys. Rev. Lett. 98, 037405 (2007)].     

Coupled-mode theory of a fiber laser-amplifier

The behavior of a fiber laser-amplifier is considered, including the effects of mode coupling, radiation losses, and noise. In the linear regime, a competitive effect between mode coupling and losses is present, in the sense that mode coupling tends to equalize the gains of different modes, while radiation losses favor low-order modes. Thus, single-mode amplification and collective amplification are predominant, respectively, in the weak and strong coupling case. In the saturated regime a mode coupling effect is found that is independent of fiber imperfections and relies on the nonlinear nature of the process.     

The quasi-confined optical phonons in wurtzite GaN/AlN multiple quantum wells

Within the framework of the dielectric-continuum model and Loudon's uniaxial crystal model, the equation of motion for p-polarization field in arbitrary wurtzite multilayer heterostructures are solved for the quasi-confined phonon (QC) modes. The polarization eigenvector, the dispersion relation, and the electron-QC interaction Fröhlich-like Hamiltonian are derived by using the transfer-matrix method. The dispersion relations and the electron-QC coupling strength are investigated for a wurtzite GaN/AlN single QW. The results show that there are infinite branches of dispersion curve with definite symmetry with respect to the center of the QW structure. The confinement of the quasi-confined phonons in the QW leads to a quantization of q_z,j characterized by an integer m that defines the order of corresponding quasi-confined modes. The QC modes are more dispersive for decreasing m. The QC modes display an interface behavior in the barrier and a confined behavior in the well. The symmetric modes have more contribution to electron-QC interaction than the antisymmetric modes. The strains have more effect on symmetry modes, and can be ignored for symmetry modes.     

Coupled-mode theory of a fiber laser-amplifier

Abstract

The behavior of a fiber laser-amplifier is considered, including the effects of mode coupling, radiation losses, and noise. In the linear regime, a competitive effect between mode coupling and losses is present, in the sense that mode coupling tends to equalize the gains of different modes, while radiation losses favor low-order modes. Thus, single-mode amplification and collective amplification are predominant, respectively, in the weak and strong coupling case. In the saturated regime a mode coupling effect is found that is independent of fiber imperfections and relies on the nonlinear nature of the process.     

Coupled modes of the torsion pendulum

The coupling of the swing modes of a torsion pendulum to the torsional mode has been solved analytically. Our solution provides a clear explanation of why a magnetic damper is effective in suppressing unwanted modes in most gravitational experiments. The analytic solution also shows that the amplitude of the mode coupling is greatest at the lowest frequencies. This explains why mode coupling at the lowest frequency is all that is observed in the experiments reported here.     

Phonon-phonon interactions in (Lu, Y)Ni2B2C

The temperature dependence of the Δ₄ acoustic and optic mode, measured at q=(0.5,0,0) in LuNi₂B₂C, YNi₂B₂C were extended to 1000 K and new measurements on the mixed compound, Y_0.5Lu_0.5Ni₂B₂C, were undertaken to explore the mode coupling in these materials. The temperature behavior of each is different and cannot be explained by a simple mode coupling scheme of the acoustic and optic modes.     

Attachment Instabilities of SF6 Inductively Coupled Plasmas under Different Coupling Intensities

Characteristics of attachment instabilities in SF6 inductively coupled plasmas are experimentally studied under different coupling intensities. Experimental results show that the instabilities only occur in H modes operating in positive feedback regions. Both the sudden mode transitions and the instabilities are influenced by the coupling intensities. With increasing absorbed power, weak and middle coupling discharges can sequently undergo sudden mode transitions and attachment instabilities. In strong coupling discharges, the sudden mode transitions disappear and only attachment instabilities exist. The strong and weak coupling discharges are the most stable and unstable, respectively.     

Infrared spectra of the superionic conductors Na,K, Rb,Ag and Ti β-aluminates

A detailed analysis of the infrared reflectivity spectra of the superionic conductors X β-aluminates (X = Na, K, Ag, Rb and Tl) is presented. A Kramers-Kronig analysis and the oscillator fits to the data have yielded the frequencies of the transverse optic and associated longitudinal optic modes. A model which ignores the coupling between the spinel blocks accounts for the number of observed infrared modes as well as the Raman modes. It is shown that even for the modes characteristic of the motions of the disordered X-ions, the wave vector selection rule is still valid. The results are discussed with the available theoretical calculations.     

双层耦合非对称反应扩散系统中的超点阵斑图

通过线性耦合Brusselator模型和Lengyel-Epstein模型,数值研究了双层耦合非对称反应扩散系统中图灵模之间的相互作用以及斑图的形成机理.模拟结果表明,合适的波数比以及相同的对称性是两个图灵模之间达到空间共振的必要条件,而耦合强度则直接影响了图灵斑图的振幅大小.为了保证对称性相同,两个图灵模的本征值高度要位于一定的范围内.只有失稳模为长波模时,才能对另一个图灵模产生调制作用,并形成多尺度时空斑图.随着波数比的增加,短波模子系统依次经历黑眼斑图、白眼斑图以及时序振荡六边形斑图的转变.研究表明失稳图灵模与处于短波不稳定区域的高阶谐波模之间的共振是产生时序振荡六边形的主要原因.     

Synchronization and control of chaos in coupled chaotic multimode Nd:YAG lasers

In this Letter we numerically investigate the dynamics of a system of two coupled chaotic multimode Nd:YAG lasers with two mode and three mode outputs. Unidirectional and bidirectional coupling schemes are adopted; intensity time series plots, phase space plots and synchronization plots are used for studying the dynamics. Quality of synchronization is measured using correlation index plots. It is found that for laser with two mode output bidirectional direct coupling scheme is found to be effective in achieving complete synchronization, control of chaos and amplification in output intensity. For laser with three mode output, bidirectional difference coupling scheme gives much better chaotic synchronization as compared to unidirectional difference coupling but at the cost of higher coupling strength. We also conclude that the coupling scheme and system properties play an important role in determining the type of synchronization exhibited by the system.     

Discrete modulational instability and bright localized spin wave modes in easy-axis weak ferromagnetic spin chains involving the next-nearest-neighbor coupling

We report a theoretical work on the properties of modulational instability and bright type nonlinear localized modes in one-dimensional easy-axis weak ferromagnetic spin lattices involving next-nearest-neighbor couplings.With a linear stability analysis, we calculate the growth rates of the modulational instability, and plot the instability regions.When the strength of the next-nearest-neighbor coupling is large enough, two new asymmetric modulational instability regions appear near the boundary of the first Brillouin zone.Furthermore, analytical forms of the bright nonlinear localized modes are constructed by means of a quasi-discreteness approach.The influence of the next-nearest-neighbor coupling on the Brillouin zone center mode and boundary mode are discussed.In particular, we discover a reversal phenomenon of the propagation direction of the Brillouin zone boundary mode.     

单模双包层光纤之间的耦合

本文求得了单模双包层光纤之间耦合系数精确的解析表达式.计算了上升内包层、匹配包层和凹陷内包层光纤耦合系数随归一化频率V的关系曲线.也给出了不同V值的耦合系数随归一化距离(D/α)的关系曲线.该公式不但能够计算x偏振模的耦合系数,而且也能计算y偏振模的耦合系数.它可用于分析折射率差较大的光纤之间能量耦合以及耦合器的偏振特性.     

Fibre Bragg Gratings Inscribed in Homemade Microstructured Fibres

Fibre Bragg gratings （FBGs） are inscribed in homemade microstructured fibres by the standard phase mask method. Enhanced couplings between the forward fundamental mode and backward cladding modes are obtained. The mode coupling and spectral characteristics are investigated experimentally. The cladding mode resonances can be affected by filling active materials into the air holes, which will be useful for the implementation of tunable photonic devices in optical fibre communication and sensing systems.     

Multiple polarization of geodesic curvature induced modes

Dispersion relations for geodesic acoustic modes are derived by using the Grad hydrodynamic equations thereby reconciling long known but not previously explained discrepancy between the results of kinetic and fluid calculations. Extended fluid theory allows a simple analysis of mode polarization and coupling. A new type of electromagnetic modes induced by geodesic compressibility is predicted. These modes are related to Alfvén and geodesic acoustic modes. While a standard geodesic acoustic mode involves poloidally and toroidally symmetric perturbations of electrostatic potential (m=n=0) and the first poloidal side-bands of plasma pressure, new modes involve side-bands of the electrostatic and vector potential as well as pressure perturbations at zeroth and second harmonics. It is shown that there exist two different values of the adiabatic constant depending on the mode polarization. Both standard (electrostatic) geodesic acoustic modes and new electromagnetic modes involve finite perturbations of parallel viscosity, which modify an effective adiabatic (compressibility) index for a toroidal plasma.     

Numerical studies of surface and bulk modes in attenuated total reflection spectra

The dispersion relations of normal modes in the layered constructions are studied. In the frequency region with negative permittivity, we investigate the normal modes near the air--metal and the air--matematerial interfaces and compare them. An extra bulk mode appearing in a pass band of two media is found near the air--matematerial interface, which is different from the air--metal case. Moreover, the bulk mode will be further proved by the attenuated total reflection (ATR) technique, in which the coupling between the incident electromagnetic waves and the normal modes is studied. For p-polarized incident waves, the ATR spectra based on the Otto and the Kretschmann configurations are obtained numerically, where the dips present the excitations of the surface and bulk modes. Furthermore, we also discuss the influences of the middle layer thickness, the incident angle and the damping term on the reflection in detail. It is found that the coupling frequencies and the maximum strength are strongly dependent on the above media parameters.     

Distinguishing the Topological Zero Mode and Tamm Mode in a Microwave Waveguide Array

The Su–Schrieffer–Heeger (SSH) model has been the subject of extensive experimental research in the context of topological photonics. Ideally, the on‐site potential and hopping strength are sufficiently accurate for implementation in photonic coupled waveguide arrays. Here, two localized edge modes, the topological zero mode and trivial Tamm mode, are proposed and demonstrated in the modified SSH model using a microwave photonic waveguide array. The system used is composed of an array of evanescently coupled ultrathin corrugated metallic “H‐bar” waveguides. Furthermore, the differences between the zero mode and Tamm mode are clearly demonstrated by microwave near‐field experiments on the coupling behavior along the propagation direction for 40 cm at the excitation frequency of 17 GHz. These findings should deepen the understanding of localized edge mode confinement mechanisms, both in coupled waveguide array systems and other topological or quantum systems.     

Degenerate band edges in optical fiber with multiple grating: efficient coupling to slow light

Degenerate band edges (DBEs) of a photonic bandgap have the form (ω-ω(D)) ∝k(2m) for integers m>1, with ω(D) the frequency at the band edge. We show theoretically that DBEs lead to efficient coupling into slow-light modes without a transition region, and that the field strength in the slow mode can far exceed that in the incoming medium. A method is proposed to create a DBE of arbitrary order m by coupling m optical modes with multiple superimposed gratings. The enhanced coupling near a DBE occurs because of the presence of one or more evanescent modes, which are absent at conventional quadratic band edges. We furthermore show that the coupling can be increased or suppressed by varying the number of excited evanescent waves.