Oscillating Guided Modes in Graphene-Based Asymmetric Waveguides

We investigate the guided modes in monolayer graphene-based waveguides with asymmetric quantum well structure induced by unequal dc voltages. The dispersion relation for the guided modes is obtained analytically, the structures of the guided modes are discussed under three distinct cases. For the cases of the classical motion and the Klein tunneling, the asymmetric structure does not influence the mode structures dramatically compared with that in the symmetric waveguide. But for the mixing case of the former two, the mode structures and the motion characteristics for the electron and the hole exhibit different behaviors at same condition. The results may be helpful for the practical application of graphene-based quantum devices.     

Oscillating Guided Modes in Graphene-Based Asymmetric Waveguides

We investigate the guided modes in monolayer graphene-based waveguides with asymmetric quantum well structure induced by unequal dc voltages. The dispersion relation for the guided modes is obtained analytically, the structures of the guided modes are discussed under three distinct cases. For the cases of the classical motion and the Klein tunneling, the asymmetric structure does not influence the mode structures dramatically compared with that in the symmetric waveguide. But for the mixing case of the former two, the mode structures and the motion characteristics for the electron and the hole exhibit different behaviors at same condition. The results may be helpful for the practical application of graphene-based quantum devices.     

用有效折射率法和有限元法分析多量子阱条形光波导

本文提出了一种快速和精确分析多量子阱(MQW)条形光波导的新算法。它基于有效折射率法和有限元法的结合。首先,由等效的平面波导替换多量子阱条形结构,用有效折射率法从原来问题得到平面波导的折射率分布;然后,用有限元法计算等效结构中E_(mn)~x模和E_(mn)~y模的传播常数和场强分布。文中给出了任意阱数和不同宽度多量子阱的对称结构和不对称结构的结果。     

Plasmonic crystal waveguides

We study the properties of electromagnetic waves propagating along the waveguides with a periodic core created by alternating metal and dielectric layers, the so-called quasi-one-dimensional plasmonic crystal waveguides. Such waveguides can be symmetric or asymmetric, depending on the cladding or substrate material properties, as well as on the termination of the periodic structure. We analyze the dispersion characteristics as well as the profiles of the guided modes for several types of waveguide structure.     

Fibre mode expansion for 2-D waveguide calculations

A numerical method is introduced which is devised to calculate the lower-order modes of optical waveguides with two-dimensional transverse index profiles embedded in a homogeneous cladding medium. We propose a method based on a series expansion using guided modes of an optical fibre. To test the method, it is applied to a three-layer optical-fibre and the results are compared with the exact analytical solutions. The error in the calculation of the modal effective indices is found to be less than 10−6. This method allows the modelling of symmetric 2 × 2 completely fused fibre couplers. Effective indices and polarization corrections are compared with results obtained by the field correction method. The adiabaticity criterion is calculated for these couplers. This revised version was published online in November 2006 with corrections to the Cover Date.     

Ray delay in gradient waveguides with asymmetric transverse refractive index profiles

Former investigations of symmetric refractive index profiles in gradient waveguides are extended to include also asymmetric profile curves. The generalized delay expression is derived and illustrated by means of various examples. Possibilities of compensating symmetric by asymmetric components of the refractive index in the delay formula lead to a number of optimum profiles for low-dispersion transmission.The pulse broadening in a waveguide with a gradient profile being distributed in a ring-shaped form over its cross section, which is to serve as an example of a guide with an asymmetric profile, is assessed.     

聚合物脊波导偏振特性

以有机聚合物脊波导为研究对象,采用半矢量有限差分光束传输法,系统地分析了不同脊高、脊宽和芯层厚度条件下对称脊波导的偏振特性,并在此基础上分析了非对称脊波导的偏振特性,研究了平板芯层短边宽度对偏振特性的影响。结果表明:非对称脊波导结构的引入对偏振特性影响较小,其双折射随平板芯层短边宽度的减小而减小。当脊高为1.3 m、平板芯层短边宽度为2.0 m时,非对称脊波导双折射能控制在可接受范围。     

Surface solitons in trilete lattices

Fundamental solitons pinned to the interface between three semi-infinite one-dimensional nonlinear dynamical chains, coupled at a single site, are investigated. The light propagation in the respective system with the self-attractive on-site cubic nonlinearity, which can be implemented as an array of nonlinear optical waveguides, is modeled by the system of three discrete nonlinear Schrödinger equations. The formation, stability and dynamics of symmetric and asymmetric fundamental solitons centered at the interface are investigated analytically by means of the variational approximation (VA) and in a numerical form. The VA predicts that two asymmetric and two antisymmetric branches exist in the entire parameter space, while four asymmetric modes and the symmetric one can be found below some critical value of the inter-lattice coupling parameter—actually, past the symmetry-breaking bifurcation. At this bifurcation point, the symmetric branch is destabilized and two new asymmetric soliton branches appear, one stable and the other unstable. In this area, the antisymmetric branch changes its character, getting stabilized against oscillatory perturbations. In direct simulations, unstable symmetric modes radiate a part of their power, staying trapped around the interface. Highly unstable asymmetric modes transform into localized breathers traveling from the interface region across the lattice without significant power loss.     

Guided modes in asymmetric metal-cladding left-handed material waveguides

We investigate guided modes in the asymmetric waveguide structure with a left-handed material (LHM) layer surrounded by air and metal. A graphical method is proposed to determine the guided modes. New properties of the oscillating and surface guided modes, such as absence of the fundamental mode, coexistence of the oscillating and surface guided modes, fast attenuation of the surface guided modes, and mode degeneracy, are analyzed in detail. We also investigate dispersive characteristics of the metal-LHM- air optical waveguide. The propagation constant increases with decreasing slab thickness for the first-order oscillating mode, which is different from that in traditional metal-cladding waveguides.     

Comparing guided modal properties of surface waves along single- and double-negative indexed slab waveguides

The guided dispersion characteristics of the fundamental symmetric and asymmetric modes of surface waves along single- and double-negative indexed slab waveguides are investigated, and a comparative analysis made when varying the single- and double-negative permittivity and permeability. While the values of the permittivity and permeability of the slab region are varied to obtain a salient picture of the guided dispersion characteristics, identical absolute product values are used for both slab cases to facilitate a reasonable comparison. In particular, in common ranges where guided mode solutions coexist for both the single- and double-negative indexed slabs, the guided mode characteristics are similar with a lower normalized frequency regime, indicating that the sign of the material parameters has a negligible effect, whereas the characteristics become quite different as the normalized frequency increases. Some other anomalous guided dispersion properties are also discussed and compared with previously reported results.     

Defect modes properties in periodic lossy multilayer containing negative index materials with symmetric and asymmetric geometric structures

In this paper, the characteristic matrix method is used to study the propagation of electromagnetic waves through one-dimensional lossy photonic crystals composed of negative and positive refractive index material layers with symmetric and asymmetric geometric structures with a defect layer at the center of the structure. First, the positive index material defect layer is considered, and the effects of the polarization and the angle of incidence on the defect mode in the transmission spectra of the both structures are investigated. The results show that the number of the defect modes in the transmission spectra depends on the geometry (symmetric or asymmetric) of the structure. In addition, it is shown that the defect mode frequency increases as the angle of incidence increases. This property is independent of the geometry of the structure. Then, for normal incidence, the negative index material defect layer is considered, and the properties of defect modes for both structures are investigated. The results can lead to designing new types of transmission narrow filters.     

Negative-index dispersion and accidental mode degeneracy in an asymmetric spoof-insulator-spoof waveguide

It has been recently demonstrated that negative-index dispersion and mode degeneracy can be achieved by manipulating a spoof-insulator-spoof(SIS) waveguide. In this paper, we propose a new SIS waveguide, which is composed of two spoof surface plasmon polaritons(SSPPs) waveguides drilled with periodic rhomboidal grooves. Both the symmetric and asymmetric cases are investigated. Our simulation results show that the asymmetric SIS waveguides are more significant.By tailoring the tilt of the rhomboidal grooves, the negative-index dispersion can be achieved and the microwave band gap(MBG) can be effectively modulated. At a critical tilt, there appears an accidental mode degeneracy at the edge of the first Brillouin zone. The excitation and propagation of the two coupled modes sustained by the asymmetric SIS waveguides are also demonstrated.     

Disappearance of modes in planar Bragg waveguides

We show how the forbidden bands of a Bragg reflector may shrink to points and how some classes of the guided modes of a planar Bragg waveguide may disappear altogether by shrinking with the forbidden bands. We derive the general conditions to determine the missing modes and explain these conditions with examples. It is possible, for example, to design a Bragg waveguide that rejects all antisymmetric modes and supports only symmetric modes for the TE polarization. We also highlight the effect of Brewster incidence on the interpretation of the missing modes for the TM polarization.     

Modes splitting in graphene-based double-barrier waveguides

The graphene-based double-barrier waveguides induced by electric field have been investigated. The guided modes can only exist in the case of Klein tunneling, and the fundamental mode is absent. The guided modes in the single-barrier waveguide split into symmetric and antisymmetric modes with different incident angles in the double-barrier waveguide.The phase difference between electron states and hole states is also discussed. The phase difference for the two splitting modes is close to each other and increases with the order of guided modes. These phenomena can be helpful for the potential applications in graphene-based optoelectronic devices.     

用科尔-霍普夫变换法求非均匀光波导导模本征方程的严格解

采用科尔-霍普夫(Cole-Hopf)变换法，将渐变折射率波导导模的本征值方程变换为里卡提(Riccati)方程，通过较简洁的数学演算导出导模的传播常量与模场分布的解析解，给出了平方律分布、对称爱泼斯坦(Epstein)分布、爱泼斯坦层的平板波导与平方律分布圆光纤4种折射率分布的计算公式。     

Slow light with symmetric gap plasmon guides with finite width metal claddings

We study the dispersion relation and the modes of a symmetric gap plasmon guide, where a dielectric planar slab is coated with finite metallic layers on both top and bottom. The finite conductivity of the metal is taken into account. The modes of the structure exhibit significant differences from those of dielectric waveguides with air or metal as the bounding media. Avoided level crossing phenomenon between the plasmon and the guided modes is shown to exist, leading to leaky modes. The structure sandwiched between two high index media is shown to lead to slow light in transmission. The group delay is shown to be larger for higher order modes.      

Design theory of high-Q optical ring resonator with asymmetric three-layered dielectrics

An optical ring resonator with asymmetric three-layered dielectrics is shown to have an anomalously highQ-factor compared with the conventional symmetric three-layered ring resonator by several orders of magnitude for suitably chosen parameters. A new leaky mode which can propagate on the asymmetric structure is found having an extremely small attenuation loss near the cut-off frequency of the guided mode, but which does not exist in the symmetric structure. Optimum values of the normalized frequency for a single-mode operation to suppress the undesired leaky modes are discussed. It is also shown that the size of the ring resonator using the asymmetric structure can be reduced considerably from that of the ordinary symmetric resonator.     

Polarization management for silicon photonic integrated circuits

Polarization management is very important for photonic integrated circuits (PICs) and their applications. Due to geometrical anisotropy and fabrication inaccuracies, the characteristics of the guided transverse‐electrical (TE) and transverse‐magnetic (TM) modes are generally different. Polarization‐dependent dispersion and polarization‐dependent loss are such manifestations in PICs. These issues become more severe in high index contrast structures such as nanophotonic waveguides made of silicon‐on‐insulator (SOI), which has been regarded as a good platform for optical interconnects because of the compatibility with CMOS processing. Recently, polarization division multiplexing (PDM) with coherent detection using silicon photonics has also attracted much attention. This trend further highlights the importance of polarization management in silicon PICs. The authors review their work on polarization management for silicon PICs using the polarization independence and polarization diversity methods. Polarization issues and solutions in PICs made of SOI nanowires and ridge waveguides are discussed.     

涂覆石墨烯的非对称并行电介质纳米线波导的模式特性分析

采用多级展开方法,对涂覆石墨烯的非对称并行电介质纳米线波导的模式特性进行了分析.首先对这种波导中的表面等离子模式进行分类,然后对七种低阶模式的有效折射率和传播长度随工作频率、几何结构参数和石墨烯费米能的依赖关系进行详细的分析.结果表明,通过改变工作频率、几何结构参数和石墨烯的费米能,可以在较大范围内调节模式的特性.与有限元法进行的对比表明,基于多级方法的半解析结果与有限元法的数值结果非常符合.研究结果可为涂覆石墨烯的非对称并行电介质纳米线的设计和制作提供一定的理论基础.