Electronic transport of graphene nanoribbons within recursive Green’s function

In this work, we introduce a recursive Green’s function method for investigating electronic transport in a graphene nanoribbons (GNRs) quantum wire with armchair (AGNR) and zigzag (ZGNR) edges which attached to two semi-infinite square lattice leads. This model reduces numerical calculations time and enables us to use Green’s function method to investigate transport in a supperlattice device. Therefore, we consider AGNR and ZGNR devices attached to metallic semi-infinite square lattice leads, taking into account the effects of longitudinal and wide of the wire. Our calculations are based on the tight-binding model, which the recursive Green’s function method is used to solve inhomogeneous differential equations. We concentrate on the electrical conductance and current for various length and wide size of the wire. Our numerical results show that the transport properties are strongly affected by the quantum interference effect and the lead interface geometry to the device. By controlling the type of contact and wire geometry, this kind of system can explain the antiresonance states at the Fermi energy. Our results can serve as a base for developments in designing nano-electronic devices.     

Thermal transport in four-terminal graphene nano-junctions

The thermal transport properties of four-terminal graphene nano-junctions (FGNJs) consisting of semi-infinite armchair-edged nanoribbon and zigzag-edged nanoribbon were calculated. The thermal transport in FGNJs is sensitive to their geometric shape. The thermal conductance of FGNJs depends on the width of semi-infinite graphene nanoribbons and center region. These thermal transport phenomena can be explained by analyzing the phonon transmission coefficient. Compared with previous thermal rectifiers, reverse modulation can be obtained by changing the width of the thermal terminal. The results provide significant physical models and theoretical validity in designing the thermal devices based on the graphene nano-junctions.     

Operator method in electromagnetic waves diffraction by semi-infinite diaphragm system in rectangular waveguide problem

Conclusions The investigations carried out show that the electromagnetic waves diffraction problem by semi-infinite diaphragm system in rectangular waveguide may be solved with the help of the operator method. This approach allows to investigate the main characteristics of these electrodynamic systems and to establish the fact of total reflection regions in semi-infinite diaphragm system and its connection with the “fading” regions in corresponding infinite structures. This physical fact is common for these objects. It is obviously that any rejection filters may be designed with the help of semi-infinite diaphragm system. However, it is necessary to have accuracy and sufficiently simple method for designing these devices, especially in millimetre wave region. The characteristics analysis method for semi-infinite and finitely element systems may be a subject of further investigations.     

Electron localization in superlattice-carbon nanotubes

Electronic transport properties of superlattice-carbon nanotubes (SCNTs) attached to semi-infinite clean metallic carbon nanotube (CNT) leads are investigated in the framework of a simple model based on mode (momentum)-space within the tight-binding approximation. This model reduces the numerical calculation time and enables us to use the transfer matrix method to investigate transport in an SCNT. We calculate the localization length and density of states (DOS) for various strengths of boron defect. Our numerical results indicate that the localization length decreases with increasing boron concentration, showing the tendency of the system towards the insulating behavior. Also, we observe a nearly stepwise dependence of the localization length on energy at small boron concentration. By controlling the layered boron concentration, the system can be tuned to yield either localized or extended states. These calculations can be generalized to the magnetic defects embedded in the device, which can act as a spin-filter. Our results can serve as a base for developments in designing nano-electronic devices.     

L型石墨纳米结的热输运

利用非平衡格林函数方法研究了由半无限长扶手椅型和锯齿型边界石墨纳米带连接而成的L型石墨纳米结的热输运性质.结果表明,L型石墨纳米结的热导依赖于L型石墨纳米结的夹角和石墨纳米带的宽度.在L型石墨纳米结的夹角从30°增加到90°再增加到150°过程中,其热导显著增大.夹角为90°的L型石墨纳米结的热导随着扶手椅型纳米带宽度增加时,在低温区热导随着宽度的增大而降低,在高温区热导随宽度的增大而升高.对于夹角为150°的L型石墨纳米结,其热导无论是在低温区还是在高温区都随着锯齿型纳米带宽度的增加而降低.利用声子透射谱对这些热输运现象进行了合理的解释.研究结果阐明了不同L型石墨纳米结中的热输运机理,为设计基于石墨纳米结的热输运器件提供了重要的物理模型和理论依据. 关键词： 石墨纳米结 热输运 热导     

Design and Characterization of Novel Comb Filters Based on Periodical Fiber Bragg Gratings

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Effect of an optical negative index thin film on optical bistability

We investigate nonlinear transmission in a layered structure consisting of a slab of positive index material with Kerr-type nonlinearity and a subwavelength layer of linear negative index material (NIM) sandwiched between semi-infinite linear dielectrics. We find that a thin layer of NIM leads to significant changes in the hysteresis width when the nonlinear slab is illuminated at an angle near that of total internal reflection. Unidirectional diodelike transmission with enhanced operational range is demonstrated. These results may be useful for NIMs characterization and for designing novel NIMs-based devices.     

Prediction of sound reflection by corrugated porous surfaces

The coupled mode (CM) and finite-element methods (FEMs) are developed and used to predict the acoustic reflection coefficient of a semi-infinite porous medium with closely spaced two-dimensional (2D) periodical corrugations. These methods are also applied to predict the reflection coefficient of a periodic array of porous corrugations installed on an acoustically rigid surface. It is shown that the predictions by the both methods agree closely. The reflection coefficient and Brewster angle of total refraction for the corrugated semi-infinite medium predicted with these methods are compared against that predicted by the Biot/Tolstoy/Howe/Twersky and extended Twersky models. A similar analysis is carried out for porous corrugations set on a rigid backing. The behavior of the reflection coefficient and the pole in the expression for the reflection coefficient located close to grazing incidence is studied.     

带多孔硅表面缺陷腔的半无限光子晶体Tamm态及其折射率传感机理

结合表面缺陷半无限光子晶体Tamm态与多孔硅光学传感机理,在光子晶体表面缺陷腔中引入多孔硅,并利用其高效的承载机制,提出基于多孔硅表面缺陷光子晶体Tamm态的折射率传感结构.在半无限光子晶体中缺陷腔与原来的周期性分层介质结构的界面上存在Tamm态,通过入射角度调制使其在缺陷腔中实现多次全反射,并在缺陷腔中加入吸收介质,使谐振波长在缺陷腔中完成衰荡,从而在反射谱中得到缺陷峰;调整光子晶体参数,使缺陷峰的半高全宽得到优化,提高其品质因数(Q值);在此基础上,根据Goos-H?nchen相位移与谐振波长的关系,建立由待测样本折射率改变所导致的多孔硅表面吸附层有效折射率变化与缺陷峰值波长漂移之间的关系模型,并分析其折射率传感特性.结果表明,此生物传感结构Q值为1429,灵敏度为546.67 nm/RIU,证明了该传感结构的有效性,可为高Q值和高灵敏度折射率传感器的设计提供一定的理论参考.     

Electronic transport through superlattice-graphene nanoribbons

In this work, we study quantum transport properties of superlattice-graphene nanoribbons (SGNRs) attached to two semi-infinite metallic armchair graphene nanoribbon (AGNR) leads. The calculations are based on the tight-binding model and Green’s function method, in which localization length, density of states (DOS) and conductance of the system are calculated, numerically. By controlling the layered boron concentration, this kind of system can separate the extended states from the localized states. Our results may have important applications for building blocks in the nano-electronic devices based on GNRs.     

Power splitting ratio couplers based on MMI structures with high bandwidth and large tolerance using silicon waveguides

We show that it is possible to obtain 2 × 2 couplers based on multimode interference (MMI) structures with nineteen new power-splitting ratios by cascading three or four MMI couplers. The other aim of this study is to use silicon waveguides, that are compatible with the existing CMOS (Complementary Metal-Oxide-Semiconductor) fabrication technology, for designing the proposed devices. The proposed MMI couplers with new power splitting ratios have simple geometries and low losses. These MMI couplers can offer valuable new possibilities for designing MMI waveguide-based photonic integrated circuits such as all-optical interconnects, microring resonators, clock distribution, Mach Zehnder Interferometer based on MMI couplers and other all-optical processing applications. The transfer matrix method (TMM) and modified effective index method (MEIM) along with the support of the 3D Beam Propagation Method (3D BPM) are used to optimize the proposed devices.     

Numerical study of localization length in disordered graphene nanoribbons

In this work, we study quantum transport properties of a defective graphene nanoribbon (DGNR) attached to two semi-infinite metallic armchair graphene nanoribbon (AGNR) leads. A line of defects is considered in the GNR device with different configurations, which affects on the energy spectrum of the system. The calculations are based on the tight-binding model and Green’s function method, in which localization length of the system is investigated, numerically. By controlling disorder concentration, the extended states can be separated from the localized states in the system. Our results may have important applications for building blocks in the nano-electronic devices based on GNRs.     

Novel neural network based optimization approach for photonic devices

In this paper, a novel optimization technique is proposed for designing photonic devices. The suggested approach relies on the use of radial bases function based artificial neural network (RBF-ANN) which shows an excellent performance in comparison with the conventional artificial neural network technique. The robustness of the suggested RBF-ANN approach is demonstrated through the numerical precision and fast convergence of the design cycle performed on a typical slanted rib waveguide polarization rotator, and ultra-flattened zero dispersion photonic crystal fiber.     

Multilayer dielectric cylindrical mirrors based on omnidirectional reflection from a one-dimensional photonic crystal

A multilayer dielectric cylindrical mirror (MDCM) based on the one-dimensional omnidirectional reflection of a photonic crystal is presented. In this case, the refractive indices of the two materials are 1.6 (polystyrene) and 4.6 (tellurium), and the corresponding optimized thicknesses are 0.75a and 0.25a. A very high reflectance over a wide frequency range is observed. In this case, a is the lattice constant of the photonic crystal. In this band, the MDCM has good reflection and focal properties. Therefore, it is feasible to use the MDCM for integrated waveguide devices. As an example, an etched diffraction grating demultiplexer based on the MDCM is also proposed. Both the operational principle and design of the device are introduced. This provides a method for designing compact integrated waveguide devices.     

Nonadiabatic electron charge pumping in coupled semiconductor quantum dots

The possibility of nonadiabatic electron pumping in the system of three coupled quantum dots (QDs) attached to the leads is discussed. We have found out that periodical changing of energy level position in the middle QD results in non-zero mean tunneling current appeared due to nonadiabatic non-equilibrium processes. The same principle can be used for fabrication of a new class of semiconductor electronic devices based on non-stationary non-equilibrium currents. As an example we propose a nanometer quantum emitter with non-stationary inverse level occupation achieved by electron pumping.     

Intelligent algorithms: new avenues for designing nanophotonic devices [Invited]

The research on nanophotonic devices has made great progress during the past decades. It is the unremitting pursuit of researchers that realize various device functions to meet practical applications. However, most of the traditional methods rely on human experience and physical inspiration for structural design and parameter optimization, which usually require a lot of resources, and the performance of the designed device is limited. Intelligent algorithms, which are composed of rich optimized algorithms, show a vigorous development trend in the field of nanophotonic devices in recent years. The design of nanophotonic devices by intelligent algorithms can break the restrictions of traditional methods and predict novel configurations, which is universal and efficient for different materials, different structures, different modes, different wavelengths, etc. In this review, intelligent algorithms for designing nanophotonic devices are introduced from their concepts to their applications, including deep learning methods, the gradient-based inverse design method, swarm intelligence algorithms, individual inspired algorithms, and some other algorithms. The design principle based on intelligent algorithms and the design of typical new nanophotonic devices are reviewed. Intelligent algorithms can play an important role in designing complex functions and improving the performances of nanophotonic devices, which provide new avenues for the realization of photonic chips.     

基于纳米光学等离子晶体的新型纳米光子学器件的发展

通过设计合适的纳米表面等离子结构,纳米光学等离子晶体具有光场增强效应,能调控近场范围内的光场分布,可用来设计新型的纳米光子学器件。介绍了纳米光学等离子晶体的原理、结构特点、制作工艺和纳米光子学器件的设计方法,对纳米光学等离子晶体和普通光子晶体做了比较。归纳了纳米光学等离子晶体的物理机制、光学特征,描述并分析了波长选择性场增强效应和束流效应等。给出几种基于纳米光学等离子晶体的纳米光子学器件应用实例。     

Interaction of electrons with the evanescent field in a free-electron device

The behaviour of free electron devices such as the modified Ledatron tubes, where the lamellar grating is replaced by a particular type of periodical structure, are compared from the point of view of the efficiency of the electron-field interaction, by assuming the periodical structure to be illuminated by one and the same evanescent field (which schematizes the field of the electron beam). The analysis takes into particular account the multiple reflections occurring in the resonant structure of the tube, and of those occurring inside the periodical structure.     

Design of Two 8-Channel Optical Demultiplexers Using 2D Photonic Crystal Homogeneous Ring Resonators

ABSTRACT

Ring resonators have always been referred to as a highly flexible structure for designing optical devices. In this paper, we have designed and evaluated two 8-channel optical demultiplexers using photonic crystal ring resonators. The purpose of this study is to investigate the flexibility of this type of resonator for designing and manufacturing optical devices based on photonic crystals. To the extent that we have investigated the literature, there is no report so far on such a study. For this purpose, two structures with the same structural parameters, but only with a difference in the type of lattice constant (square or triangular) are used. Both structures have a common photonic band gap within a proper range for telecommunication applications used in wavelength-division multiplexing (WDM) systems. Both designed structures have an average crosstalk of ?26 dB. For the demultiplexer structure with a square lattice constant, the quality factor and the transmission coefficient are 3,046 and 93.7% respectively, and its channel spacing is 1.97 nm. For the structure with a triangular lattice constant, the quality factor and the transmission coefficient are 1577.7 and 94.5%, respectively and its channel spacing is equal to 4 nm. To obtain the photonic band gap of the structures, the plane wave expansion (PWE) method is used and the output spectrum of the structures is obtained using the finite-difference time-domain (FDTD) method. The good results obtained in this study is through designing and simulating optical demultiplexer structures only by creating a change in the type of lattice constant used. This undoubtedly justifies the high flexibility of ring resonators, when used in the design of optical devices, as well as their suitability for the use in WDM systems     

Large lateral shift in a multi-layer waveguide with indefinite media

We explore the basic properties of lateral shift in a multi-layer waveguide including isotropic metamaterial and indefinite media. Effective index method is used to calculate the refractive index of indefinite media and stationary-phase approach is applied to analysis the lateral shift of reflected light. The influences of thicknesses of isotropic metamaterial and indefinite media, off-diagonal parameters of indefinite media, the loss of metamaterial and the permeability of lateral shift on the lateral shift are discussed based on simulation results, which give us a guidance in designing multi-functional devices by use of the lateral shift in the proposed structure.