Electronic transport through self-assembled quantum disks

In this paper we study a single electron tunneling through a vertically stacked self-assembled quantum disks structure using a transfer matrix technique in the framework of effective mass approximation. In the disks, the electron is confined both laterally and vertically; we separate the motion in the vertical and lateral directions within the adiabatic approximation and treat the energy levels of the latter as an effective confining potential. The influence of a constant applied electric field is taken into account using an exact Airy-function formalism and the current density is calculated at zero temperature. By increasing the widths of the barriers, we find the peaks of the current density shift toward lower voltage region; meanwhile, they can become even sharper. (C) 2004 Elsevier Ltd. All rights reserved.

Excitonic energy of vertically stacked self-assmbled InAs quantum dots

We have studied the exciton states in vertically stacked self-assembled quantum disks within the effective mass approximation. The energy spectrum of the electron and hole is calculated using the transfer matrix formalism in the adiabatic approximation. The Coulomb interaction between the electron and the hole is treated accurately by the direct diagonalization of the Hamiltonian matrix. The effect of the vertical alignment of the disks on the ground energy of heavy- and light-hole exciton is presented and discussed. The binding energy is discussed in terms of the probability of the ground wave function. The ground energy of heavy- and light-hole excitons as a function of the magnetic field is presented and the effect of the disk size (the radius of disks) on the exciton energy is discussed.

Optical characteristics of DBR with inhomogeneous graded interfaces

Optical properties of Al0.9Ga0.1As/Al gamma Ga1-gamma As/GaAs/Al chi Ga1-chi As DBR with inhomogeneous graded interfaces has been investigated by using characteristic matrix method. The refractive index model and the analytic characteristic matrix of graded interfaces are obtained. The reflectance spectrum and the reflective phase shift are calculated for GaAs/Al-0.9 Ga-0.1 As DBR and graded interfaces DBR by using characteristic matrix method. The effect of graded interfaces on the optical properties of DBR is discussed. The result shows an extra graded phase matching layer must he added in front of the graded interfaces DBR to fulfil the conditions of phase matching at central wavelength. The accurate thickness of phase matching layer is calculated by optical thickness approximation method.

Tunable Fano resonance in photonic crystal slabs

A three dimensional analysis of a special class of anisotropic materials is presented. We introduce an extension of the Scattering Matrix Method (SMM) to investigate the behavior of anisotropic Photonic Crystal Slabs (PhCS) subject to external radiation. We show how the Fano effect can play a fundamental role in the realization of tunable optical devices. Moreover, we show how to utilize electron injection, electric field and temperature as parameters to control the Fano resonance shift in both isotropic and anisotropic materials as Si and Potassium Titanium Oxide Phosphate (KTP). We will see that because Fano modes are sensitive and controllable, a broad range of applications can be considered. (c) 2006 Optical Society of America

Photon localization and lasing in disordered GaNxAs1-x optical superlattices

We propose an approach to fabricate a disordered optical superlattice using microcracking faces in GaNxAs1-x epilayers. Laser action is observed and the emission exhibits random laser behaviors. A transfer-matrix simulation suggests photon localization occurs at the lasing modes.

Tunneling transmission in two quantum wires coupled by a magnetically defined barrier

A numerical analysis of an electron waveguide coupler based on two quantum wires coupled by a magnetically defined barrier is presented with the use of the scattering-matrix method. For different geometry parameters and magnetic fields, tunneling transmission spectrum is obtained as a function of the electron energy. Different from that of conventional electron waveguide couplers, the transmission spectrum of the magnetically coupled quantum wires does not have the symmetry with regard to those geometrically symmetrical ports, It was found that the magnetic field in the coupling region drastically enhances the coupling between the two quantum wires for one specific input port while it weakens the coupling for the other input port. The results can be well understood by the formation of the edge states in the magnetically defined barrier region. Thus, whether these edge states couple or decouple to the electronic propagation modes in the two quantum wires, strongly depend on the relative moving directions of electrons in the propagating mode in the input port and the edge states in the magnetic region. This leads to a big difference in transmission coefficients between two quantum wires when injecting electrons via different input ports. Two important coupler specifications, the directivity and uniformity, are calculated which show that the system we considered behaves as a good quantum directional coupler. (C) 1997 American Institute of Physics.

Impact of diode facet reflectivity on the fiber grating external cavity semiconductor laser

The scattering matrix method is used to analyze the multiple reflection effect between the laser diode facet and the fiber grating facet by considering the fiber grating external cavity laser diode (FGECL) as a four-mirror cavity laser. When neglecting other important parameters such as butt-coupling distance between the diode and the fiber facets, coupling efficiency, external cavity length, it is shown that low reflectivity is not a crucial factor for the laser characteristics such as SMSR. Experimentally high SMSR fiber grating external cavity laser is fabricated with a relatively large residual facet reflectivity (about 1%), which is coincident with our simulation results.

On detection wavelength and electron-hole wave function overlap of type II InAs/In-x Ga1-x Sb superlattice infrared photodetector

In this paper, the detection wavelength and the electron-hole wave function overlap of InAs/IrxGa1-xSb type II superlattice photodetectors are numerically calculated by using the envelope function and the transfer matrix methods. The band offset is dealt with by employing the model solid theory, which already takes into account the lattice mismatch between InAs and InxGa1-xSb layers. Firstly, the detection wavelength and the wave function overlap are investigated in dependence on the InAs and InxGa1-xSb layer thicknesses, the In mole fraction, and the periodic number. The results indicate that the detection wavelength increases with increasing In mole fraction, InAs and InxGa1-xSb layer thicknesses, respectively. When increasing the periodic number, the detection wavelength first increases distinctly for small periodic numbers then increases very slightly for large period numbers. Secondly, the wave function overlap diminishes with increasing InAs and InxGa1-xSb layer thicknesses, while it enhances with increasing In mole fraction. The dependence of the wave function overlap on the periodic number shows the same trend as that of the detection wavelength on the periodic number. Moreover, for a constant detection wavelength, the wave function overlap becomes greater when the thickness ratio of the InAs over InxGa1-xSb is larger.

Transverse trapping forces of focused Gaussian beam on ellipsoidal particles

A modified T-matrix method is presented to compute the scattered fields of various realistically shaped particles; then the radiation forces on the particles can be calculated via the Maxwell stress tenser integral. Numerical results of transverse trapping efficiencies of a focused Gaussian beam on ellipsoidal and spherical particles with the same volume are compared, which show that the shape and orientation of particles affect the maximal transverse trapping force and the displacement corresponding to the maximum. The effect of the polarization direction of the incident beam on the transverse trapping forces is also revealed. (c) 2007 Optical Society of America.

Radiation forces of a highly focused radially polarized beam on spherical particles

A radially polarized beam focused by a high-numerical-aperture (NA) objective has a strong longitudinal and nonpropagating electric field in the focal region, which implies that it is suitable for axial optical trapping. In this paper, we use the vectorial diffraction integral to represent the field distribution of the radially polarized beam focused by a high-NA objective and then employ the T-matrix method to compute the radiation forces on spherical particles. Effects of different parameters, such as the size of the sphere, the inner radius of the radially polarized beam, and the NA of the objective, on the radiation forces are presented.

Trapping of low-refractive-index particles with azimuthally polarized beam

Azimuthally polarized beams, focused by a high-numerical-aperture (NA) objective lens, form a hollow intensity distribution near the focus, which is appropriate for trapping low-refractive-index particles, in contrast to common linearly polarized or radially polarized beams. In this paper, the field distribution of the azimuthally polarized beam focused by a high-NA objective is described by the vectorial diffraction integral, and then the radiation forces on spherical particles with different parameters such as radius and refractive index are calculated by the T-matrix method. Numerical results show that the azimuthally polarized beam not only can steadily trap low-refractive-index particles at the focus center but also can trap multiple high-refractive-index particles around the focus center by virtue of the hollow-ring configuration. The range of the sizes of low-refractive-index particles that can be trapped steadily are presented, corresponding to different parameters such as the NA of the objective and the relative refractive index, based on which the NA of the objective can be selected to trap the appropriate size of particles. (C) 2009 Optical Society of America

功能性有机薄膜晶体管的研究

有机薄膜晶体管由于具有操作温度低、可以大面积加工和兼容柔性基底等优点，在有源矩阵显示、集成电路和化学传感器等方面显示出广泛的应用潜力。基于这个原因，有关有机半导体材料和器件的研究已经成为当前的热点。从实际应用的角度考虑，当前有机薄膜晶体管的器件性能和单一的工作模式还不能充分的满足实际的需要。因此如何提高有机薄膜晶体管的器件性能、开发多工作模式的器件成为有机电子学领域研究人员的重要挑战。本论文的工作以有机薄膜晶体管器件为主要研究对象。在对二种栅绝缘层的有机薄膜晶体管的研究基础上，提出了有机／无机双介电层有机薄膜晶体管的概念，解决器件的闭值电压和栅源漏电的问题；通过的对有机薄膜晶体管中电荷注入的研究的基础上，提出了含有缓冲层的有机薄膜晶体管，显著的改善了电荷注入效率，提高了器件性能。在对异质结型器件研究的基础上，获得了耗尽型和双极型有机薄膜晶体管，满足实际应用对多种工作模式有机薄膜晶体管的需求。论文的主体工作主要分为基于不同栅绝缘层的有机薄膜晶体管的研究，有机薄膜晶体管中接触效应的研究以及异质结型有机晶体管三个部分其具体内容如下：（l）基于不同栅绝缘层有机薄膜晶体管的研究。通过对有机晶体管中常用的高介电和低介电绝缘层对器件性能的影响进行了系统的研究，在有机晶体管构建中提出了有机／无机双介电层的概念。这一概念包含两个方面意义：一方面采用的双绝缘层分别为一层有机材料和一层无机材料的双介电层；另一方面无机介电层使用的是高介电常数的材料，有机部分为低介电常数材料。有机半导体沉积在这种方法构建的栅绝缘层基底上，改善了有机半导体的薄膜形态，提高了有机半导体层和无机基底的乳附力，从而达到了优化了器件电子性质的目的。与此同时解决了在当前含有高介电绝缘栅的有机晶体管中存在的高漏电的问题。通过这种方法构建的有机薄膜晶体管具有低阂值电压、低漏电、高迁移率等优点。由于我们在有机绝缘层的加工中采用了化学修饰的方法，该方法有加工简便、性质稳定好所以具有很好的实际应用前景。（2）有机薄膜晶体管中接触效应的研究。通过实验中采用TML（Transfer－Line一Method）的方法系统的研究了酞著铜薄膜晶体管中的接触效应，首次获得了酞著铜和金电极接触电阻定量化的结果。通过改变不同的电极材料，研究了不同的源漏电极对有机晶体管器件性能的影响。在此基础上为了能减小有机晶体管中的接触电阻，改善电荷注入效率，我们采用了在源漏电极和有机半导体之间插入一层高电导率的有机材料的方法，有效的降低了金属和有机半导体的注入势垒，增大了电荷隧穿几率，从而显著的改善了有机晶体管的器件性质。（3）异质结型有机晶体管。为了推动有机晶体管在集成互补电路中的应用，多工作模式的有机晶体管是需要的。在这里，我们把有机的异质结引入到有机晶体管的制备中，分别获得了高性能的耗尽型和双极型有机晶体管。通过了一系列的设计实验，我们提出了双沟道的载流子输运模型，为异质结型晶体管中双载流子的输运机制提供了理论上的解释。此外，我们通过有机异质结厚度的调节，进而改变沟道区的载流子密度，从而可以有效的调节了器件的阂值电压。当前的研究结果的意义在于利用的有机的异质结效应可以获得不同阂值电压的全塑型有机晶体管，为其实际中的广泛应用开辟了道路。此外我们还在同一基底上构建了基于一个常开型（耗尽）和一个常关型（累积）两个有机晶体管的反向器电路，从而表明异质结型有机晶体管在集成电路方面具有广泛的应用潜力。综上所述，本论文对目前有机薄膜晶体管器件中存在的问题进行了深入的研究。在此基础上，针对不同的问题分别提出了三种功能性的有机晶体管，优化了器件的性能。当前的研究也将为加速有机薄膜晶体管的在实际中的应用做出贡献。

Rashba electron's ballistic transport in two-dimensional quantum waveguide

The ballistic transport of Rashba electrons in a straight structure in two-dimensional electron gas is studied. It is found that there is no mixing between the wave functions of spin up and spin down states, and the transfer matrix is independent for the spin in every interface. The influence of the structure and Rashba coefficient on the electron transport is investigated. Our results indicate that the transmission probabilities are independent of the sign and magnitude of the Rashba coefficient and it depends on the shape of the structure, especially the stub width. The antiresonance is found, where the quasiconfined state is formed in the center part of the structure.

Magnetic frustration induced quantum phase transitions in the S=1/2 vertically asymmetric diamond chain

This work was supported by the National Basic Research Program of China (973 Program) grant No. G2009CB929300 and the National Natural Science Foundation of China under Grant Nos. 60521001 and 60776061.

Quantum coherent networks: A theoretical study

Electron transport in quantum coherent networks (interacting quantum waveguide arrays) is investigated theoretically with use of the scattering-matrix method. The scattering matrix for the basic unit of networks, the cross junction with Square or rounded corners, is derived using the mode-matching technique, The overall scattering matrix for the network is obtained by the composition of the scattering matrices associated with each unit of the network, For a uniform network, the transmission spectra are calculated in the single-mode regime and an found notably dependent on the junction geometry. Small reflection for the input terminal and uniform output for some output ports are obtained, which means that the quantum coherent network can be used as a distributing net for the electron waves. Cross junctions with rounded corners of large radii are found to play a negative role in the device application of quantum coherent networks. (C) 1997 American Institute of Physics.