Zhongyuan Yu

THE STRAIN DISTRIBUTIONS AND CARRIER'S CONFINING POTENTIALS OF SELF-ORGANIZED InAs/GaAs QUANTUM DOT

On the basis of the finite element approach, we systematically investigated the strain field distribution of conical-shaped InAs/GaAs self-organized quantum dot using the two-dimensional axis-symmetric model. The normal strain, the hydrostatic strain and the biaxial strain components along the center axis path of the quantum dots are analyzed. The dependence of these strain components on volume, height-over-base ratio and cap layer (covered by cap layer or uncovered quantum dot) is investigated for the quantum grown on the (001) substrate. The dependence of the carriers confining potentials on the three circumstances discussed above is also calculated in the framework of eight-band k (.) p theory. The numerical results are in good agreement with the experimental data of published literature.

Dependence of elastic strain field on the self-organized ordering of quantum dot superlattices

A systematic investigation of the strain distribution of self-organized, lens-shaped quantum dot in the case of growth direction on (001) substrate was presented. The three-dimensional finite element analysis for an array of dots was used for the strain calculation. The dependence of the strain energy density distribution on the thickness of the capping layer was investigated in detail when the elastic characteristics of the matrix material were anisotropic. It is shown that the elastic anisotropic greatly influences the stress, strain, and strain energy density in the quantum dot structures. The anisotropic ratio of the matrix material and the combination with different thicknesses of the capping layer, may lead to different strain energy density minimum locations on the capping layer surface, which can result in various vertical ordering phenomena for the next layer of quantum dots, i.e. partial alignment, random alignment, and complete alignment.

Valence band structures of InAs/GaAs quantum rings using the Fourier transform method

The valence band structures of strained InAs/GaAs quantum rings are calculated, with the four-band k ? p model, in the framework of effective-mass envelope function theory. When determining the Hamiltonian matrix elements, we develop the Fourier transform method instead of the widely used analytical integral method. Using Fourier transform, we have investigated the energy levels as functions of the geometrical parameters of the rings and compared our results with those obtained by the analytical integral method. The results show that the energy levels in the quantum rings change dramatically with the inner radius, outer radius, average radius, width, height of the ring and the distance between two adjacent rings. Our method can be adopted in low-dimensional structures with arbitrary shape. Our results are consistent with those in the literature and should be helpful for studying and fabricating optoelectronic devices.

Tunable terahertz optical antennas based on graphene bowtie structures

Highly tunable optical bowtie antennas in teraherz range are proposed, which employ grapheme plasmons using numerical simulations. The resonance frequency of graphene antennas and the field enhancement factor are calculated compared to that of the metallic antenna with the same size.

Full-vector analysis of photonic crystal fiber and 2D plane photonic crystals waveguides by finite difference method

An important class of optical waveguides are those refractive index profile is not continuous, such as Bragg fiber, photonic crystal fiber and 2-D photonic crystal waveguides. These microstructure fibers and waveguides become more and more important in the future optical devices for their novel and spurious optical characteristics. To give a full understand of these devices, the exact mode fields solver is very critical. In this paper, we use the full-vector finite difference approach free of spurious modes to solve mode field characteristics of the photonic crystal and 2-D plane photonic crystal waveguides. The photonic band structures within an irreducible Brillouin zone are investigated for both in plane and out plane propagation. The out of plane propagation can be used for the photonic crystal fiber. The coupled difference equations are in terms of the transverse magnetic filed components. Based on the appropriate transparent boundary conditions, a unique set of couple five-point difference equation are developed, and an efficient numerical technique to solve the deterministic equations by the Eispark in Matlab. For the in plane propagation, the guides mode are either TE or TM modes, the difference equations are uncoupled. Using the appropriate period boundary condition and combined with the transparent boundary condition, we derived the five point difference equations that can be used for the 2-D plane photonic crystal waveguides. Based on these finite difference equations, we analyzed the optical mode field characteristics of the crystal fiber and the plane optical crystal waveguides. The filling materials either dielectric or air are also analyzed. Good agreements are obtained compared the numerical results with the experiment data and the published literature. The mode fields solver can also be used for the other waveguides such as Bragg fibers.

The multi-mode behavior of vertical-cavity surface-emitting lasers under a spatially periodical current injection

A two-dimensional spatially independent rate equation model of vertical-cavity surface-emitting lasers (VCSELs) is derived and then used to analysis the multi-mode behaviour of VCSELs. The transverse mode characteristics of VCSELs, the carrier distribution in both the radial and the azimuthal directions, and the effects of the azimuthal non-uniformity of the injection current on the transverse mode behaviours are investigated in detail. By using both Bessel and Fourier expansion of carrier density, the 2D spatially independent rate equations for transverse mode are formulated, which take into account carrier diffusion both in the radial and in the azimuthal direction as well as gain non-uniformity in the lateral direction. The equations are numerical solved self-consistently using the Runge-Kutta method for different spatial periodic injection current. Results show that a proper current injection profile can separate the sine mode and cosine mode of the same order transverse modes observably. It is found that an injection current with periodic change in the azimuthal direction is favourable for the excitation of the modes whose mode profile match the current profile best. The results are useful to the design and control of transverse mode characteristics of a VCSEL.

Optimization of index modulation profile of sampling period for sampled fiber Bragg gratings

Using the simple inverse Flourier transformation of the target channels, we can get the index modulation structure of the sampled period for the sampled fiber Bragg gratins (FBGs). In this method, the enable channels are identical wavelength operation while the unable channels are almost suppressed completely, and the enable and unable channels can be established at will based on the applications. However, the efficient of the sampled FBGs of which the index modulation is obtained by the simple inverse Fourier transformation is very slow. To get a high efficient, a particle swarm optimization algorithm is applied to design the phase of the target spectrum. Combing the two methods together, a high efficient multi-level phase sampled FBG that each channel is suppressed or not at will, can be obtained. We also use the particle swarms optimizations (PSO) algorithm to optimize the pure phase sampling profiles of the sampled FGB. Results showed that a much high efficient of the FBG can be obtained compared with that obtained from the simple inverse Fourier transformation technology. If appropriate grating period chirp and sampled period chirp is applied to such a grating, a novel FBGs based OADM or interleaver devices with dispersion or dispersion slope compensation can be designed.

Tunable Graphene Plasmonics Waveguide

To exploit the extraordinary properties of graphene for the optoelectronic applications, we investigate the dispersion characteristics of the graphene plasmonics waveguide. Our result indicates that the effective index and propagation length can be tuned by the conductivity of graphene.

A density functional theory study on the electronic and magnetic properties of (Mn,N)-codoped ZnO

A first-principles study has been performed to evaluate the electronic and magnetic properties of the Zn1-xMnxO1-yNy system. Doping Mn atoms introduces local magnetic moments, while doping N atoms introduces carriers. It is worth noting that intrinsic Mn-doped ZnO favors antiferromagnetic (AFM) ordering, and this cannot be changed by raising Mn ions concentration continuously. However, by the codoping N and Mn, it is possible to change the ground state from no-metallic AFM to half-metallic ferromagnetic (FM) and make ZnO as a dilute magnetic semiconductor. We have succeeded in describing the change (from AFM to FM) by using the magnetic interaction that is hole-mediated FM due to the hybridization between N 2p and Mn 3d states. Furthermore, the most stable configurations are found to be -O-Mn-N-Mn-O-.Our results are in good agreement with other theoretical results that are additional holes carriers is one of the possible mechanisms.

Determination on wave function of quantum structures using finite-difference time domain

With the interest in quantum structures, there is a need to have a flexible method that can help us to determine eigenfunctions for these structures. In this article, we present a method that accomplishes this by using the simulation of the Schrödinger equation based on finite-difference time-domain (FDTD). We choose one-and two-dimensional finite square well potential, and one-and two-dimensional harmonic oscillator potential as examples. Giving the initial condition, we determine the eigenfrequencies through a Fourier transform of the time domain data collected at the center point in the problem space. Another simulation implements a discrete Fourier transform at the eigenfrequencies at every point in the problem space, hence, the eigenfunctions can be constructed.