Showing papers by "Morten Willatzen published in 2009"

Optical properties and optimization of electromagnetically induced transparency in strained InAs/GaAs quantum dot structures

Abstract: Using multiband k ·p theory we study the size and geometry dependence on the slow light properties of conical semiconductor quantum dots. We find the V-type scheme for electromagnetically induced transparency EIT to be most favorable and identify an optimal height and size for efficient EIT operation. In case of the ladder scheme, the existence of additional dipole allowed intraband transitions along with an almost equidistant energy-level spacing adds additional decay pathways, which significantly impairs the EIT effect. We further study the influence of strain and band mixing comparing four different k ·p band-structure models. In addition to the separation of the heavy and light holes due to the biaxial-strain component, we observe a general reduction in the transition strengths due to energy crossings in the valence bands caused by strain and bandmixing effects. We furthermore find a nontrivial quantum dot size dependence of the dipole moments directly related to the biaxial-strain component. Due to the separation of the heavy and light holes the optical transition strengths between the lower conduction and upper most valence-band states computed using one-band model and eight-band model show general qualitative agreement, with exceptions relevant for EIT operation.

Electromagnetic-wave propagation along curved surfaces

Abstract: We show that Maxwell's equations for a nonmagnetic, isotropic, but electrically inhomogeneous medium in the absence of charges or current sources lead to a wave equation governing surface electromagnetic wave propagation along a general curved, smooth surface which, when recasted using an appropriate choice of curvilinear coordinates u{sup 1},u{sup 2},u{sup 3}, can be fully separated in the spatial dimensions. It is shown that surface electromagnetic wave solutions decay exponentially away from the surface (along the u{sup 3} coordinate) with the same decay rate independent of the shape of the surface. Transmission and reflection coefficients governing scattering of electromagnetic waves on a varying surface shape are derived. Two test cases of a Gaussian-shaped and a sinusoidal-shaped surface are solved in details and discussed numerically in terms of transmission and reflection coefficients including dependencies on surface-shape parameters in the wavelength range 250-750 nm. The present method for determining surface electromagnetic wave propagation along complex-shaped metal-dielectric surfaces allows better insight into the importance of surface geometry as well as considerably faster computational speeds than those provided by standard numerical methods.

Curved nanowire structures and exciton binding energies

Abstract: Growth of quantum-confined semiconductor structures is a complicated process that may lead to imperfect and complex shapes as well as geometrical nonuniformities when comparing a large number of intended identical structures. On the other hand, the possibility of tuning the shape and size of nanostructures allows for extra optimization degrees when considering electronic and optical properties in various applications. This calls for a better understanding of size and shape effects. In the present work, we express the one-band Schrodinger equation in curved coordinates convenient for determining eigenstates of curved quantum-wire and quantum-dash structures with large aspect ratios. Firstly, we use this formulation to solve the problem of single-electron and single-hole states in curved nanowires. Secondly, exciton states for the curved quantum-wire Hamiltonian problem are found by expanding exciton eigenstates on a product of single-particle eigenstates. A simple result is found for the Coulomb matrix elements of an arbitrarily curved structure as long as the radius-of-curvature is much larger than the cross-sectional dimensions. We use this general result to compute the groundstate exciton binding energy of a bent nanowire as a function of the bending radius-of-curvature. It is demonstrated that the groundstate exciton binding energy increases by 40 meV as the radius-of-curvature changes from 20 to 2 nm while keeping the total length (and volume) of the nanowire constant.

Implementation of the Kurganov-Tadmor Highresolution Semi-Discrete Central Scheme for Numerical Solution of the Evaporation Process in Dry Expansion Evaporators

Abstract: A set of partial differential equations is derived in mass flow, enthalpy, pressure, and evaporator wall temperature based on the continuity equation, Navier-Stokes equations, and the energy equation for the refrigerant in addition to the heat exchange equation for the wall (the latter accounts for convective heat exchange with the refrigerant and the ambient). The combination of pressure and enthalpy thermodynamic variables allows for complete specification of the thermodynamic state spatially and temporally in the different zones in the evaporator. Details on the implementation of the Kurganov-Tadmor scheme for the governing equations are given and results are shown for step-responses in inlet mass-flow and outlet volume-flow.

Parameter sensitivity study of a Field II multilayer transducer model on a convex transducer

Abstract: The influence of different model parameters describing a multilayer transducer model is addressed by altering each single simulation parameter within ±20 % in steps of 2 % and by calculating the pressure and the intensity at a field point located 112 mm from the source. The simulations are compared with a hydrophone measured pressure pulse and intensity from a single element of a 128 element convex medical transducer. Results show that mainly the lens material and the ceramic material are of importance for errors in the pressure pulse prediction. Specifically the thickness, the density, and the stiffness constants are of significance. Among the results it is found that a −4 % change in lens stiffness yields a 6 % relative error change and a −4 % change in ceramic stiffness yields a −1.2 % relative error change. When calculating intensity the piezoceramic and electronic driving circuits are of importance, where a similar change in the lens and the ceramic stiffness shows a −0.1% and a −12% relative error change, respectively.