C. de Oliveira

PROBING OF THE QUANTUM DOT SIZE DISTRIBUTION IN CDTE-DOPED GLASSES BY PHOTOLUMINESCENCE EXCITATION SPECTROSCOPY

We studied confinement effects in CdTe quantum dots by means of photoluminescence excitation spectroscopy. We show that by changing the detection energy we can resolve the spectrum of quantum dots of different sizes inside their much broader size distribution in CdTe‐doped glass. The spectra obtained show several well‐resolved lines. There is excellent agreement between the photoluminescence excitation spectra peak energies and calculations of the confined energy transitions based on a modified multiband envelope function model.

CdTe quantum dots by melt heat treatment in borosilicate glasses

CdTe quantu m dots in borosilicate glasses were produced and their growth kinetics, the effects of quantum confinement for electrons and phonons and the time resolved optical transmission were studied. Quantum dots in the range of 10 to 25 with a 5% size dispersion were grown. The growth kinetics were studied by small angle X-ray scattering using synchrotron light. A spherical k .p model for the electron confinement explains absorption data and selection rules, and a dielectric continuum model for the phonon confinement could explain the resonant longitudinal and surface optical phonons, as well as their overtone combinations. The time response of absorption was studied by a pump-probe measurement on the femtosecond time scale. For samples showing a large amount of surface traps the recovery time can be as fast as 300 fs, while for an almost surface trap free sample it remains on the 1-3 ps time scale.

LMI relaxations for robust H2 performance analysis of polytopic linear systems

This paper investigates the evaluation of H2 guaranteed costs of linear uncertain systems in polytopic domains by means of a particular type of polynomial parameter-dependent Lyapunov function. This class of functions, which depend on the dynamic matrix of the system, has been recently proposed for robust stability analysis and is extended here to cope with H2 guaranteed costs, providing an alternative LMI relaxation which presents a good trade-off between computational burden and accurate results. The results are illustrated by several numerical examples

Assessing stability of time-delay systems using rational systems

In this paper, we show how stability of an infinite-dimensional linear time-delay system can be assessed by studying the stability of an associated finite-dimensional linear system, constructed after substituting the exponential function in the characteristic equation of the delay-system by a high enough finite power of the bilinear transformation.

Schur stability of polytopic systems through positivity analysis of matrix-valued polynomials

This paper is concerned with robust stability of uncertain discrete-time linear systems. The matrix defining the linear system (system matrix) is assumed to depend afflnely on a set of time-invariant unknown parameters lying on a known polytope. Robust stability is investigated by checking whether a certain integer power k of the uncertain system matrix has spectral norm less than one. This peculiar stability test is shown to be equivalent to the positivity of a homogeneous symmetric matrix polynomial with known coefficients and degree indexed by K. A unique feature is that no extra variables need to be added to the problems being solved. Numerical experiments reveal that the value of K needed to test robust stability is mostly independent of the system dimension but grows sharply as the eigenvalues of the uncertain system approach the unit circle. By identifying the proposed stability test with a particular choice of a parameter-dependent Lyapunov function, extra variables can be introduced that can help mitigate such convergence problems for systems of small dimension.

PHOTOINDUCED INTERSUBBAND TRANSITION IN UNDOPED HGCDTE MULTIPLE QUANTUM WELLS

We present photoinduced intersubband absorption measurements in HgCdTe undoped quantum wells. The transition energies and the linewidths are well described by a full 8×8 k⋅p Kane model calculation. Also, based on this model we show that different in‐plane effective masses for the first and second electron subbands should be considered in order to properly fit the low energy side of the experimental spectra. The experimental results can be explained using the calculated intersubband oscillator strength with no exciton enhancement.

Asymptotically exact H 2 guaranteed cost computation by means of a special parameter-dependent Lyapunov function

This paper deals with guaranteed, H2 norm computation for continuous-time uncertain linear systems in convex bounded domains. As main result, we show that a special choice for the parameter-dependent Gramian solution allows us to establish a convergent sequence of polynomially parameter-dependent linear matrix inequalities whose solutions are constant symmetric matrices parametrized in terms of an integer k. Then, we propose some linear matrix inequality relaxations to numerically compute the H2 guaranteed cost and we investigate the trade off between accuracy of the results and computational effort.

Parameter-dependent Lyapunov functions for robust stability analysis of time-varying systems in polytopic domains

The robust stability of linear continuous-time uncertain systems in polytopic domains is investigated in this paper. The uncertain parameters are assumed as time-varying with bounded rates of variation. The robust stability conditions are obtained from the definition of a Lyapunov function with a particular structure, depending on integer powers K of the dynamic uncertain matrix of the system. As a consequence, parametrized linear matrix inequalities conditions are obtained in terms of k. As K grows, the robust stability conditions can take into account bounds on the successive time-derivatives of the uncertain parameters, reducing the conservativeness of the evaluations. Numerical examples illustrate the effectiveness of the proposed methodology.

Proposal of methodology for the modeling and control of manipulators

Industrial applications demand that robots operate in agreement with the position and orientation of their end effector. It is necessary to solve the kinematics inverse problem. This allows the displacement of the joints of the manipulator to be determined, to accomplish a given objective. Complete studies of dynamical control of joint robotics are also necessary. Initially, this article focuses on the implementation of numerical algorithms for the solution of the kinematics inverse problem and the modeling and simulation of dynamic systems. This is done using real time implementation. The modeling and simulation of dynamic systems are performed emphasizing off-line programming. In sequence, a complete study of the control strategies is carried out through the study of several elements of a robotic joint, such as: DC motor, inertia, and gearbox. Finally a trajectory generator, used as input for a generic group of joints, is developed and a proposal of the controllers implementation of joints, using EPLD development system, is presented.