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Optical experiments on electric field tunable AlAs/GaAs coupled quantum wells clearly demonstrate that the optical nature of these structures can be directly controlled by an applied electric field. In this work we are concerned with calculation and analysis of the first electron and holOe states in these structures. We take into account the effects of the heterostrucuture geometry and of the external uniform electric field applied perpendicularly to the layers, which may lead to an anticrossing of low-lying electron levels in the system. Calculations are performed within the tight-binding supercell formalism with interactions between atomic orbitals up to second nearest-neighbors. Our results show that for GaAs layers less than 30 Å wide, application of electric fields takes the structures from typeII to type I heteroestructure behavior. Our estimated value of the electric field intensity needed to cause this transition is in good agreement with the experimental results for comparable heteroestructures geometries .

A systematic study of the longitudinal effective mass associated with either shallow impurity states or conduction Landau levels under in-plane magnetic fields is presented for Fibonacci superlattices. We analyze the relation between the mean longitudinal effective masses and the localization length of the corresponding electronic or impurity states. Under appropriate conditions we show that the mean longitudinal effective mass weakly depends on the nature of the electron state and localization length, and is essentially related to the Cantor-like energy spectra or ``band structure.''

A systematic study of the ground-state binding energies of a hydrogenic impurity in quantum dots subjected to external electric and magnetic fields is presented. The quantum dot is modeled by superposing a lateral parabolic potential and a square-well potential and the energies are calculated via a variational approach within the effective-mass approximation. The interplay between the confinement effects due to the applied fields and the quantum-size confinements on the binding energies is analyzed. The role played by the impurity position along the well direction on the impurity energies is also discussed. We have shown that by changing the strength of the external electric and magnetic fields, a large spread in the range of the donor binding energy may be obtained, for a particular choice for the lateral confinement. The presented results could be used to tailor energy states in optoelectronic devices.

The donor binding energies and density of impurity states of a hydrogenic impurity in a quantum dot are presented within the effective-mass approximation following a variational procedure. The emphasis is placed on the dependence of the binding energy on the volume of the dot and on the impurity position. We show that the results for the donor binding energy in the quantum dot goes to the exact limits of a square-cross-sectional quantum-well wire and a quantum well when appropriate limits are considered. Comparing the donor binding energies for cubic and spherical quantum dots, we found that the values are very close provided the dots have similar volumes.

Optical experiments on electric-field-tunable AlAs/GaAs heterostructures demonstrate that the optical nature of these structures can be directly controlled by an applied electric field. We present calculations performed within the tight-binding supercell formalism, taking into account the effects of the heterostructure geometry and of the external uniform electric field. We describe the temporal and spatial evolution of the relevant electronic states. The electric-field intensity needed to cause the indirect–direct transition is obtained in good agreement with the experimental results for comparable heterostructure geometry. We present simple and reliable expressions to estimate the switching time scales involved in such processes.

Submitted by Guilherme Lemeszenski (guilherme@nead.unesp.br) on 2014-02-26T17:25:21Z No. of bitstreams: 1 WOS000238575800050.pdf: 160330 bytes, checksum: 10f14bebba39a540b2177930d32d5295 (MD5) Made available in DSpace on 2014-02-26T17:25:21Z (GMT). No. of bitstreams: 1 WOS000238575800050.pdf: 160330 bytes, checksum: 10f14bebba39a540b2177930d32d5295 (MD5) Previous issue date: 2006-06-01 Submitted by Vitor Silverio Rodrigues (vitorsrodrigues@reitoria.unesp.br) on 2014-05-20T13:26:30Z No. of bitstreams: 1 WOS000238575800050.pdf: 160330 bytes, checksum: 10f14bebba39a540b2177930d32d5295 (MD5) Made available in DSpace on 2014-05-20T13:26:30Z (GMT). No. of bitstreams: 1 WOS000238575800050.pdf: 160330 bytes, checksum: 10f14bebba39a540b2177930d32d5295 (MD5) Previous issue date: 2006-06-01 Coherent properties and Rabi oscillations in two-level donor systems, under terahertz excitation, are theoretically investigated. Here we are concerned with donor states in bulk GaAs and GaAs-(Ga,Al)As quantum dots. We study confinement effects, in the presence of an applied magnetic field, on the electronic and on-center donor states in GaAs- (Ga,Al)As dots, as compared to the situation in bulk GaAs, and estimate some of the associated decay rate parameters. Using the optical Bloch equations with damping, we study the time evolution of the Is and 2p(+) states in the presence of an applied magnetic field and of a terahertz laser. We also discuss the role played by the distinct dephasing rates on the photocurrent and calculate the electric dipole transition moment. Results indicate that the Rabi oscillations are more robust as the total dephasing rate diminishes, corresponding to a favorable coherence time. Univ Fed Fluminense, Dept Fis, BR-24210340 Niteroi, RJ, Brazil Univ Estadual Feira Santana, Dept Fis, BR-44031460 Feira de Santana, BA, Brazil Univ Estadual Paulista, Fac Ciências, Dept Matemat, BR-17033360 Bauru, SP, Brazil Univ Estadual Campinas, Inst Fis, BR-13083970 Campinas, SP, Brazil Univ Fed Rio de Janeiro, Inst Fis, BR-21945970 Rio de Janeiro, Brazil Univ Estadual Paulista, Fac Ciências, Dept Matemat, BR-17033360 Bauru, SP, Brazil

The effects of both electric and magnetic fields (applied perpendicular to the interfaces) on the donor transition energies on a GaAs‐Ga1−xAlxAs quantum well are studied by following a variational calculation within the effective‐mass approximation with two‐parameter variational envelope hydrogenic wave functions for the 1s‐ and 2p+‐like donor states. A detailed analysis of the far‐infrared intradonor absorption spectra, taking into account a proper consideration of the impurity‐doping profile, is performed and results are compared with recent experimental data. The agreement between the reported magnetospectroscopic data and the average photon energy at the width at half‐maximum of the calculated infrared‐absorption spectra is quite apparent. It is unambiguously shown that the absorption spectra must be evaluated with an adequate choice of the variational envelope wave function for a correct quantitative understanding of the experimental measurements.