J. M. Villas-Boas

Coherent control of tunneling in a quantum dot molecule

Quantum dot ~QD! structures provide a three-dimensional confinement of carriers. Electrons and holes in the QD can occupy only a set of states with discrete energies, just as in an atom, and can thus be used to perform ‘‘atomic physics’’ experiments in solid-state structures. One advantage of QD’s is that they provide different energy scales and physical features which can be easily varied over a wide range of values. Most important, perhaps, is that QD’s also allow the control of direct quantum-mechanical electronic coupling with not only composition but externally applied voltages. These flexible systems represent therefore the ideal for theoretical and experimental investigations, where the interactions between light and matter can be studied in a fully controlled, wellcharacterized environment, and with excellent optical and electrical probes. These features make semiconductor QD’s promising candidates for applications in electro-optical devices such as QD lasers, 1,2 and in quantum information processing. 3‐ 6 In the latter case, one can exploit the optical excitation in a QD, 3,5 or its spin state, 4,6 as qubits. These high

Electrical control of interdot electron tunneling in a double InGaAs quantum-dot nanostructure.

We employ ultrafast pump-probe spectroscopy to directly monitor electron tunneling between discrete orbital states in a pair of spatially separated quantum dots. Immediately after excitation, several peaks are observed in the pump-probe spectrum due to Coulomb interactions between the photogenerated charge carriers. By tuning the relative energy of the orbital states in the two dots and monitoring the temporal evolution of the pump-probe spectra the electron and hole tunneling times are separately measured and resonant tunneling between the two dots is shown to be mediated both by elastic and inelastic processes. Ultrafast (<5u2009u2009ps) interdot tunneling is shown to occur over a surprisingly wide bandwidth, up to ∼8u2009u2009meV, reflecting the spectrum of exciton-acoustic phonon coupling in the system.

Spin polarization control via magnetic barriers and spin-orbit effects

We investigate the spin-dependent transport properties of two-dimensional electron gas (2DEG) systems formed in diluted magnetic semiconductors and in the presence of Rashba spin-orbit interaction in the framework of the scattering matrix approach. We focus on nanostructures consisting of realistic magnetic barriers produced by the deposition of ferromagnetic strips on heterostructures. We calculate spin-dependente conductance of such barrier systems and show that the magnetization pattern of the strips, the tunable spin-orbit coupling, and the enhanced Zeeman splitting have a strong effect on the conductance of the structure. We describe how these effects can be employed in the efficient control of spin polarization via the application of moderate fields. PACS numbers: 71.70.Ej, 73.23.Ad, 72.25.-b, 72.10.-d Spin-orbit coupling in semiconductors intrinsically connects the spin of an electron to its momentum, 1 providing a pathway for electrically initializing and manipulating electron spins for applications in spintronics 2,3 and spin-based quantum information processing. 4 This coupling can be regulated with quantum confinement in semiconductor heterostructures through band structure engineering, as well as by the application of external electric fields, as in the celebrated spin field-effect transistor proposed by Datta and Das. 5 Using diluted magnetic semiconductors (DMS) in such systems provides an additional degree of control of the transport properties. In particular, when an external magnetic field is applied, the magnetic dopant spins align, giving rise to a strong exchange field that acts on the electron spin. This sd exchange interaction between the electron spin in the conduction band and the localized magnetic ions induces a giant Zeeman splitting. In this communication we investigate the spindependent transport properties of two-dimensional electron gas (2DEG) systems formed in diluted magnetic semiconductors and take into account the electric-field– dependent Rashba spin-orbit (SOI) interaction. We focus our attention on nanostructures consisting of realistic magnetic barriers produced by the deposition of ferromagnetic strips near the heterostructures, 6 providing a relatively strong inhomogeneous magnetic field on the 2DEG. We show how the conductance of the 2DEG depends strongly on the magnetization pattern of the strips, as well as on geometry and externally applied electric fields. We demonstrate that significant spin polarization (exceeding 50%) can be obtained at low temperatures for ferromagnetic strips of typical dimensions and magnetization.

Selective coherent destruction of tunneling in a quantum-dot array

The coherent manipulation of quantum states is one of the main tasks required in quantum computation. In this paper we demonstrate that it is possible to control coherently the electronic position of a particle in a quantum-dot array. By tuning an external ac electric field we can selectively suppress the tunneling between dots, trapping the particle in a determined region of the array. The problem is treated non-perturbatively by a time-dependent Hamiltonian in the effective mass approximation and using Floquet theory. We find that the quasienergy spectrum exhibits crossings at certain field intensities that result in the selective suppression of tunneling.

Quantum dot structures grown on Al containing quaternary material for infrared photodetection beyond 10μm

Different InAs quantum dot structures grown on InGaAlAs lattice matched to InP were investigated for quantum dot infrared photodetectors. Extremely narrow photocurrent peaks were observed, demonstrating great potential for fine wavelength selection. Structures which can detect radiation beyond 10μm were developed. Polarization dependence measurements showed that the structures have a zero-dimensional character and are suitable for detection of normal incident light. On the other hand, structures containing coupled quantum wells showed a hybrid two-dimensional/zero-dimensional behavior.

Luminescence spectra of quantum dots in microcavities. III. Multiple quantum dots

The coherent coupling of a semiconductor quantum dot (QD) exciton to the optical mode of a microcavity has been intensely investigated throughout the last years in cavity quantum electrodynamics (CQED) experiments [1–20] and theory [21–40]. In some of these works, the experimental spectral function of the strongly coupled QD–cavity system was directly compared to a theoretical model [11, 12, 19, 23], and the agreement is excellent. It was assumed that most of the light escapes the system via the radiation pattern of the cavity mode, and the experimental spectra were compared to the spectral function calculated from the cavity occupation. This detection geometry is known in atomic cQED as “end emission” or “forward emission” [41]. In atomic systems, negligible light escapes the cavity through the cavity mode, that is to say, the cavity photon lifetime is so long as to be considered infinite. Light is then detected in the so-called “side-emission”, where the radiation pattern of the emitter is probed instead. With microcavities, the situation is reversed: the cavity mode is measured often with an emitter of a much longer lifetime. In the spontaneous emission regime of a system in strong-coupling, this makes measurements of the Rabi doublet in the photoluminescence more difficult, unless some cavity feeding makes the quantum state of the system photon-like, since changing the nature of the excitation is, in this case, equivalent to changing the channel of detection [23]. In the nonlinear regime, this also hinders manifestations of the Jaynes–Cummings ladder. All the transitions between its rungs have the same intensity in the exciton emission. In the cavity emission, however, the photon has two paths to be emitted, one with the dot in its ground state, the other with the dot in its excited states [28]. These two paths interfere destructively when the initial and final states are out of phase, which is the case for two out of the four possible transitions in the Jaynes–Cummings ladder. On the other hand, these two paths interfere constructively when the initial and final states are in phase, or, up to a photon, indistinguishable. In the dressed-state picture, the cavity photon to be emitted decouples from the polaritons and carries away little information from the coupled system, being more like a cavity photon the higher the number of excitations. The dot photon, on the other hand, does not decouple from the system, regardless the number of excitations: the dot cannot de-excite without altering fundamentally the state of the entire system. As a result, the dot photon carries more information of the coupled system. Summarizing, the dot is essentially a quantum emitter whereas the cavity is essentially a classical emitter. It is therefore interesting to detect directly the dot emission. The ratio R of dot-vs-cavity emitted photons depends on the populations of the dot (resp. cavity), n1 (resp. na) and their rate of emission, γ1 (resp. γa):

Nonperturbative electron dynamics in crossed fields

Intense AC electric fields on semiconductor structures have been studied in photon-assisted tunneling experiments with magnetic field applied either parallel (B_par) or perpendicular (B_per) to the interfaces. We examine here the electron dynamics in a double quantum well when intense AC electric fields F, and tilted magnetic fields are applied simultaneously. The problem is treated non-perturbatively by a time-dependent Hamiltonian in the effective mass approximation, and using a Floquet-Fourier formalism. For B_par=0, the quasi-energy spectra show two types of crossings: those related to different Landau levels, and those associated to dynamic localization (DL), where the electron is confined to one of the wells, despite the non-negligible tunneling between wells. B_par couples parallel and in-plane motions producing anti-crossings in the spectrum. However, since our approach is non-perturbative, we are able to explore the entire frequency range. For high frequencies, we reproduce the well known results of perfect DL given by zeroes of a Bessel function. We find also that the system exhibits DL at the same values of the field F, even as B_par non-zero, suggesting a hidden dynamical symmetry in the system which we identify with different parity operations. The return times for the electron at various values of field exhibit interesting and complex behavior which is also studied in detail. We find that smaller frequencies shifts the DL points to lower field F, and more importantly, yields poorer localization by the field. We analyze the explicit time evolution of the system, monitoring the elapsed time to return to a given well for each Landau level, and find non-monotonic behavior for decreasing frequencies.

Laser Induced Energy Transfer in Colloidal System of Quantum Dots Coupled by FRET

Fluorescence resonant energy transfer in a system composed by two colloidal quantum dots was studied in this work. Simulations were performed applying the density matrix operator and the time evolution of the system was observed.

Probing ultrafast charge and spin dynamics in a quantum dot molecule

We apply ultrafast pump-probe photocurrent spectroscopy to directly probe few Fermion charge and spin dynamics in an artificial molecule formed by vertically stacking a pair of InGaAs self-assembled quantum dots. As the relative energy of the orbital states in the two dots are energetically tuned by applying static electric fields, pronounced anticrossings are observed arising from electron tunnel couplings. Time resolved photocurrent measurements performed in the vicinity of these anticrossings provide direct information on the comparative roles of elastic and inelastic resonant tunneling processes between the two quantum dots forming the molecule. Resonant pumping of the neutral exciton in the upper dot with circularly polarized light facilitates ultrafast initialization of hole spin qubits over timescales limited only by the laser pulse duration (<5ps) and a near perfect Pauli spin-blockade with a near unity suppression of absorption (>96%) for spin forbidden transitions. Such a spin selective photocurrent response opens the way to probe spin dynamics in the system over ultrafast timescales.

Exceptionally Narrow-Band Quantum Dot Infrared Photodetector

InGaAlAs/InGaAs/InGaAlAs/InAs/InP quantum-dot structures have been investigated for the development of infrared photodetectors capable of generating photocurrent peaks exceptionally narrow for sharp wavelength discrimination. Our specially designed structure displays a photocurrent peak at 12