Marcelo Z. Maialle

On the origin of the blueshift from type-II quantum dots emission using microphotoluminescence

We have studied type-II InP/GaAs self-assembled quantum dots by microphotoluminescence spectroscopy. Sharp spectral features were observed on top of a broad emission band. They are associated to statistical fluctuations from the ensemble of dots. Photoluminescence measurements as a function of the excitation intensity revealed markedly distinct behaviors: the broadband contour shows a large blueshift while the energy positions of the sharp features remain basically constant. We show that the large blueshift of the broad emission band in type-II quantum dots is not due to the barrier interface potential variation, but to the state filling of higher-energy states.

Photocurrent Calculation of Intersubband Transitions to Continuum-Localized States in GaAs/AlGaAs Multiquantum Wells for Mid-Infrared Photodetector

In this paper, we propose a system based on GaAs heterostructure where it is possible to generate photocurrent with mid-infrared radiation. This system is based on a central quantum well (CQW) embedded in a superlattice. Because of the CQW, which acts as a defect, there are localized states between the mini-bands in the continuum of the conduction band. Unlike the usual systems where the final states are delocalized, the oscillator strength due to the transitions between electrons occupying the ground-state to these continuum-localized states is enhanced. An applied electrical bias mixes the mini-band states with the localized state in the continuum, and due to the combined effects of strong oscillator strength and high transmission coefficients, narrow and sharp peaks are observed in the photocurrent when exciting these final states. We calculate and present results of the absorption and photocurrent for a system built to operate at 4.1 μm and discuss their dependence with the bias applied to the system and with the intensity of the incident radiation.

Infrared photocurrent with one- and two-photon absorptions in a double-barrier quantum well system

We present a theoretical investigation of a double-barrier quantum-well infrared photodetector (QWIP) having two-color selectivity. The quantum well is placed between a pair of potential barriers in order to increase selectivity through modulation of the continuum states. This also leads to a potential decrease in the dark current. Calculations are carried in the effective-mass approximation using a single-electron hamiltonian. The approach used to obtain the photocurrent yields the observation of single as well as many-photon transitions in a unified manner, by naturally accounting for real and virtual processes through intermediate states that take part in the generation of photocurrent. The two-color selectivity of the calculated photocurrent spectra comes from both one- and two-photon transitions. The performance of the system studied is compared to the results for the isolated quantum well and the advantages of the double barrier are pointed out.

Multiple-photon peak generation near the ˜10 μm range in quantum dot infrared photodetectors

We present results from simulations of the photocurrent observed in recently fabricated InAs quantum dot infrared photodetectors that respond with strong resonance peaks in the ∼10μm wavelength range. The results are in good agreement with experimental data generated earlier. Multiphoton scattering of electrons localized in the quantum dots are not only in accordance with the observed patterns, but are also necessary to explain the photocurrent spectrum obtained in the calculations.

Role of multilevel Landau-Zener interference in extreme harmonic generation

Motivated by the observation of multiphoton electric dipole spin resonance processes in InAs nanowires, we theoretically study the transport dynamics of a periodically driven five-level system, modeling the level structure of a two-electron double quantum dot. We show that the observed multiphoton resonances, which are dominant near interdot charge transitions, are due to multilevel Landau-Zener-Stuckelberg-Majorana interference. Here a third energy level serves as a shuttle that transfers population between the two resonant spin states. By numerically integrating the master equation we replicate the main features observed in the experiments: multiphoton resonances (as large as 8 photons), a robust odd-even dependence, and oscillations in the electric dipole spin resonance signal as a function of energy level detuning.

Compensation effect on the CW spin-polarization degree of Mn-based structures

We investigated the effects of Mn ions on the spin dynamics of electrons confined in a semiconductor quantum well nearby a Mn-based ferromagnetic layer. Circularly polarized Hanle and time-resolved photoluminescence (PL) measurements were carried out on a set of samples with different Mn delta-doping concentrations. We observe a strong influence of the Mn layer for both the electron lifetime and its spin-relaxation time for high-Mn concentrations, when the electrons significantly overlap with Mn ions. Our results also show that the circular-polarization degree obtained by simple continuous-wave PL measurements is not sufficient to determine the relaxation dynamics due to a compensation effect of the lifetime and the spin-relaxation time.

Exploring Parity Anomaly for Dual Peak Infrared Photodetection

In this paper, we show a superlattice quantum well infrared photodetector (S-QWIP) grown by metal-organic vapor phase epitaxy with two narrow photocurrent peaks in the mid infrared range due to transitions between the ground state from a quantum well and two excited states localized in the continuum. The structure composed of InGaAs/InAlAs quantum-well lattice matched to InP with a central quantum well acting as an artificial defect. The potential profile is carefully chosen to explore the parity anomaly of the continuum localized states and also to reduce the thermoexcited electrons decreasing the dark current. The photocurrent spectrum shows two peaks with transition energies of 300 and 460 meV (Δλ/λ of 0.13 and 0.12) at 12 K. The peak detectivity is 1.23×1010 Jones at 30 K and +5 V. When compared with a regular multiquantum well sample designed to generate photocurrent at the same wavelength, the S-QWIP shows an increase of 15 K on its background-limited performance temperature and a lower dark current for temperatures above 200 K.

Exciton spin precessions in a biased double quantum dot

We investigated the electric field effects on the spin precessions of excitons in a double quantum dot embedded in a semiconductor nanowire under an applied magnetic field. The electric field moves the carriers in the dots along the nanowire axis, modifying their confinement and therefore the effective g factors and the electron-hole exchange interaction. We obtain the time evolution of the excitonic spin and show, from the spin precession spectra, how the applied electric field affects the excitonic spin dynamics.

Generation and control of spin-polarized photocurrents in GaMnAs heterostructures

Photocurrents are calculated for a specially designed GaMnAs semiconductor heterostructure. The results reveal regions in the infrared range of the energy spectrum, in which the proposed structure is remarkably spin-selective. For such photon energies, the generated photocurrents are strongly spin-polarized. Application of a relatively small static bias in the growth direction of the structure is predicted to efficiently reverse the spin-polarization for some photon energies. This behavior suggests the possibility of conveniently simple switching mechanisms. The physics underlying the results is studied and understood in terms of the spin-dependent properties emerging from the particular potential profile of the structure.

Terahertz-field-induced tunneling current with nonlinear effects in a double quantum well coupled to a continuum

We have theoretically investigated the tunneling current induced by a terahertz (THz) field applied to an asymmetric double quantum well. The excitation couples an initially localized state to a nearby continuum of extended states. We have shown that the calculated current has similar features as those present in the optical spectra, such as interference effects due to the interaction between the continuum and the localized states, in addition to many-photon transition effects. The induced current is calculated as a function of the intensity of the THz field. A second THz field is used to yield nonlinear processes, useful to control the interference effects. We believe that part of the issues studied here can be useful for the integration of novel switching mechanisms based on optics (THz) and electronic current.