Ensemble Of Self-assembled Quantum Dots Research Articles

Dephasing of InAs quantum dot p -shell excitons studied using two-dimensional coherent spectroscopy

The dephasing mechanisms of p-shell and s-shell excitons in an InAs self-assembled quantum dot ensemble are examined using two-dimensional coherent spectroscopy (2DCS). 2DCS provides a comprehensive picture of how the energy level structure of dots affects the exciton dephasing rates and recombination lifetimes. We find that at low temperatures, dephasing of s-shell excitons is lifetime limited, whereas p-shell excitons exhibit significant pure dephasing due to scattering between degenerate spin states. At elevated temperatures, quadratic exciton-phonon coupling plays an important role in both s-shell and p-shell exciton dephasing. We show that multiple p-shell states are also responsible for stronger phonon dephasing for these transitions

Detuning dependence of Rabi oscillations in an InAs self-assembled quantum dot ensemble

Inhomogeneous broadening in ensembles of semiconductor quantum dots (QDs) has hindered coherent operations due to the detuning effects, caused by the large fluctuation in the QD transition energy due to size dispersion. This difficulty is especially evident when using femtosecond laser pulses for excitation. Here, the authors successfully measure detuning-dependent coherent evolution for a QD ensemble by employing prepulse two-dimensional coherent spectroscopy. The dephasing mechanism is found to be significantly different from that observed using picosecond lasers. This work presents a novel method to explore collective phenomena, such as superfluorescence, as well as motivating future studies of coherent interactions in the femtosecond regime.

Coherent Control of the Exciton-Biexciton System in an InAs Self-Assembled Quantum Dot Ensemble.

Coherent control of a strongly inhomogeneously broadened system, namely, InAs self-assembled quantum dots, is demonstrated. To circumvent the deleterious effects of the inhomogeneous broadening, which usually masks the results of coherent manipulation, we use prepulse two-dimensional coherent spectroscopy to provide a size-selective readout of the ground, exciton, and biexciton states. The dependence on the timing of the prepulse is due to the dynamics of the coherently generated populations. To further validate the results, we performed prepulse polarization dependent measurements and confirmed the behavior expected from selection rules. All measured spectra can be excellently reproduced by solving the optical Bloch equations for a 4-level system.

Spin noise of electrons and holes in (In,Ga)As quantum dots: Experiment and theory

The spin fluctuations of electron and hole doped self-assembled quantum dot ensembles are measured optically in the low-intensity limit of a probe laser in absence and presence of longitudinal or transverse static magnetic fields. The experimental results are modeled by two complementary approaches based either on semiclassical or quantum mechanical descriptions. This allows us to characterize the hyperfine interaction of electron and hole spins with the surrounding bath of nuclei on time scales covering several orders of magnitude. Our results demonstrate (i) the intrinsic precession of the electron spin fluctuations around the effective nuclear Overhauser field caused by the host lattice nuclear spins, (ii) the comparably long time scales for electron and hole spin decoherence, as well as (iii) the dramatic enhancement of the spin lifetimes induced by a longitudinal magnetic field due to the decoupling of nuclear and charge carrier spins.

Photon-photon correlation statistics in the collective emission from ensembles of self-assembled quantum dots

We present a theoretical analysis of the intensity autocorrelation for the spontaneous emission from a planar ensemble of self-assembled quantum dots. Using the quantum jump approach, we numerically simulate the evolution of the system and construct photon-photon delay time statistics that approximates the second order correlation function of the field. The form of this correlation function in the case of collective emission from a highly homogeneous ensemble qualitatively differs form that characterizing an ensemble of independent emitters (inhomogeneous ensemble of uncoupled dots). The signatures of collective emission in the intensity correlations are observed also in the case of an inhomogeneous but sufficiently strongly coupled ensemble. Thus, we show that the second order correlation function of the emitted field provides a sensitive test of cooperative effects.

Transport spectroscopy of non-equilibrium many-particle spin states in self-assembled quantum dots

Self-assembled quantum dots (QDs) are prominent candidates for solid-state quantum information processing. For these systems, great progress has been made in addressing spin states by optical means. In this study, we introduce an all-electrical measurement technique to prepare and detect non-equilibrium many-particle spin states in an ensemble of self-assembled QDs at liquid helium temperature. The excitation spectra of the one- (QD hydrogen), two- (QD helium) and three- (QD lithium) electron configuration are shown and compared with calculations using the exact diagonalization method. An exchange splitting of 10 meV between the excited triplet and singlet spin states is observed in the QD helium spectrum. These experiments are a starting point for an all-electrical control of electron spin states in self-assembled QDs above liquid helium temperature.

Determination of spin polarization in InAs∕GaAs self-assembled quantum dots

The spin polarization of electrons trapped in InAs self-assembled quantum dot ensembles is investigated. A statistical approach for the population of the spin levels allows one to infer the spin polarization from the measured values of the addition energies. From the magnetocapacitance spectroscopy data, the authors found a fully polarized ensemble of electronic spins above 10T when B∥[001] and at 2.8K. Finally, by including the g-tensor anisotropy, the angular dependence of the spin polarization with the magnetic field B orientation and strength could be determined.

Quantum light memory using quantum dot spins in a microdisk cavity via Raman process

A solid state quantum circuit where an ensemble of self-assembled quantum dots in a microdisk cavity served as long-lived quantum light memory, is investigated. It is shown that via laser coupling Raman process, the coherent transfer between the light field (qubits) and the ensemble spin states of the quantum dots can be efficient and fast. The coherence properties of the system are analyzed, which enables us to obtain a long coherence time.

Probing the indium clustering in InGaAs∕GaAs quantum wells by room temperature contactless electroreflectance and photoluminescence spectroscopy

Room temperature contactless electroreflectance (CER) supported by photoluminescence (PL) has been proposed as a fast and nondestructive ex situ technique for testing the effect of atom clustering in quantum wells (QWs). The indium clustering in InGaAs∕GaAs QWs was achieved by increasing the growth temperature. It has been shown that this effect causes significant changes in the spectral response. While the line shape of the GaAs-related CER feature remains unaffected there appear broad resonances similar to those for naturally inhomogeneous ensemble of self-assembled quantum dots instead of sharp and intensive lines characteristic for QWs. Additionally, the PL signal exhibits a quantum-dot-like behavior as well, i.e., strongly broadened Gaussian-like peaks with linear excitation power dependence on their intensity and the occurrence of the state filling effect for high excitation.

Spectral and threshold performance of patterned quantum dot lasers

Semiconductor quantum dots have been widely researched as a means of improving the performance of optoelectronic devices. Self-assembly has been the dominant method of fabricating quantum dots because of its relative ease compared to more explicit techniques. We have developed a method for fabricating quantum dots in a more explicit manner using electron beam lithography and selective-area metal-organic chemical vapor deposition crystal growth. By eliminating the dependence on strain-driven self-assembly, we can avoid the size distribution and resulting inhomogeneously broadened emission spectrum associated with self-assembled quantum dot ensembles. We report on the threshold and spectral properties of patterned quantum dot lasers. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Carrier emission from the electronic states of self-assembled indium arsenide quantum dots

We have used the new technique of high resolution (Laplace) transient spectroscopy to examine the electronic states of ensembles of self-assembled quantum dots of InAs in a GaAs matrix. These have been produced by solid source MBE. We have monitored the s and p state occupancies as a function of time under thermal excitation over a range of temperatures after electrons have been captured by the quantum dots with different Fermi level positions. This can provide more information about the interaction of the dots with the host matrix than is possible with optical techniques and gives new fundamental insights into how such dots may operate in electronic devices such as memory and sensors. The increase in resolution of Laplace transient spectroscopy over conventional experiments reveals quite specific rates of carrier loss which we attribute to tunnelling at low temperatures and a combination of thermal emission and tunnelling as the temperature is increased.

Precise measurement of the fraction of charged dots in self-assembled quantum dot ensembles using ultrafast pump-probe techniques

We demonstrate that polarization-resolved ultrafast pump-probe spectroscopy can be used to measure accurately (<10%) the fraction of charged dots in quantum dot (QD) ensembles. The method is applied to study the dependence of charging levels in thermally annealed InGaAs QDs, grown both as nominally undoped and n-type modulation-doped structures. We also show that the method can be used to study the spectral distribution of charges (or charging profiles) in quantum dot ensembles.

Impact of carrier redistribution on the photoluminescence of CdTe self-assembled quantum dot ensembles

We report on the photoluminescence (PL) measurements of two structurally different CdTe self-assembled quantum dot (QD) samples. These include CdTe QD's connected with the uniform two-dimensional wetting layer (WL) and fully developed CdTe QD's that are isolated from each other. Both temperature and excitation power dependencies found for the ensemble of fully developed QD's reflect that of individual QD's. In this case, no signature of intradot carrier redistribution is observed even at 120 K. In contrast, both continuous-wave and time-resolved PL show the presence of strong thermally induced carrier redistribution for the structure, where the dots are connected by a uniform two-dimensional WL. In particular, three distinct temperature regions are identified. At temperatures below 30 K, dots in the ensemble are isolated from each other: the emission occurs only from QD's occupied initially by both electron and hole. When temperature increases $(Tg30\mathrm{K}),$ the carriers from QD's are thermally activated into the uniform WL and are subsequently captured by other QD's. Finally, above 100 K the nonradiative recombination in the WL prevents the capture of thermally activated carriers. These results, while providing a thorough understanding of the optical properties of QD structures, give also important information about structural complexity of epitaxially grown II-VI semiconductor QD's.

Spectral engineering of carrier dynamics in In(Ga)As self-assembled quantum dots

Time-resolved photoluminescence upconversion with 200 fs resolution is used to investigate the carrier capture, energy relaxation, and radiative recombination in two self-assembled quantum-dot ensembles with distinctly different sizes and energy spectra. When carriers are excited into the wetting layer at low density and low lattice temperature, the relaxation time to the ground state of the larger dots is ∼1 ps, but the corresponding time for the smaller dots with larger energy spacings is ∼7 ps. This, along with the observed temperature dependence, suggests phonon participation in the relaxation process. At low temperatures, the radiative recombination time in the smaller dots is approximately twice that of the larger dots. The reduced oscillator strength in the smaller dots may be due to a reduced electron–hole wave-function overlap in the smaller dots, in addition to a size-dependent super-radiance effect.

Electron distribution and level occupation in an ensemble ofInxGa1−xAs/GaAsself-assembled quantum dots

We presented capacitance-voltage characteristics and electron-filling reflectance measurements to investigate electron distribution in ${\mathrm{In}}_{0.5}{\mathrm{Ga}}_{0.5}\mathrm{As}$ self-assembled quantum dot ensemble. First, the electronic structures of the quantum dots were constructed by capacitance-voltage profile. Coulomb charging effects on the electronic structures were also discussed. Then, the electron level occupations were investigated by the electron-filling reflectance spectra. Due to the correlated carrier transfer among the quantum dots and the n-type GaAs environment, the electron level filling is found to be inhomogeneous near the Fermi level. Finally, electron thermal population in the quantum dot levels was also investigated. The activation energies for the thermal population were found to be close to the level splitting. This means that electrons thermally populated to higher state do not require Coulomb charging energy.

Carrier thermalization within a disordered ensemble of self-assembled quantum dots

The electroluminescence (EL) and photocurrent (PC) spectra of p-i-n structures containing ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{A}\mathrm{s}/\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}$ self-assembled quantum dots (QD's) are studied from 10 to 290 K. Comparison between the EL and PC shows a Stokes Shift, i.e., the QD emission is redshifted with respect to the QD absorption in PC. The magnitude of the Stokes Shift depends on the temperature and on the extent of the dot energy dispersion, as measured by the QD absorption linewidth in different samples. The origin of the Stokes shift is discussed in terms of carrier thermalization effects by analogy with carrier distribution in disordered quantum wells.

Temperature effects on the radiative recombination in self-assembled quantum dots

Several ensembles of self-assembled quantum dots (QDs) based on the AlInAsAl/GaAs and InGaAs/GaAs material systems have been investigated using photoluminescence (PL), PL excitation (PLE) and time-resolved PL (TRPL). The influence of the temperature is measured by monitoring sharp spectral features (as narrow as ∼ 90 μeV) obtained when probing the PL of small QD ensembles (few hundreds QDs). Thermionic emission of the photocarriers out of the QD potential is found to be the dominant mechanism leading to the thermal quenching of the PL and temperature-independent linewidths are observed up to the onset of the PL quenching.

Photocarrier recombination in AlyIn1-yAs/AlxGa1-xAs self-assembled quantum dots.

Ensembles of self-assembled quantum dots (QD's) are investigated using photoluminescence (PL) and resonant time-resolved PL in the visible spectrum. Faster carrier relaxation/recombination dynamics (\ensuremath{\sim}250 ps) are observed for resonant excitations. The influence of the temperature and the intensity of excitation is also measured by monitoring sharp spectral features (\ensuremath{\sim}200 \ensuremath{\mu}eV) obtained when probing the PL of small ensembles (\ensuremath{\sim}600 QD's). Temperature-independent linewidths are observed up to 70 K (${\mathit{k}}_{\mathit{B}}$T\ensuremath{\gg}200 \ensuremath{\mu}eV). For higher excitation intensity, however, the collective background, emitted by the ensemble of QD's, is enhanced relative to the amplitude of the individual sharp spectral features.