Isolated Quantum Dots Research Articles

Spin hot spots in vertically coupled few-electron isolated quantum dots

The effects of spin-orbit (SO) coupling arising from the confinement potential in single and two vertically-coupled quantum dots have been investigated. Our work indicates that a dot containing a single electron shows the lifting of the degeneracy of dipole-allowed transitions at B=0 due to the SO coupling which disappears for a dot containing two electrons. For coupled dots with one electron in each dot, the optical spectra is not affected by the coupling and is the same as the dot containing one electron. However, for the case of two coupled dots where one partner dot has two interacting electrons while the other dot has only one electron, a remarkable effect is observed where the oscillator strength of two out of four dipole-allowed transition lines disappears as the distance between the dots is decreased.

Singularities in the magneto-photoluminescence spectra from isolated quantum dots

We have optically investigated quantum dots containing a few electrons each. In zero magnetic field, the photoluminescence (PL) should reflect the occupied density of states of such artificial atoms. However, the few-electron response to recombination of one of these electrons with a localized hole appears to be enhanced as the energy of the electrons approaches the highest occupied state. This phenomenon resembles the Fermi-edge singularity observed in the PL spectra from n-type two- and one-dimensional electron systems. Like in the case of doped n-type quantum wires, this singularity is seen to dominate the PL spectrum of the quantum dots at a low magnetic field with a sharp fall in intensity at a specific magnetic field.

Isolated quantum dot in application to terahertz photon counting

We present an experimental study of a large quantum dot (QD) capacitively coupled to an aluminum single electron transistor (SET) and irradiated with terahertz radiation from a blackbody source. The SET is used as a noninvasive electrometer sensitive to a single-charge fluctuation on the QD. Qualitatively different regimes of QD confinement have been identified from the SET response. We demonstrate that the state of a nearly isolated QD can potentially be used for counting individual terahertz photons.

Effect of the longitudinal and transverse stacking period of InAs/GaAs quantum dots on the distribution of strain field

A systematic investigation is made on the influence of the longitudinal and transverse period distributions of quantum dots on the elastic strain field. The results showed that the effects of the longitudinal period and transverse period on the strain field are just opposite along the direction of center-axis of the quantum dots, and under proper conditions, both effects can be eliminated. The results demonstrate that in calculating the effect of the strain field on the electronic structure, one must take into account the quantum dots period distribution, and it is inadequate to use the isolated quantum dot model in simulating the strain field.

Frequency shift imaging of quantum dots with single-electron resolution

We employ atomic force microscope-based frequency shift microscopy to study the electronic properties of quantum dots formed in carbon nanotubes. The nontransport detection scheme of frequency shift allows us to probe nearly isolated quantum dots in a few electron regime. At 4K, we observe Coulomb oscillations of quantum dots with single-electron resolution and extract the charging energy of a quantum dot.

Fast exciton spin relaxation in single quantum dots

Exciton spin relaxation is investigated in single epitaxially grown semiconductor quantum dots in order to test the expected spin relaxation quenching in this system. We study the polarization anisotropy of the photoluminescence signal emitted by isolated quantum dots under steady-state or pulsed non-resonant excitation. We find that the longitudinal exciton spin relaxation time is strikingly short ($\leq$100 ps) even at low temperature. This result breaks down the picture of a frozen exciton spin in quantum dots.

Wave function mapping at Fano resonances and conductance switching in open quantum dots

We numerically simulate the effects of a controllable impurity induced by an atomic microscope tip scanned on an open quantum dot with highly transmitting channels. The coupled quantum dot – tip system may result in a transistor like device, for which the conductance switching is a function of the position, spatial dimensions and potential of the perturbation induced by the tip. For realistic dimensions we show that energy shifts of the resonances deliver a bona fide imaging of low energy isolated quantum dot states, as well as of higher energy states coupled to the continuum (Fano resonances). (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Electronic spectrum of a two-dimensional quantum dot array in the presence of electric and magnetic fields in the Hall configuration

We report calculations of the electronic spectrum of a two-dimensional lattice of coupled quantum dots, subject to external electric and magnetic fields in the Hall configuration. The quantum dots array was modeled by a periodic superposition of truncated, parabolic potential wells. By adopting the Landau gauge, a single-particle Hamiltonian was formulated, and its eigenfunctions were obtained as appropriately symmetrized, magnetic field-dependent Bloch functions. The magnetic field was consistently included in the corresponding Wannier functions, which were approximated by the eigenvectors of an isolated quantum dot in the presence of the external magnetic field, and multiplied by the Peierls's phase.

Modeling room-temperature lasing spectra of 1.3-μm self-assembled InAs∕GaAs quantum-dot lasers: Homogeneous broadening of optical gain under current injection

We studied the injection current dependence of room-temperature lasing spectra of a 1.3-μm self-assembled InAs∕GaAs quantum-dot laser both experimentally and theoretically. Starting from the ground-state lasing with a few longitudinal modes, the spectra showed splitting, broadening, excited-state lasing, and quenching of the ground-state lasing as the current increased. We could explain this unique current dependence by numerical simulation based on our quantum-dot laser theory, taking into account the inhomogeneous and homogeneous broadening of the optical gain as well as the carrier relaxation processes in the spatially isolated quantum dots. Through the simulation, we found that the homogeneous broadening of the ground state is kept between 5 and 10 meV under the ground-state lasing, while it increases up to 20 meV under the excited-state lasing.

Molecular epitaxy and the electronic properties of Ge/Si heterosystems with quantum dots

We present the results of a study of the processes of preparation and the determination of the electrical and optical characteristics of arrays of Ge nanoislands in Si (artificial “atoms”) with a discrete energy spectrum, which is manifested all the way up to room temperature. We give a brief analysis of the current ideas about the mechanisms of the initial stage of self-formation and ordering of ensembles of nanoclusters in the heteroepitaxy of Ge on Si. The main factors that determine the spectrum of states are the size quantization and Coulomb interaction of the carriers. It is shown that a new factor arising in an array of quantum dots and which distinguishes it from the situation for an isolated quantum dot is the presence of Coulomb correlations between islands. The rate of emission and the cross section for trapping of holes are determined as functions of the energy level depth. The values of the cross sections are several orders of magnitude greater than the known values in Si. The electron transport along the layers of quantum dots is occurs via hopping conduction, the value of which oscillates as the degree of occupation of the occupation of the islands by holes; this can serve as a working principle for electronic data transmission circuits based on quantum dots. It is shown that Ge quantum dots can be used to construct a photodetector tunable over the near- and mid-IR ranges.

Optical properties of single semiconductor quantum dots

We review the electronic and optical properties of single self-organized semiconductor quantum dots. A particular attention is paid to the description of the fundamental electron/hole levels, which govern the ground state interband transition. We show that a fine description of the fundamental transition goes far beyond a simple atomic-like picture, and that the optical properties of an isolated quantum dot are dependent upon different weak couplings, which reflect its intrinsic solid state nature and should be incorporated in any theoretical scheme aiming at describing its response to light. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Rabi oscillations in a semiconductor quantum dot: Influence of local fields

The influence of electron-hole dipole-dipole interactions (local fields) on the excitonic Rabi oscillations in an isolated quantum dot (QD) has been theoretically investigated. An analysis for the QD interaction with monochromatic field and ultrashort Gaussian pulse has been performed. On the basis of optical Bloch equations the Rabi oscillation dynamics has been investigated. As a result, the bifurcation and essentially anharmonic regimes in the Rabi oscillations in a QD exposed to the monochromatic field have been predicted. The strong dependence of the period of Rabi oscillations on the QD depolarization has been revealed. For the Gaussian pulse it has been shown that the final state of inversion as a function of the pulse peak strength demonstrates step-like transitions.

Electronic structure and transport properties of quantum dots

AbstractThe subject of this paper are electronic properties of isolated quantum dots as well as transport properties of quantum dots coupled to two electronic reservoirs. Thereby special focus is put on the effects of Coulomb interaction and possible correlations in the quantum dot states. First, the regime of sequential tunneling to the reservoirs is investigated. It is shown that in case degenerate states participate in transport, the resonance positions in the differential conductance generally depend on temperature and the degree of degeneracy. This effect can be used to directly probe degeneracies in a quantum dot spectrum. A further effect, characteristic for sequential tunneling events, is the complete blocking of individual channels for transport. A generalisation of the well known spin blockade is found for correlated dot states transitions through which are not directly spin‐forbidden. In the second part, the electronic structure of spherical quantum dots is calculated. In order to account for correlation effects, the few‐particle Schrödinger equation is solved by an exact diagonalization procedure. The calculated electronic structure compares to experimental findings obtained on colloidal semiconductor nanocrystals by Scanning Tunneling Spectroscopy. It is found that the electric field induced by the tunneling tip is gives rise to a Stark effect which can break the spherical symmetry of the electronic ground state density which is in agreement with wave‐function mapping experiments. The symmetry breaking depends on the competition between exchange energy and the Stark energy. Moreover, a systematic dependence on particle number is found for the excitation energies of optical transitions which explains recent experimental findings on self‐organized quantum dots. In the last part, co‐tunneling in the Coulomb blockade regime is studied. For this end the tunneling current is calculated up to the forth order perturbation theory in the tunnel coupling by a real‐time Green's function approach for the non‐equilibrium case. The differential conductance calculated for a quantum dot containing up to two interacting electrons shows complex signatures of the excitation spectrum which are explained by a combination of co‐tunneling and sequential tunneling processes. Thereby the calculations show a peak structure within the Coulomb blockade regime which has also been observed in experiment.

Electronic structure and transport properties of quantum dots

The subject of this paper are electronic properties of isolated quantum dots as well as transport properties of quantum dots coupled to two electronic reservoirs. Thereby special focus is put on the effects of Coulomb interaction and possible correlations in the quantum dot states. First, the regime of sequential tunneling to the reservoirs is investigated. It is shown that in case degenerate states participate in transport, the resonance positions in the differential conductance generally depend on temperature and the degree of degeneracy. This effect can be used to directly probe degeneracies in a quantum dot spectrum. A further effect, characteristic for sequential tunneling events, is the complete blocking of individual channels for transport. A generalisation of the well known spin blockade is found for correlated dot states transitions through which are not directly spin-forbidden. In the second part, the electronic structure of spherical quantum dots is calculated. In order to account for correlation effects, the few-particle Schrödinger equation is solved by an exact diagonalization procedure. The calculated electronic structure compares to experimental findings obtained on colloidal semiconductor nanocrystals by Scanning Tunneling Spectroscopy. It is found that the electric field induced by the tunneling tip is gives rise to a Stark effect which can break the spherical symmetry of the electronic ground state density which is in agreement with wave-function mapping experiments. The symmetry breaking depends on the competition between exchange energy and the Stark energy. Moreover, a systematic dependence on particle number is found for the excitation energies of optical transitions which explains recent experimental findings on self-organized quantum dots. In the last part, co-tunneling in the Coulomb blockade regime is studied. For this end the tunneling current is calculated up to the forth order perturbation theory in the tunnel coupling by a real-time Green's function approach for the non-equilibrium case. The differential conductance calculated for a quantum dot containing up to two interacting electrons shows complex signatures of the excitation spectrum which are explained by a combination of co-tunneling and sequential tunneling processes. Thereby the calculations show a peak structure within the Coulomb blockade regime which has also been observed in experiment.

Anomalous Stark effect in the ground state of the confined hydrogen atom in a spherical quantum dot

We present an accurate numerical calculation for the energy levels and resonance widths of the quasibound states of a confined hydrogen atom in an isolated quantum dot subjected to an external electric field. Resonance positions and widths are reported for a wide range of dot sizes to demonstrate that Stark resonances in a confined hydrogen atom lead to interesting phenomena as a consequence of the quantum confinement of the atom, contrary to the Stark effect on free atoms.

Effects of biexcitons on exciton decoherence processes inInxGa1−xAsquantum dots

Exciton decoherence (dephasing) properties are examined for the effects of biexcitons in ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ quantum dots. The dephasing time of higher exciton states is measured from the linewidths of discrete exciton absorption lines by employing a high-energy-resolution microphotoluminescence excitation method in single isolated quantum dots. The excitation intensity dependence and temperature dependence of the exciton linewidth are used to consider the effects of exciton-biexciton and exciton-phonon interactions. The excitation intensity dependence reveals that the exciton-biexciton interaction contributes crucially to the exciton dephasing processes. A numerical calculation undertaken with a density-matrix method interprets this result as an enhancement of the power broadening effect due to coherent coupling between excitons and biexcitons. The temperature dependence indicates that the exciton-biexciton interaction dominates the exciton-phonon interaction below 40 K. These results provide a guideline for achieving a long-lived coherence in quantum dots, which is a key to the implementation of quantum information processing.

Excitons in coupled quantum dots

Properties of excitons in vertically coupled GaAs/AlGaAs quantum dots were investigated using the variational method within the envelope function and effective mass approximations. It was found that when the thickness of the spacer layer becomes less than about one exciton Bohr radius, both the exciton binding energy and the fundamental optical transition energy are reduced compared to those in isolated quantum dots. This is a result of increased space extension of exciton due to the penetration of carrier wave functions into the spacer layer and corresponding reduction in confinement energy which dominates over the Coulomb interaction between the electron and the hole.

Electron Spin Flip by Antiferromagnetic Coupling between Semiconductor Quantum Dots

We have investigated antiferromagnetic coupling between semiconductor quantum dots. Electron spin is observed to flip at 80 ps after photoexcitation via the interdot-exchange interaction. The spin relaxation time under the antiferromagnetic order is extended to 10–12 ns, one order of magnitude longer than that in isolated quantum dots. The antiferromagnetic order exists at temperatures lower than 50–80 K. The photoluminescence experiments for various carrier densities show that antiferromagnetic coupling disappears when the electron pairing probability is low. A model calculation based on the Heitler–London approximation supports the finding that the antiferromagnetic order is observable at low temperature.

Electron spin evolution induced by interaction with nuclei in a quantum dot

We study the decoherence of a single electron spin in an isolated quantum dot induced by hyperfine interaction with nuclei for times smaller than the nuclear spin relaxation time. The decay is caused by the spatial variation of the electron envelope wave function within the dot, leading to a non-uniform hyperfine coupling $A$. We show that the usual treatment of the problem based on the Markovian approximation is impossible because the correlation time for the nuclear magnetic field seen by the electron spin is itself determined by the flip-flop processes. The decay of the electron spin correlation function is not exponential but rather power (inverse logarithm) law-like. For polarized nuclei we find an exact solution and show that the precession amplitude and the decay behavior can be tuned by the magnetic field. The decay time is given by $\hbar N/A$, where $N$ is the number of nuclei inside the dot. The amplitude of precession, reached as a result of the decay, is finite. We show that there is a striking difference between the decoherence time for a single dot and the dephasing time for an ensemble of dots.

Artificial molecules in coupled and single quantum dots

An exactly solvable model has been proposed for the artificial molecules composed of two electrons confined in double coupled and single isolated quantum dots. This model allows us to study systematically the spontaneous symmetry breaking, which results from the electron-electron correlation in the artificial molecules. By comparing the exact and Hartree-Fock results we have shown that---in the barrier-separated coupled quantum dots---the correlation increases with the increasing barrier thickness, which leads to the localization of both the electrons in the different dots, i.e., the vanishing probability of finding both the electrons in the same dot. In the single quantum dot, the correlation increases with the increasing dot size, which leads to a formation of a Wigner molecule. We have found a remarkable similarity of the electron density distribution in both the types of the artificial molecules.