Neutral Quantum Dots Research Articles

Light-polarization-independent nuclear spin alignment in a quantum dot

We report optical pumping of neutral quantum dots leading to nuclear spin alignment with direction insensitive to polarization and wavelength of light. Measurements of photoluminescence of both "dark" and "bright" excitons in single dots reveal that nuclear spin pumping occurs via a virtual spin-flip transition between these states accompanied by photon emission. The sign of the nuclear spin polarization is determined by asymmetry in the exciton energy spectrum, rather than by the sign of the exciton spin polarization.

Radiative cascade from quantum dot metastable spin-blockaded biexciton

We detect a novel radiative cascade from a neutral semiconductor quantum dot. The cascade initiates from a metastable biexciton state in which the holes form a spin-triplet configuration, Pauli-blockaded from relaxation to the spin-singlet ground state. The triplet biexciton has two photon-phonon-photon decay paths. Unlike in the singlet-ground state biexciton radiative cascade, in which the two photons are co-linearly polarized, in the triplet biexciton cascade they are crosslinearly polarized. We measured the two-photon polarization density matrix and show that the phonon emitted when the intermediate exciton relaxes from excited to ground state, preserves the exciton's spin. The phonon, thus, does not carry with it any which-path information other than its energy. Nevertheless, entanglement distillation by spectral filtering was found to be rather ineffective for this cascade. This deficiency results from the opposite sign of the anisotropic electron-hole exchange interaction in the excited exciton relative to that in the ground exciton.

Enhancing the transdermal delivery of rigid nanoparticles using the simultaneous application of ultrasound and sodium lauryl sulfate

The potential of rigid nanoparticles to serve as transdermal drug carriers can be greatly enhanced by improving their skin penetration. Therefore, the simultaneous application of ultrasound and sodium lauryl sulfate (referred to as US/SLS) was evaluated as a skin pre-treatment method for enhancing the passive transdermal delivery of nanoparticles. We utilized inductively coupled plasma mass spectrometry and an improved application of confocal microscopy to compare the delivery of 10- and 20-nm cationic, neutral, and anionic quantum dots (QDs) into US/SLS-treated and untreated pig split-thickness skin. Our findings include: (a) ∼0.01% of the QDs penetrate the dermis of untreated skin (which we quantify for the first time), (b) the QDs fully permeate US/SLS-treated skin, (c) the two cationic QDs studied exhibit different extents of skin penetration and dermal clearance, and (d) the QD skin penetration is heterogeneous. We discuss routes of nanoparticle skin penetration and the application of the methods described herein to address conflicting literature reports on nanoparticle skin penetration. We conclude that US/SLS treatment significantly enhances QD transdermal penetration by 500–1300%. Our findings suggest that an optimum surface charge exists for nanoparticle skin penetration, and motivate the application of nanoparticle carriers to US/SLS-treated skin for enhanced transdermal drug delivery.

All-optical spin switching in neutral or charged magnetic quantum dots

The spin dynamics in a single semiconductor quantum dot doped with a single Mn atom are analyzed. We consider a neutral and a negatively charged dot and in both cases we concentrate on the light hole-to-conduction band transition. Both electrons and light holes couple to the Mn spin via the strong exchange interaction. After the excitation by an ultra short laser pulse oscillatory spin dynamics take place, where electron or hole can flip their spin accompanied by a change of the Mn spin. The Mn spin dynamics can be controlled by excitation with additional pulses. Starting from a given initial state we demonstrate that the Mn spin can be flipped all-optically into a steady final state in both neutral or charged quantum dots.

Coherent spin precession of electrons and excitons in charge tunable InP quantum dots

Coherent spin precession of electrons and excitons is observed in charge tunable InP quantum dots under the transverse magnetic field by means of time-resolved Kerr rotation. In a quantum dot doped by one electron, spin precession of the doped electron in the quantum dot starts out of phase with spin precession of the doped electrons in a GaAs substrate just after a trion is formed and persists for more than 2 ns even after the trion recombines. Simultaneously spin precession of a trion (hole) starts. Observation of spin precession of both a doped electron and a trion (hole) confirms creating coherent superposition of an electron and a trion as the initialization process of spin of doped electrons in quantum dots. In a neutral quantum dot, the exciton spin precession starts out of phase with spin precession of the doped electrons in a GaAs substrate and the precession frequency does not converge to 0 at the zero field limit. It contains the electron–hole exchange interaction and corresponds to the splitting between bright and dark excitons under the transverse magnetic field.

Controlling the Polarization Eigenstate of a Quantum Dot Exciton with Light

We demonstrate optical control of the polarization eigenstates of a neutral quantum dot exciton without any external fields. By varying the excitation power of a circularly polarized laser in microphotoluminescence experiments on individual InGaAs quantum dots we control the magnitude and direction of an effective internal magnetic field created via optical pumping of nuclear spins. The adjustable nuclear magnetic field allows us to tune the linear and circular polarization degree of the neutral exciton emission. The quantum dot can thus act as a tunable light polarization converter.

Strong optical field study of a single self-assembled quantum dot

We review the investigation of a single quantum dot driven by a strong optical field. By coherent pump-probe spectroscopy, we demonstrate the Autler–Townes splitting and Mollow absorption spectrum in a single neutral quantum dot. Furthermore, we also show the typical Mollow absorption spectrum by driving a singly charged quantum dot in a strong optical coupling regime. Our results show all the typical features of an isolated atomic system driven by a strong optical field, such as the AC stark effect, Rabi side bands and optical gain effect, which indicate that both neutral and charged quantum dots maintain the discrete energy level states even at high optical field strengths.

Spin orientation in charge-tunable InP quantum dots

Spin polarization in charge-tunable InP quantum dots (QDs) was investigated as a function of the longitudinal magnetic field under the linearly polarized excitation. In neutral QDs, photoluminescence (PL) polarizes resonantly at two magnetic fields where bright and dark excitons anticross each other. The resonant spin-orientation structure turns from up to down with the change of the observation photon energy, reflecting the size-variable g-factor of holes in InP/InGaP QDs. In one-electron doped QDs, on the other hand, PL polarization linearly increases with the increase of magnetic field. In two-electron doped QDs, small differential structure coming from the electron–hole anisotropic exchange interaction is superposed on the linear increase of PL polarization versus magnetic field. The linear slope observed for trions and tetraons is smaller than that expected for holes uncorrelated with the electron spins and thermalized in Zeeman sublevels and is reduced by the electron–hole exchange interaction at low temperatures.

Spin orientation of excitons, trions and tetraons in charge tunable InP quantum dots

AbstractIn the longitudinal magnetic field, photoluminescence (PL) polarization in InP quantum dots (QDs) dramatically changed depending on the number of doped electrons. Under linearly polarized excitation, PL polarizes resonantly at two magnetic fields where bright and dark excitons anticross each other in neutral QDs. On the other hand, PL polarization monotonously increases with the increase of longitudinal magnetic field in one‐ and two‐electron doped QDs, where thermal spin orientation of a photoexcited hole determines the PL polarization of a trion and a tetraon. At low temperatures, however, the electron‐hole exchange interaction reduces the thermal spin orientation of holes. In two‐electron doped QDs, the anisotropic electron‐hole exchange interaction gives small structure in the magnetic‐field‐dependent circular polarization at the low field region. Time‐resolved circular polarization clearly shows the dynamical spin polarization process in charged QDs in the longitudinal magnetic field. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Energy and Optical Absorption Spectra of Multiply Charged Anisotropic Quantum Boxes

Using a simple model of a two-dimensional rectangular quantum box we study the efiects of size and anisotropy on the energy and photoluminescence spectra of neutral and charged quantum dots. The competition of symmetries and energy/length scales of the free exciton or trion and of the conflning potential is analyzed. The numerical calculations consisted of the diagonalization of the few-electron-hole Hamiltonian matrices in the full conflguration-interaction basis, with the simultaneous resolution of the conserved orbital and spin quantum numbers.

Spin quantum beats of bright and dark excitonic states in neutral InAs quantumdots

The energy eigenvalues and eigenfunctions of heavy-hole (HH) excitons and theexpression of spin quantum beats (QBs) in neutral InAs quantum dots (QDs) inan oblique magnetic field have been calculated and derived in terms of a spinHamiltonian model. It is shown from the oscillating curves of spin QBs that theamplitude and period of QBs are determined by the magnitude and direction of themagnetic field, and that the impact of the coupling component of electrons and holes,δ0, on QBs should not be neglected even if in an intense magnetic field likeB = 4 T, so longas the angle θ≤30°; θ is the angle between the direction of the magnetic field and thez-direction (quantized direction). The influence of anisotropicg factors of electron and hole on QBs has also been studied. The calculated results show thatg-factormarkedly affected the amplitude and periodicity of QBs and the periodic oscillating of QBs vanishesat Δgx = ge,x−gh,x = 0.6, which shows that whether or not the periodic signal of QBs canbe observed is related to the ‘matching’ of the electron and holeg-factors.

Valence-band mixing in neutral, charged, and Mn-doped self-assembled quantum dots

We analyze the optical emission of single II-VI quantum dots containing 0 or 1 magnetic atom (manganese) and a controlled number of carriers (0, $\ifmmode\pm\else\textpm\fi{}1$ electron). The emission of these quantum dots presents a large degree of linear polarization. This linear polarization is attributed to a valence-band mixing and we show that in nonmagnetic quantum dots combining both a shape anisotropy and an anisotropic in-plane strain distribution, the linear polarization direction of the exciton are controlled by an interplay between valence-band mixing and electron-hole (e-h) exchange interaction. Similarly, under strong transverse magnetic field, the direction of the linearly polarized emission of the charged excitons is simultaneously controlled by the valence-band mixing and the direction of the magnetic field. In quantum dots containing a Mn atom, the valence-band mixing allows simultaneous hole-Mn spin flips coupling bright and dark excitons. These spin flips are responsible for linearly polarized transitions in the emission of the charged excitons at zero magnetic field.

Hole spin relaxation in neutral InGaAs quantum dots: Decay to dark states

The authors report measurements of hole spin relaxation in neutral InGaAs quantum dots using polarization-dependent time-resolved photoluminescence experiments. The single-particle hole spin relaxation was isolated from other spin flip processes in the electron-hole system by detecting the initial transfer of population from optically active to dark states. The results indicate that electron-hole exchange interactions play a negligible role in the carrier spin kinetics, and are consistent with a mechanism of hole spin relaxation via phonon-mediated virtual scattering between confined quantum dot states.

Redistribution of photogenerated carriers in neutral and charged InAs quantum dot systems

Time-integrated and time-resolved photoluminescence spectra of neutral and negatively charged self-assembled InAs quantum dots (QDs) were studied. Obtained spectra have indicated that the redistribution of carriers in QDs occurs in all samples, but the temperature dependence of spectra are quite different for neutral and charged QDs. To clarify the origin of these behaviors, a model calculation based on two possible redistribution mechanisms has been carried out, and compared with experiments to show that the carrier tunneling between neighboring QDs is suppressed in charged QDs.

Coherent spin dynamics in semiconductor quantum dots

The anisotropic exchange interaction (AEI) between electrons and holes is shown to play a central role in quantum dots (QDs) spin dynamics. In neutral QDs, AEI is at the origin of spin quantum beats observed under resonant excitation between the lowest energy doublet of linearly dipole-active eigenstates. In negatively charged QDs, AEI is at the origin of QD emission with opposite helicity to the optic al excitation, under non-resonant excitation conditions. Finally, the possibility of leaving a spin information in the system after recombination of the photo-injected electron-hole pair is discussed with respect to the type and the level of the doping. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Tunneling-induced dephasing in InP quantum dots

We observed the tunneling process of photo-excited holes in neutral InP quantum dots and Pauli blocking of charged InP quantum dots. A highly sensitive heterodyne-detected photon echo method enabled us to observe the signal from one layer of self-assembled InP quantum dots under the electric field. The electric field could control the charging or neutralization of the InP quantum dots and hence the photon echo signal decreased considerably with the increase of electron doping. The photon echo of neutral InP quantum dots under the electric field showed tunneling-induced dephasing, which decays non-exponentially reflecting the non-Markovian nature of the tunneling process.

Molecular spintronics: Coherent spin transfer in coupled quantum dots

Time-resolved Faraday rotation has recently demonstrated coherent transfer of electron spin between quantum dots coupled by conjugated molecules. Using a transfer Hamiltonian ansatz for the coupled quantum dots, we calculate the Faraday rotation signal as a function of the probe frequency in a pump-probe setup using neutral quantum dots. Additionally, we study the signal of one spin-polarized excess electron in the coupled dots. We show that, in both cases, the Faraday rotation angle is determined by the spin transfer probabilities and the Heisenberg spin exchange energy. By comparison of our results with experimental data, we find that the transfer matrix element for electrons in the conduction band is of order 0.08 eV and the spin transfer probabilities are of order 10%.

Efficient electron spin detection with positively charged quantum dots

We report the application of time- and polarization-resolved photoluminescence up-conversion spectroscopy to the study of spin capture and energy relaxation in positively and negatively charged, as well as neutral InAs self-assembled quantum dots. When compared to the neutral dots, we find that carrier capture and relaxation to the ground state is much faster in the highly charged dots, suggesting that electron–hole scattering dominates this process. The long spin lifetime, short capture time, and high radiative efficiency of the positively charged dots, indicates that these structures are superior to both quantum well and neutral quantum dot light-emitting diode spin detectors for spintronics applications.

Excited States in optically‐gated charged single InAs quantum dots

Using a micro-spectroscopy technique, resonant excitation of the excited states of single neutral and charged InAs quantum dots are observed. Photo-depletion is used to efficiently control the presence of excess carriers in the quantum dot. Due to their different Coulomb energy shifts, the charged and neutral states of the same quantum dot can be selectively excited. Photoluminescence excitation spectroscopy allows to compare the structure of the excited sates in neutral and charged quantum dots. Time-resolved coherent spectroscopy shows that the dephasing time of the excited states is longer when the quantum dot is charged.

Electron scattering in GaAs/n‐AlGaAs selectively doped heterojunctions with charged and neutral InGaAs quantum dots

The scattering process of electrons has been studied in selectively doped GaAs/n-AlGaAs inverted heterojunctions, where InGaAs dots are embedded in the vicinity of a GaAs channel. By performing both capacitance and Hall measurements, the percentage Poc of charged dots filled with an electron is determined as a function of the gate voltage Vg and is found to vary from about 30% to 90% with Vg. We have also examined both mobilities μ and the ratio of the quantum lifetime τq to the transport lifetime τt of electrons. It is found that μ increases with Vg(Poc), suggesting that the scattering by a charged dot is weaker than that by a neutral dot. It is also found that the ratio τq/τt decreases with Vg(Poc), indicating that the scattering potential associated with a charged dot is of a longer range as compared with the scattering by a neutral dot.