Excitons In Quantum Dots Research Articles

Colloidal Quantum Dots: Think Outside the (Particle-in-a-)Box

This Perspective discusses recent work on mechanisms by which organic ligands affect the electronic structure and exciton dynamics of colloidal quantum dots (QDs). Much of the work described here uses some combination of steady-state absorption, transient absorption, steady-state photoluminescence, and transient photoluminescence spectroscopies to characterize QD–ligand complexes. Ligands affect the ground-state electronic structure of QDs via mixing of the frontier orbitals at the QD–ligand interface and influence the dynamics of excitonic decay by mediating charge trapping or by participating in charge transfer. This Perspective highlights strategies to address the various forms of structural and chemical heterogeneity of QD ensembles in identifying the mechanisms of these ligand-mediated processes. Finally, four-wave mixing techniques are discussed as promising methods for direct measurement of ligand-mediated nonradiative dissipation of the QD exciton.

Generation and dynamics of phononic cat states after optical excitation of a quantum dot

We study theoretically the fluctuation properties of optical phonons generated after optical excitation of a quantum dot. If the quantum dot exciton is optically manipulated by ultrafast laser pulses, the electronic system and the phonon system can become entangled, which strongly influences the fluctuation properties of the phonons. When reduced to the phonon system, such an entanglement corresponds to a mixed phonon state. We discuss excitations with one or two ultrafast laser pulses. For a single pulse excitation, in general, a statistical mixture of two coherent states is found. For more pulses, a statistical mixture of superpositions of coherent states builds up in the phonon system. With the help of the Wigner function, which provides an intuitive picture of the generated phonon states, we explain how these states are formed depending on the excitation conditions and illustrate their time evolution. From this the fluctuation properties of the corresponding states can be well interpreted and the conditions for obtaining phonon squeezing are identified.

Electronic states and the resonant optical non-linearity of exciton in a narrow band InSb quantum dot

Binding energy, interband emission energy and the non-linear optical properties of exciton in an InSb/InGa x Sb 1− x quantum dot are computed as functions of dot radius and the Ga content. Optical properties are obtained using the compact density matrix approach. The dependence of non-linear optical processes on the dot sizes is investigated for different Ga concentrations. The linear, third order non-linear optical absorption coefficients, susceptibility values and the refractive index changes of the exciton are calculated for different concentrations of gallium content. It is found that gallium concentration has great influence on the optical properties of InSb/InGa x Sb 1− x dots.

Phonon effects on the radiative recombination of excitons in double quantum dots

We study theoretically the radiative recombination of excitons in double quantum dots in the presence of carrier-phonon coupling. We show that the phonon-induced pure dephasing effects and transitions between the exciton states strongly modify the spontaneous emission process and make it sensitive to temperature, which may lead to nonmonotonic temperature dependence of the time-resolved luminescence. We show also that, under specific resonance conditions, the biexcitonic interband polarization can be coherently transferred to the excitonic one, leading to an extended lifetime of the total coherent polarization, which is reflected in the nonlinear optical spectrum of the system. We study the stability of this effect against phonon-induced decoherence.

Temperature-Dependent Optical Properties of InAs/GaAs Self-Assembled Quantum Dots: Spectroscopic Measurements and an Eight-Band Study

The temperature-dependent optical properties of InAs/GaAs self-assembled quantum dots are studied by spectroscopic measurements along with the corresponding theoretical calculations. We observe the redshift of photoluminescence peak energy with increasing temperature and the thermally activated quenching of each state, which result from the efficient redistribution of carriers in quantum dots. Meanwhile, the electronic structures of the InAs/GaAs quantum dots are investigated by a detailed theoretical study in terms of an eight-band k·p model, taking strain effects into account. The calculated transition energies of the excitons are in reasonable agreement with the results of the photoluminescence spectra. According to the spatial distribution of carriers, it is found that the evolution of photogenerated excitons in quantum dots with temperature mainly relies on the electrons rather than the holes.

Dynamics of above-barrier state excitons in multi-stacked quantum dots

We report the relaxation dynamics from above-barrier exciton states to the lowest one in multi-stacked quantum dots (QDs). The photoluminescence decay time increases because of the excitation of higher exciton states, which is attributed to the wider miniband width of above-barrier excitons and the localization of the envelope functions in the barrier layers. The existence of the above barrier minibands makes carrier transport along the growth direction possible and eliminates a difficulty with close QD stacking. These results demonstrate an effective approach to achieve high efficiency QD devices.

Optical orientation of bright excitons in InAs/GaAs quantum dots: Influence of a Faraday magnetic field and the dark exciton states

We study the injection of polarized bright and dark excitons in quantum dots, under nonresonant or resonant excitation, by polarization-resolved photoluminescence experiments on an ensemble of self-assembled InAs/GaAs quantum dots. The importance of the polarized dark exciton creation on the optical emission under magnetic field is discussed. Under circular excitation, we observe the expected increase and saturation of the polarization rate with a magnetic field applied in Faraday geometry. Strikingly, the polarization rate slightly decreases for magnetic fields greater than \ensuremath{\sim}1.5 T; the feature is more pronounced for higher interband energies and is attributed to a more efficient initial polarization of the dark exciton states. This interpretation is confirmed by the lack of decrease of the polarization rate for quantum dots excited at exact resonance through a 1LO-phonon-assisted transition. Finally, we measure the bright exciton exchange energy as a function of interband emission energy, we measure a decrease from 65 to 30 \ensuremath{\mu}eV in the range 1.28--1.35 eV, and we obtain an estimate of the dark exciton splitting.

Efficient Charge Separation in Multidimensional Nanohybrids

We report unidirectional charge transfer in multidimensional nanohybrids, consisting of a quantum dot, an electronically active molecular linker, and a carbon nanotube. After covalent attachment to the nanotube, only emission consistent with the negatively charged quantum dot exciton ion rather than the neutral exciton is observed, showing nearly monoexponential recombination kinetics and an average lifetime of 3.5 ns. Using kinetic models, we explain how charge transfer is biased at the expense of other decay pathways.

Hole Transfer from Single Quantum Dots

Photoinduced hole transfer dynamics from single CdSe/CdS(3ML)/CdZnS(2ML)/ZnS(2ML) core/multishell quantum dots (QDs) to phenothiazine (PTZ) molecules were studied by single QD fluorescence spectroscopy to investigate the static and dynamic heterogeneities of the hole transfer process as well as its effect on the blinking dynamics of QDs. Ensemble-averaged transient absorption and fluorescence decay measurements show that excitons in QDs dissociate by transferring the valence band hole to PTZ with a time constant of 50 ns for the 1:1 PTZ-QD complex, and the subsequent charge recombination process (i.e., electron transfer from the conduction band of the reduced QD to oxidized PTZ to regenerate the complex in the ground state) occurs mainly on the 100 to 1000 ns time scale. Single QD-PTZ complexes show pronounced correlated fluctuations of fluorescence intensity and lifetime with time. In addition to the dynamic fluctuation, there are considerable heterogeneities of average hole transfer rate among different QD-PTZ complexes. The hole transfer process has little effect on the statistics of the off-states, which is often believed to be positively charged QDs with a valence band hole. Instead, it increases the probability of weakly emissive or "gray" states.

Hot Exciton Relaxation Dynamics in Semiconductor Quantum Dots: Radiationless Transitions on the Nanoscale

The ability to confine electrons and holes in semiconductor quantum dots (QDs) in the form of excitons creates an electronic structure which is both novel and potentially useful for a variety of applications. Upon optical excitation of the dot, the initial excitonic state may be electronically hot. The relaxation dynamics of this hot exciton is the primary event which controls key processes such as optical gain, hot carrier extraction, and multiple exciton generation. Here, we describe femtosecond state-resolved pump/probe experiments on colloidal CdSe quantum dots that provide the first quantitative measure of excitonic state-to-state transition rates. The measurements and modeling here reveal that there are multiple paths by which hot electrons and hot holes relax. The immediate result is that there is no phonon bottleneck for electrons or holes for excitons in quantum dots. This absence of phonon-based relaxation is confirmed by independent measurements of weak exciton–phonon coupling between the vario...

Dynamics of elastic and inelastic energy transfer between quantum dots in a microcavity

We develop a theory of exciton energy transfer dynamics between two quantum dots in a planar microcavity. We study the dynamics of quantum dot excitons in three different cases: (i) a single dot coupled only to cavity modes, (ii) two dots coupled to cavity modes, and (iii) two dots coupled to cavity modes and to acoustic phonons. In the latter case, we focus on the process of phonon-assisted inelastic exciton energy transfer between the quantum dots. We take into account phonon effects by introducing a light-matter Hamiltonian with operators describing the exciton-photon-phonon coupling and we truncate the Hilbert space by assuming that at most one excitation is present in the system. Using this approach we simulate the exciton dynamics with realistic parameters in the zero temperature limit. From the dynamics we extract the dependence of the characteristic energy transfer rate as a function of the interdot separation. This theoretical approach can be used to optimize exciton energy transfer by designing structures with engineered photon and phonon density of states.

Efficient quantum dot single photon extraction into an optical fiber using a nanophotonic directional coupler

We demonstrate a spectrally broadband and efficient technique for collecting emission from a single InAs quantum dot directly into a standard single mode optical fiber. In this approach, an optical fiber taper waveguide is placed in contact with a suspended GaAs nanophotonic waveguide with embedded quantum dots, forming a broadband directional coupler with standard optical fiber input and output. Efficient photoluminescence collection over a wavelength range of tens of nanometers is demonstrated, and a maximum collection efficiency of 6% (corresponding single photon rate of 3.0 MHz) into a single mode optical fiber is estimated for a single quantum dot exciton.

External electric field effect on the optical rectification coefficient of an exciton in a spherical parabolic quantum dot

External electric field effects on the optical rectification coefficient of an exciton confined in a spherical parabolic quantum dot are theoretically investigated. To this end, energy eigenvalues and eigenfunctions of the system are calculated, using the direct matrix diagonalization method. The compact-density matrix approach and an iterative method are used to find the optical rectification coefficient of a typical GaAs parabolic quantum dot. The results show that the optical rectification coefficient strongly depends on the confinement frequency and the magnitude of the electric field. Moreover, the peak value of this optical quantity is shifted to the aspect of high energy when the influence of the electric field is considered.

Micro-Pump–Probe Spectroscopy of an Exciton in a Single Semiconductor Quantum Dot Using a Heterodyne Technique

A heterodyne detection scheme was applied to micro-pump–probe spectroscopy to study the transient response of an exciton in a single semiconductor quantum dot. The elimination of the pump pulse in the frequency domain provided us a direct probing of the energy relaxation process of the quantum state excited by the pump pulse. The excitonic Rabi oscillations were also monitored by this technique, even in the presence of a strong pump pulse. This spectroscopic technique is suitable for the observation of the optical response and coherent manipulation of a semiconductor quantum dot.

Optically Induced Rotation of an Exciton Spin in a Semiconductor Quantum Dot

We demonstrate control over the spin state of a semiconductor quantum dot exciton using a polarized picosecond laser pulse slightly detuned from a biexciton resonance. The control pulse follows an earlier pulse, which generates an exciton and initializes its spin state as a coherent superposition of its two nondegenerate eigenstates. The control pulse preferentially couples one component of the exciton state to the biexciton state, thereby rotating the exciton's spin direction. We detect the rotation by measuring the polarization of the exciton spectral line as a function of the time difference between the two pulses. We show experimentally and theoretically how the angle of rotation depends on the detuning of the second pulse from the biexciton resonance.

Electron spin relaxation in a single InAs quantum dot measured by tunable nuclear spins

The electron-spin dynamics of positively charged excitons in a single InAs quantum dot (QD) were measured by time-resolved photoluminescence spectroscopy. Using alternating \ensuremath{\sigma}${}^{+}/$\ensuremath{\sigma}${}^{\ensuremath{-}}$ pulse sequences to excite the QD, the lattice nuclear spins remained randomly oriented. This method enables us to check the theory of electron-spin relaxation in the randomly distributed frozen fluctuation of the nuclear field. The experimental results are in qualitative agreement with theoretical prediction, showing that the electron-spin polarization decreases to a minimum value at first, and then increases again up to a steady value of about 1/3 of its initial value.

Erratum: Theory of excitons in cubic III-V semiconductor GaAs, InAs and GaN quantum dots: Fine structure and spin relaxation [Phys. Rev. B83, 235323 (2011)

Erratum: Theory of excitons in cubic III-V semiconductor GaAs, InAs and GaN quantum dots: Fine structure and spin relaxation [Phys. Rev. B<b>83</b>, 235323 (2011)

Combined effects of hydrostatic pressure and temperature on nonlinear properties of an exciton in a spherical quantum dot under the applied electric field

Within the framework of the effective-mass approximation, we have calculated the combined effects of hydrostatic pressure, temperature and applied electric field on an exciton confined in a typical GaAs/Ga 0.7Al 0.3As quantum dot. Several inputs of the confinement potential, hydrostatic pressure, temperature, and applied electric field have been considered. Our findings suggest that (1) the effect of the confinement strength is dominant over the electric field effect, (2) the oscillator strength is an increasing function of the hydrostatic pressure, (3) the absorption coefficients and energy difference depend strongly on the hydrostatic pressure but weakly on the temperature, (4) the absorption coefficients with considering excitonic effects are stronger than those without considering excitonic effects and the absorption peak will move to the right side induced by the electron–hole interaction, (5) the applied electric field may effect either the size or the position of absorption peaks of excitons.

Exciton relaxation and coupling dynamics in a GaAs/AlxGa1−xAs quantum well and quantum dot ensemble

Exciton inter- and intra-actions in a GaAs/AlGaAs quantum well (QW) and quantum dot (QD) ensemble are studied using optical two-dimensional Fourier transform spectroscopy. We measure population dynamics for times up to 300 ps and temperatures up to 50 K and observe biexponential decay for both QW and QD excitons, strong QW $\ensuremath{\rightarrow}$ QD relaxation, and weak QD $\ensuremath{\rightarrow}$ QW activation. The population dynamics are modeled using a system of rate equations that incorporate radiative and nonradiative decay, coupling between bright and dark exciton states, and QW $\ensuremath{\leftrightarrow}$ QD coupling. The fast decay rates are attributed to exciton-bound hole spin flips between optically active and inactive states and are similar for the QW and QDs, indicating excitons are weakly localized in the QDs. The QW $\ensuremath{\rightarrow}$ QD relaxation rate increases with temperature, and QD $\ensuremath{\rightarrow}$ QW excitation is observed at temperatures $\ensuremath{\geqslant}$35 K.

Strong coupling of two interacting excitons confined in a nanocavity–quantum dotsystem

We present a study of the strong coupling between radiation and matter, considering asystem of two quantum dots, which are in mutual interaction and interact with asingle mode of light confined in a semiconductor nanocavity. We take into accountdissipative mechanisms such as the escape of the cavity photons, decay of the quantumdot excitons by spontaneous emission, and independent exciton pumping. It isshown that the mutual interaction between the dots can be measured off-resonanceonly if the strong coupling condition is reached. Using the quantum regressiontheorem, a reasonable definition of the dynamical coupling regimes is introduced interms of the complex Rabi frequency. Finally, the emission spectrum for relevantconditions is presented and compared with the above definition, demonstratingthat the interaction between the excitons does not affect the strong coupling.