Ensembles Of Semiconductor Quantum Dots Research Articles

Modeling the Induction of Excitons upon the Continuous-Wave Laser Excitation of an Ensemble of Semiconductor Quantum Dots

A theoretical investigation is performed for the possibility and conditions of the stimulated induction of excitons in a thin layer of semiconductor quantum dots upon excitation by continuous-wave laser radiation.

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.

Photon echo in the ensemble of semiconductor quantum dots spread on a glass substrate

Simple procedure to prepare samples containing semiconductor quantum dots was developed. Test photon echo measurements in the ensemble of quantum dots spread on a glass substrate were performed to study optical dephasing processes.

The method of surface plasmon-polariton pulses generation via cooperative effects in a waveguide spaser

Cooperative effects arising under conditions of a 0-D model for a dense ensemble of semiconductor quantum dots located in a dielectric layer near a flat metal surface are considered. The threshold conditions of the effect are determined and the values of the complex refractive index of the dielectric are chosen, which make it possible to increase the efficiency of the formation of surface plasmon-polariton (SPP) pulses.

Photon echo transients from an inhomogeneous ensemble of semiconductor quantum dots

An ensemble of quantum dot excitons may be used for coherent information manipulation. Due to the ensemble inhomogeneity any optical information retrieval occurs in form of a photon echo. We show that the inhomogeneity can lead to a significant deviation from the conventional echo timing sequence. Variation of the area of the initial rotation pulse, which generates excitons in a dot sub-ensemble only, reveals this complex picture of photon echo formation. We observe a retarded echo for {\pi}/2 pulses, while for 3{\pi}/2 the echo is advanced in time as evidenced through monitoring the Rabi oscillations in the time-resolved photon echo amplitude from (In,Ga)As/GaAs self-assembled quantum dot structures and confirmed by detailed calculations.

Quantum coherence induces pulse shape modification in a semiconductor optical amplifier at room temperature

Coherence in light–matter interaction is a necessary ingredient if light is used to control the quantum state of a material system. Coherent effects are firmly associated with isolated systems kept at low temperature. The exceedingly fast dephasing in condensed matter environments, in particular at elevated temperatures, may well erase all coherent information in the material at timescales shorter than a laser excitation pulse. Here we show for an ensemble of semiconductor quantum dots that even in the presence of ultrafast dephasing, for suitably designed condensed matter systems quantum-coherent effects are robust enough to be observable at room temperature. Our conclusions are based on an analysis of the reshaping an ultrafast laser pulse undergoes on propagation through a semiconductor quantum dot amplifier. We show that this pulse modification contains the signature of coherent light–matter interaction and can be controlled by adjusting the population of the quantum dots via electrical injection.

Electrical transport in ensembles of semiconductor quantum dots

The novelty of nano physics is associated with the basic changes that take place in systems that consist of particles when their size is reduced to the nano scale. While the corresponding changes in the individual particles are relatively well understood the understanding of the effect of these changes on the properties of ensembles of such particles is still at a rudimentary level. This is in particular so for the electrical transport in such systems. In this paper we will try to evaluate the effect of the changes in the density and properties of the individual particles on the corresponding global electrical transport in the ensembles. This evaluation is based on our experimental results and their analyses as obtained on ensembles of silicon nano crystallites that are embedded in an insulating matrix.

Spin relaxation in nanowires by hyperfine coupling

Hyperfine interactions establish limits on spin dynamics and relaxation rates in ensembles of semiconductor quantum dots. It is the confinement of electrons which determines nonzero hyperfine coupling and leads to the spin relaxation. As a result, in nanowires one would expect the vanishing of this effect due to extended electron states. However, even for relatively clean wires, disorder plays a crucial role and makes electron localization sufficient to cause spin relaxation on the time scale of the order of 10 ns. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Vector solitons in semiconductor quantum dots

A theory of an optical vector soliton of self-induced transparency in an ensemble of semiconductor quantum dots is considered. By using the perturbative reduction method, the system of the Maxwell–Liouville equations is reduced to the two-component coupled nonlinear Schrödinger equations. It is shown that a distribution of transition dipole moments of the quantum dots and phase modulation changes significantly the pulse parameters. The shape of the optical two-component vector soliton with the sum and difference of the frequencies in the region of the carrier frequency is presented. The vector soliton can be reduced to the breather solution of self-induced transparency with a different profile. Explicit analytical expressions in the presence of single-excitonic and biexcitonic transitions for the optical vector soliton are obtained with realistic parameters which can be reached in current experiments.

Electrical transport mechanisms in three dimensional ensembles of silicon quantum dots

In this review, we try to derive a comprehensive understanding of the transport mechanisms in three dimensional ensembles of Si quantum dots (QDs) that are embedded in an insulating matrix. This understanding is based on our systematic electrical measurements as a function of the density of Si nanocrystallites as well as on a critical examination of the available literature. We conclude that in ensembles of low density QDs, the conduction is controlled by quantum confinement and Coulomb blockade effects while in the high density regime, the system behaves as a simple disordered semiconductor. In between these extremes, the transport is determined by the clustering of the QDs. In view of the clustering, two types of transitions in the electrical and optical properties of the system are identified. In order to understand them, we introduce the concept of “touching.” The application of this concept enables us to suggest that the first transition is a local carrier deconfinement transition, at which the concentration of the non “touching” QDs reaches its maximum, and that the other transition is associated with the onset of percolation in a continuous disordered network of “touching” QDs. It is hoped that our conclusions for the entire possible density range will provide guidance for the discussion and understanding of the transport in ensembles of semiconductor QDs in general and in ensembles of Si and Ge QDs in particular.

Superfluorescent pulsed emission from biexcitons in an ensemble of semiconductor quantumdots

Picosecond time-resolved photoluminescence from biexcitons in CuCl quantum dots(QDs) embedded in a NaCl matrix has been measured using an optical Kerrgate method. Ultrafast pulsed emission from the biexciton states was observedfor the first time, only under resonant two-photon excitation of biexcitons. Thisimplies that complete population inversion between the biexciton and excitonstates is necessary in order to trigger the pulsed emission. In addition, the natureof the dependence of the time profiles of the pulsed emission on the excitationintensity reveals that the peak intensity is directly proportional to the square ofthe number of excited QDs. We conclude that this phenomenon is caused bysuperfluorescence, that is, the cooperative spontaneous radiative decay of many isolatedexcited states coupled by a resonant electromagnetic wave. Such a phenomenon hasbeen observed for the first time in an ensemble of semiconductor QDs in thisstudy. The results presented in this paper show that it is possible to control themicroscopic coherent dynamics of electronic excited states in a QD ensemble.

Electrical Transport Phenomena in Systems of Semiconductor Quantum Dots

While a fairly good understanding of optical and transport properties that are associated with single quantum dots has emerged in recent years the understanding of the relation between these properties and the observed macroscopic optical and electrical properties of solid ensembles of such dots is still at a very rudimentary level. This is in particular so in regard to the transport properties where the interplay between inter-dot conduction and the connectivity of the dots network determines the macroscopic observations. Reviewing the basic concepts and issues associated with these two essential ingredients, and considering some recent experimental observations on quantum dot ensembles of CdSe and Si, an effort is made here to derive a whole-but-simple physical basis for the understanding of the transport and the optoelectronic properties of solid state ensemble of semiconductor quantum dots.

Meyer-Neldel-like manifestation of the quantum confinement effect in solid ensembles of semiconductor quantum dots

We report clear crystallite size dependencies of the transport and phototransport properties in solid-state ensembles of semiconductor quantum dots. By finding a Meyer-Neldel-like behavior for the former and by comparing the experimental results with computer simulations for the latter, we show that the above dependencies are associated with the quantum confined induced variation of the band gap in the individual dots. These findings go beyond the available knowledge of interparticle conduction mechanisms by providing a basis for the corresponding physical statistics of such quantum dot ensembles.

Statistical analysis of time-resolved emission from ensembles of semiconductor quantum dots: Interpretation of exponential decay models

We present a statistical analysis of time-resolved spontaneous emission decay curves from ensembles of emitters, such as semiconductor quantum dots, with the aim of interpreting ubiquitous non-single-exponential decay. Contrary to what is widely assumed, the density of excited emitters and the intensity in an emission decay curve are not proportional, but the density is a time integral of the intensity. The integral relation is crucial to correctly interpret non-single-exponential decay. We derive the proper normalization for both a discrete and a continuous distribution of rates, where every decay component is multiplied by its radiative decay rate. A central result of our paper is the derivation of the emission decay curve when both radiative and nonradiative decays are independently distributed. In this case, the well-known emission quantum efficiency can no longer be expressed by a single number, but is also distributed. We derive a practical description of non-single-exponential emission decay curves in terms of a single distribution of decay rates; the resulting distribution is identified as the distribution of total decay rates weighted with the radiative rates. We apply our analysis to recent examples of colloidal quantum dot emission in suspensions and in photonic crystals, and we find that this important class of emitters is well described by a log-normal distribution of decay rates with a narrow and a broad distribution, respectively. Finally, we briefly discuss the Kohlrausch stretched-exponential model, and find that its normalization is ill defined for emitters with a realistic quantum efficiency of less than 100%.

Photoinduced fluorescence intensity oscillation in a reaction-diffusion cell containing a colloidal quantum dot dispersion

The nonlinear spontaneous oscillation of photoluminescence (PL) intensity in an ensemble of semiconductor quantum dots (QDs), which differs from the fluorescence intermittency of a single QD, is investigated. The PL intensity in a QD dispersion slowly oscillates with time under continuous illumination. The oscillatory behavior is found to vary with changing QD concentration, solvent viscosity, volume fraction of irradiated region, and irradiation intensity. On the basis of the Gray-Scott model [Chemical Oscillation and Instabilities: Non-linear Chemical Kinetics (Clarendon, Oxford, 1994); J. Phys. Chem. 89, 22 (1985); Chem. Eng. Sci. 42, 307 (1987)], and its comparison with the experimental results, it is revealed that the following processes are important for PL oscillation: (1) mass transfer of QDs between the illuminated and dark regions, (2) autocatalytic formation of vacant sites on QD surfaces via photodesorption of ligand molecules, and (3) passivation of vacant sites via photoadsorption of water molecules.

Optical Properties of Dense and Diluted Ensembles of Semiconductor Quantum Dots

We examine collective effects in an ensemble of close-packed ultrasmall CdSe quantum dots by comparison of optical properties of dense and diluted thin film composites. Studies of absorption spectra, photoluminescence excitation spectra and electroabsorption of diluted and condensed structures along with numerical simulation confirmed that in a dense ensemble extended electron states occur by analogy with Anderson transitions in disordered solids. Therefore evolution from isolated quantum dots to quantum dot solids in case of strong confinement results in a drastic modification of optical and electronic properties.

Optical Properties of Dense and Diluted Ensembles of Semiconductor Quantum Dots

Optical Properties of Dense and Diluted Ensembles of Semiconductor Quantum Dots

Effect of size dispersion on the optical absorption of an ensemble of semiconductor quantum dots

The linear optical absorption of an ensemble of semiconductor quantum dots randomly positioned in an insulating matrix is studied theoretically and experimentally for the CdTe/glass system. The calculation of the effective dielectric function of the system, whose imaginary part determines the absorption, is based on a modified Maxwell-Garnett formalism using a diagram technique to calculate the average renormalized polarizability of spherical quantum dots. This approach takes into account both the fluctuations of the polarization interaction due to the random positions of the spheres and their size dispersion. A comparison of the theoretical and experimental spectra permits determination of the mean quantum dot size and concentration. The dependence of these parameters on the postgrowth annealing time of the samples is not consistent with the existing theories on the spinodal decomposition of solid solutions.