Article

Favoritism of quantum dot inter-Coulombic decay over direct and multi-photon ionization by laser strength and focus

Authors:
To read the full-text of this research, you can request a copy directly from the authors.

Abstract

We study the dynamics of a two-electron system undergoing resonant excitation and inter-Coulombic decay (ICD) in a pair of quantum dots. The influence of the focus of the exciting laser on the ICD process is investigated for a π-pulse with a close look on competing processes, i.e., direct ionization and multi-photon excitations. We scan through the field strength up to six Rabi cycles to show that ICD is still verifiable after several population inversions. With novel analyses, we determine for the first time populations of the different continuum states and thus conclude on the importance of several multi-photon excitation channels. Finally, we look into the influence of complex absorbing potentials on the dynamics.

No full-text available

Request Full-text Paper PDF

To read the full-text of this research,
you can request a copy directly from the authors.

... The theoretical frameworks that enables to capture the electron dynamics while maintaining the single-electron picture in an explicitly timedependent fashion can be multi-reference descriptions Email address: annika.bande@helmholtz-berlin.de (Annika Bande) as the time-dependent multiconfiguration selfconsistent field method (TD-MCSCF), also known as multiconfiguration time-dependent Hartree-Fock (MCTDHF) [8,9,10,11,12,13,14], and single-reference descriptions like time-dependent coupled cluster (TD-CC) [15,16,2,6], hybrid timedependent density functional theory/configuration interaction (hybrid TDDFT/CI) [17,18], also known as time-dependent Tamm-Dankov approximation (TD-TDA) [19], and last but not least, timedependent configuration interaction (TD-CI) [20,21,22,23,24,25,26,27]. ...
... above presented local densities of the many-body wavefunction, ρ CI (t) and ∆ 0 (t), also time-dependent NTO densities, ρ NTO (t) are introduced as defined in Eq. (11). Furthermore, the time-dependent exciton descriptors like the hole and particle positions, their size (Eq. ...
Article
Full-text available
We report the animation and analysis of laser-driven many-electron dynamics by means of time-dependent local densities and quantities derived from the one-particle transition density matrix, such as time-dependent natural transition orbital densities, particle and hole positions and exciton size. The time-dependent configuration interaction method was used to revisit studies on hydrogen molecule and lithium cyanide by Saalfrank and coworkers and our study shines new light on the optical transitions in these small molecules to benchmark the implemented time-dependent exciton properties. One focus of our simulations is the comparison of the local densities to the quantities of the two-body exciton wavefunction for resonant excitations leading either to state-to-state transitions or to the creation of a wave packet.
Article
In this work, we investigate the capability of using real-time time-dependent density functional theory (RT-TDDFT) in conjunction with a complex absorbing potential (CAP) to simulate the intermolecular Coulombic decay (ICD) processes following the ionization of an inner-valence electron. We examine the ICD dynamics in a series of noncovalent bonded dimer systems, including hydrogen-bonded and purely van der Waals (VdW)-bonded systems. In comparison to previous work, we show that RT-TDDFT simulations with a CAP correctly capture the ICD phenomenon in systems exhibiting a stronger binding energy. The calculated time scales for ICD of the studied systems are in the range of 5–50 fs, in agreement with previous studies. However, there is a breakdown in the accuracy of the methodology for the pure VdW-bonded systems. Overall, the presented RT-TDDFT/CAP methodology provides a powerful tool for differentiating between competing electronic relaxation pathways following inner-valence or core ionization without necessitating any a priori assumptions.
Chapter
Chemical modelling covers a wide range of disciplines, and this book is the first stop for any chemist, materials scientist, biochemist, or molecular physicist wishing to acquaint themselves with major developments in the applications and theory of chemical modelling. Containing both comprehensive and critical reviews, it is a convenient reference to the current literature. Coverage includes, but is not limited to, considerations towards rigorous foundations for the natural-orbital representation of molecular electronic transitions, quantum and classical embedding schemes for optical properties, machine learning for excited states, ultrafast and wave function-based electron dynamics, and attosecond chemistry.
Article
Full-text available
Interatomic or intermolecular Coulombic decay (ICD) is a nonlocal electronic decay mechanism occurring in weakly bound matter. In an ICD process, energy released by electronic relaxation of an excited atom or molecule leads to ionization of a neighboring one via Coulombic electron interactions. ICD has been predicted theoretically in the mid nineties of the last century, and its existence has been confirmed experimentally approximately ten years later. Since then, a number of fundamental and applied aspects have been studied in this quickly growing field of research. This review provides an introduction to ICD and draws the connection to related energy transfer and ionization processes. The theoretical approaches for the description of ICD as well as the experimental techniques developed and employed for its investigation are described. The existing body of literature on experimental and theoretical studies of ICD processes in different atomic and molecular systems is reviewed.
Article
The interparticle Coulombic decay process in paired quantum dots is studied by electron dynamics calculations. We consider a pair of Coulomb-coupled one-electron charged gallium arsenide quantum dots embedded in a nanowire. The two-electron decay process is approximately described by a single active electron model. Within this model, we employ the time-dependent wavepacket approach to the Fermi golden rule (introduced in the context of vibrational predissociation) to calculate autoionization rates, which are compared to exact rates obtained from fully correlated two-electron dynamics calculations. We found that the approximate decay rates agree well with the exact results in the limit of sufficiently separated quantum dots. Finally, we explore whether the short-range behavior of the new model can be further enhanced by the inclusion of local exchange effects by means of regularization of the Coulomb-potential based on a Jastrow-Slater wavefunction. The proposed method may open a route to study the interparticle Coulombic decay in more intricate systems, e.g., paired metal-nanoparticle—quantum dot systems.
Article
The interparticle Coulombic decay process (ICD) in a Coulomb-coupled pair of quantum dots (QDs) was predicted to feature electronic relaxation within one QD in conjunction with ionization of the other. In this work the QD model is extended from a pair to a triad of one excited and two ionizable QDs and in total three electrons. Analytical Wigner-Weisskopf expressions for the decay rates are formulated and confirmed with numerical electron dynamics calculations, suggesting a rate enhancement by a factor two that may be relevant for the competitiveness of ICD in QDs. Particularly, we compare two energetic scenarios, one allowing only for single ionization of the QD triad and one, not yet discussed in the community, potentially allowing for double ionization.
Article
The inter-Coulombic decay (ICD) process, where one electronically-excited species relaxes, while the neighboring one is concomitantly ionized, has been recently discovered likewise in atomic, molecular, biological, and nanostructured systems. Any theoretical prediction of it relies strongly on an accurate treatment of the involved resonance and continuum states. Here, we describe laser-induced ICD in quantum dots with electron dynamics at a multiconfiguration time-dependent Hartree-Fock level for the first time for a two-dimensional continuum, such that ICD control with laser polarization is within reach. This explicit electron dynamics is possible through the efficient Multigrid POTFIT tensor decomposition of the Coulomb interaction on a grid. Conclusively, ICD turns out to be much faster in laterally-arranged self-assembled or lithographic quantum dots connected to a two-dimensional wetting-layer continuum than in previously investigated dots in nanowires.
Article
Recently, highly accurate multi-configuration time-dependent Hartree electron dynamics calculations demonstrated the efficient long-range energy transfer inter-Coulombic decay (ICD) process to happen in charged semiconductor quantum dot (QD) pairs. ICD is initiated by intraband photoexcitation of one of the QDs and leads to electron emission from the other within a duration of about 150 ps. On the same time scale electronically excited states are reported to relax due to the coupling of electrons to acoustic phonons. Likewise, phonons promote ionisation. Here, the QDs' acoustic breathing mode is implemented in a frozen-phonon approach. A detailed comparison of the phonon effects on electron relaxation and emission as well as on the full ICD process is presented, which supports the previous empirical finding of ICD being the dominant decay channel in paired QDs. In addition the relative importance of phonon–phonon, phonon–electron and electron–electron interaction is analysed.
Article
Full-text available
We recently predicted that the interatomic Coulombic electron capture (ICEC) process, a long-range electron correlation driven capture process, is achievable in gated double quantum dots (DQDs). In ICEC an incoming electron is captured by one quantum dot (QD) and the excess energy is used to remove an electron from the neighboring QD. In this work we present systematic full three-dimensional electron dynamics calculations in quasi-one dimensional model potentials that allow for a detailed understanding of the connection between the DQD geometry and the reaction probability for the ICEC process. We derive an effective one- dimensional approach and show that its results compare very well with those obtained using the full three-dimensional calculations. This approach substantially reduces the computation times. The investigation of the electronic structure for various DQD geometries for which the ICEC process can take place clarify the origin of its remarkably high probability in the presence of two-electron resonances.
Article
Full-text available
A quantum emitter efficiently coupled to a nanophotonic waveguide constitutes a promising system for the realization of single-photon transistors, quantum-logic gates based on giant single-photon nonlinearities, and high bit-rate deterministic single-photon sources. The key figure of merit for such devices is the beta-factor, which is the probability for an emitted single photon to be channeled into a desired waveguide mode. Here we report on the experimental achievement of beta = 98.43 +- 0.04% for a quantum dot coupled to a photonic-crystal waveguide. This constitutes a nearly ideal photon-matter interface where the quantum dot acts effectively as a 1D "artificial" atom since it interacts almost exclusively with just a single propagating optical mode. The beta-factor is found to be remarkably robust to variations in position and emission wavelength of the quantum dots. Our work demonstrates the extraordinary potential of photonic-crystal waveguides for highly efficient single-photon generation and on-chip photon-photon interaction.
Article
Full-text available
In this paper we investigated the interatomic Coulombic decay (ICD) of a resonance singlet state in a model potential for two few-electron semiconductor quantum dots (QDs) by means of electron dynamics. We demonstrate that ICD is the major decay process of the resonance for the singlet wave function and compare the total and partial decay widths as a function of the QD separation with that from our previous study on the corresponding triplet states [1].
Article
Full-text available
We use time‐, wavelength‐, temperature‐, polarization‐resolved luminescence to elucidate the nature of the absorbing and ‘‘band edge’’ luminescing states in 32 Å diameter wurtzite CdSe quantum crystallites. Time‐resolved emission following picosecond size‐selective resonant excitation of the lowest excited state shows two components—a temperature insensitive 100 ps component and a microsecond, temperature sensitive component. The emission spectrum, showing optic phonon vibrational structure, develops a ∼70 wave number red shift as the fast component decays. Photoselection shows the slow component to be reverse polarized at 10 K, indicating this component to be the result of a hole radiationless transition. The 100 ps emitting state is repopulated thermally as temperature increases from 10 to 50 K. All available data are interpreted by postulating strong resonant mixing between a standing wave molecular orbital delocalized inside the crystallite and intrinsic surface Se lone pair states. The apparent exciton transition is assigned to a ∼130 wave number wide band of eigenstates with the hole localized principally on the surface. The band contains strongly emitting ‘‘doorway’’ states and weakly emitting ‘‘background’’ states. The hole becomes mobile among these states as T increases to 50 K. It is suggested that such resonant mixing may be general in II–VI and III–V crystallites.
Article
Full-text available
A problem of interacting charge carriers confined in quasi-one-dimensional (1D) semiconductor nanostructures has been studied. We have derived an analytical 1D formula for the effective interaction potential between the confined charge carriers. We have applied both the 1D model with the effective potential and the full three-dimensional (3D) approach to an electron pair confined in a single and double quantum dot as well as to an exciton confined in a quantum wire. Comparing the results of the 1D and 3D approaches we have discussed the applicability of the effective 1D interaction potential to the real 3D nanostructures. We have shown that the present effective interaction leads to accurate results for weakly coupled multiple quantum dots and wire-like nanostructures, i.e., the quantum wires and dots with the lateral confinement much stronger than the longitudinal one.
Article
Full-text available
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.
Article
Full-text available
A study of quantum box transitions coupled to three-dimensionally confined photonic modes in pillar microcavities is presented, focusing on the conditions for achieving a vacuum-field Rabi splitting. For a single InAs quantum box the oscillator strength is a factor of ten too small for being in strong coupling. A calculation of exciton states localized to monolayer fluctuations in quantum wells leads to much larger values of the oscillator strengths. Single localized excitons embedded in state-of-the-art micropillars can be in strong-coupling regime with a vacuum-field Rabi splitting.
Article
Full-text available
A ten-stacked self-assembled InAs/GaAs quantum-dot infrared photodetector operated in the 2.5–7 μm range by photovoltaic and photoconductive mixed-mode near-room-temperature operation (⩾250 K) was demonstrated. The specific peak detectivity D∗ is 2.4×108 cm Hz1/2/W at 250 K. The use of high-band-gap Al0.3Ga0.7As barriers at both sides of the InAs quantum-dot structure and the long carrier recombination time are the key factors responsible for its near-room-temperature operation. © 2001 American Institute of Physics.
Article
Full-text available
The reflection and transmission properties of different complex absorbing potentials (CAPs) are studied using WKB and scaling procedures which make the results transferable to any mass and kinetic energy. Explicit formulas are obtained which describe the reflection and transmission properties of monomial CAPs −iηxn with high accuracy. These properties are now well understood. The approximate results are compared to exact analytical results available for quadratic CAPs, and to numerical results obtained by wave packet propagation followed by an energy resolved analysis. The approximate, but accurate, description of the action of the CAP is finally used to determine optimal CAP parameters. CAP length, strength, and order can now be chosen in such a way that the sum of reflection and transmission is minimized. Optimal parameters are compiled for different energies and energy intervals. © 1996 American Institute of Physics.
Article
Full-text available
The recently proposed scheme for representing multidimensional potential energy surfaces as a linear combination of products of one‐dimensional functions is extended. The extensions prove to be important if one proceeds to higher dimensions. An iteration procedure is introduced which can further improve the representation. The product representation of potential energy surfaces is especially well suited to be employed within the framework of the multiconfiguration time‐dependent Hartree (MCTDH) approximation. The potential representation scheme cannot only be used to represent given analytical potential energy surfaces, but also to interpolate multidimensional surfaces on given, e.g. ab initio, product grid points. The product representation method is applied to the three‐dimensional S1 electronic surface of NOCl and to a six‐dimensional model Coulomb potential. To check the quality of the NOCl surface representation, the photoabsorption spectrum for an excitation from the S0 to the S1 surface is computed. Weight functions are shown to be easily implemented and, in the case of the NOCl surface, allow a substantial reduction of the number of required expansion coefficients. Exploiting the underlying symmetries of the potential under consideration can further reduce the computational effort, as is shown in the example of the Coulomb potential. Finally, the NOCl S1 potential surface defined on 616 ab initio points is interpolated, as an example for the product interpolation scheme. © 1996 American Institute of Physics.
Article
Full-text available
We have investigated the intraband absorption within the conduction band of InAs/GaAs quantum dots. The islands obtained by self-organized epitaxy are modulation doped with a silicon planar doping 2 nm below the dot layer plane. The dots exhibit infrared absorption polarized along the growth axis in the midinfrared spectral range. The absorption is maximum around 150 meV with a large broadening around 130 meV. This broadening is attributed to size fluctuations within the one dot layer plane and the consequent variation of the electron confinement energy with the dot size. The magnitude of the absorption along the growth axis for the one dot layer plane is ≈ 2.5×10−2% which corresponds to an equivalent absorption cross section σz ≈ 3.1×10−15 cm2. We show that the intraband absorption can also be clearly observed using a photoinduced infrared absorption technique with the doped quantum dots. © 1997 American Institute of Physics.
Article
Full-text available
Catalyst-assisted growth of semiconductor nanowires has opened up several new and exciting possibilities for low-dimensional semiconductor structures. The authors review progress on the realization of quantum dots in semiconductor nanowires, and their characterization by transport spectroscopy. Emphasis is placed on the wide range electronic properties exhibited due to flexibility of the growth process in terms of nanostructure composition and size. Particular attention is placed on studies of spin in few-electron quantum dots.
Article
Full-text available
Some 'Keldysh-like' theories are analysed leading to a more pragmatic definition of the tunnelling regime of ionization of atoms. Rather than using the more extreme definition of tunnelling (i.e. gamma <<1, F<<Fat), a more precise definition of the Keldysh gamma parameter ( gamma (0.5) and of the laser field F (F(FBSI) is proposed, where FBSI is the barrier suppression ionization field. New experimental results of the ionization of mercury atoms by an intense CO2 laser pulse are presented. This experiment, along with some previous experiments, have been analysed, and may be classified as tunnel ionization under this definition.
Article
Full-text available
The spectral properties of Hamilton operators perturbed by a complex absorbing potential (CAP) are studied. For a wide class of CAPS proper eigenvalues of the perturbed Hamilton operator converge to Siegert resonance eigenvalues of the unperturbed Hamiltonian with decreasing CAP strength. The errors in the calculation of complex resonance energies caused by the additional CAP and by finite basis set representation are examined. In order to minimize these errors a scheme of approximations is provided. The application of this method allows for the use of real L2 basis sets. The feasibility and accuracy of the proposed method is demonstrated by calculations of resonance energies of a model potential and of the 2 Pi g shape resonance of N2.
Article
Full-text available
We theoretically investigate how to control the Rabi oscillation of excitons of the coupling quantum dots by manipulating static electric fields. Our results show that, for a single-photon process, when direct excitons change into indirect excitons with a bias applied on the sample, the Rabi oscillation rarely alters. However, for the two-photon process, a pronounced enhancement of Rabi oscillation is observed, which can be utilized as the logic gate in quantum information.
Article
Full-text available
Thermal generation rate in quantum dots (QD) can be significantly smaller than in quantum wells, rendering a much improved signal to noise ratio. QDs infrared photodetectors were implemented, composed of ten layers of self-assembled InAs dots grown on GaAs substrate. Low temperature spectral response shows two peaks at low bias, and three at a high one, polarized differently. The electronic level structure is determined, based on polarization, bias, and temperature dependence of the transitions. Although absorbance was not observed, a photoconductive signal was recorded. This may be attributed to a large photoconductive gain due to a relatively long lifetime, which indicates, in turn, a reduced generation rate. (C) 1998 American Institute of Physics. [S0003-6951(98)02140-8].
Article
Full-text available
The two-electron resonance states of spherically symmetric artificial atoms (quantum dots) are investigated using the complex scaled full configuration-interaction method. The one-electron confining potential term of the quantum dot is presented by a Gaussian one-electron confining potential. Contrary to natural atoms, the single electron excited states of an artificial atom become resonance states for appropriately chosen external confining potential parameters. Moreover, the external confining potential of an artificial atom can be tuned to provide efficient photodetectors, which are extremely sensitive even to a weak external radiation with a specific wavelength. In order to illustrate efficiency of such a photodetector, we calculate the ionization rates corresponding to the interaction of an artificial atom with external laser field. The mechanism for photoionization through a short-lived autoionizing singly excited resonance state is discussed.
Chapter
Full-text available
Introduction and history“Pointwise“ representations in one dimensionMultidimensional DVRs and applicationsCaveatsSummary and conclusions
Article
Full-text available
The nuclear dynamics accompanying the excitation to and the subsequent decay of an electronic state is discussed. Particular attention is paid to cases, in which the whole process cannot be divided into two steps (excitation and decay) since the excitation and the decay times are of the same order of magnitude. The recently introduced time-dependent formulation of the theory describing the wave packets' dynamics is extended to include the excitation process. The wave packets can be related to the intensity of the emitted particles. Most of the resulting integrals can actually be performed by employing eigenstates of the Hamiltonians corresponding to the involved potential energy surfaces. This leads to the so called "time-independent" formulation of the theory. Computational details of the implementation of the corresponding "time-dependent" and "time-independent" methods are presented. Illustrative applications are given to illuminate both the influence of the excitation process and the lifetime of the decaying state. It emerges that the intuitive interpretation of the spectra (within the above two step model) may fail. Insight into the process is gained by studying the evolution of the spectra as a function of time. The appearance of "atomic lines" due to dissociative decaying and final states is investigated in some detail.
Article
Full-text available
A comprehensive theory developed previously for optical heating in semiconductors is applied to the calculation of laser damage thresholds in Ge, Si, InSb, and GaAs. The calculated thresholds agree well with the experimental values over a broad range of laser wavelengths and pulse durations. The results demonstrate that the dynamic nature of the material’s optical and transport properties with changing temperature and laser‐generated carrier density has a significant effect on the heating process, particularly at short pulse durations.
Article
Full-text available
The multiconfigurational time-dependent Hartree (MCTDH) approximation to the time-dependent Schrodinger equation is tested for a realistic three-dimensional example, the photodissociation of NOCl. The working equations of the MCTDH scheme introduced earlier are discussed in some detail. A computational scheme is presented which allows for efficient numerical MCTDH calculations. This scheme is applied to the photodissociation of NOCl after excitation to the S1 surface. The results are compared to the results of an exact wave-packet dynamics calculation. Fast convergence of the MCTDH results toward the exact one is found as the number of configurations is increased. The computation times of the MCTDH calculations are found to be much shorter than those of the exact calculation. Even MCTDH calculations including sufficiently many configurations for a fully converged (quasiexact) description require over two orders of magnitude less CPU time than an exact calculation. The so-called "natural populations" that are computed along with the MCTDH wave packet serve as a check of the degree of convergence without the need to resort to an exact (or more accurate) calculation.
Article
In recent years the laser-induced interatomic Coulombic decay (ICD) process in paired quantum dots has been predicted [J. Chem. Phys. 138 (2013) 214104]. In this work we target the enhancement of ICD by scanning over a range of strong-field laser intensities. The GaAs quantum dots are modeled by a one-dimensional double-well potential in which simulations are done with the space-resolved multi-configuration time-dependent Hartree method including antisymmetrization to account for the fermions. As a novelty a complementary state-resolved ansatz is developed to consolidate the interpretation of transient state populations, widths obtained for the ICD and the competing direct ionization channel, and Fano peak profiles in the photoelectron spectra. The major results are that multi-photon processes are unimportant even for the strongest fields. Further, below- to pulses display the highest ICD efficiency while the direct ionization becomes less dominant.
Article
In electron dynamics calculations the interatomic Coulombic decay (ICD) process has recently been shown to take place in two vertically-aligned quantum dots (QDs). Energy emitted during the relaxation of one electron in one QD is converted into kinetic energy of another electron ejected from a neighboring QD. As the electronic structure of QDs can be controlled by their geometries, we prove here in thorough scans of the transversal and vertical QD confinement potentials’ widths that geometries are likewise control parameters for ICD. Such a comprehensive investigation has been enabled by a significant development of the calculations in terms of speed achieved among others by optimization of the grid and Coulomb interaction operator representations. As key result of this study we propose two cigar-shaped singly-charged GaAs QDs vertically aligned in the direction of their long side for a most efficient QD ICD realization useful for an infrared photodetector.
Article
Luminescent solar concentrators serving as semitransparent photovoltaic windows could become an important element in net zero energy consumption buildings of the future. Colloidal quantum dots are promising materials for luminescent solar concentrators as they can be engineered to provide the large Stokes shift necessary for suppressing reabsorption losses in large-area devices. Existing Stokes-shift-engineered quantum dots allow for only partial coverage of the solar spectrum, which limits their light-harvesting ability and leads to colouring of the luminescent solar concentrators, complicating their use in architecture. Here, we use quantum dots of ternary I-III-VI2 semiconductors to realize the first large-area quantum dot-luminescent solar concentrators free of toxic elements, with reduced reabsorption and extended coverage of the solar spectrum. By incorporating CuInSexS2-x quantum dots into photo-polymerized poly(lauryl methacrylate), we obtain freestanding, colourless slabs that introduce no distortion to perceived colours and are thus well suited for the realization of photovoltaic windows. Thanks to the suppressed reabsorption and high emission efficiencies of the quantum dots, we achieve an optical power efficiency of 3.2%. Ultrafast spectroscopy studies suggest that the Stokes-shifted emission involves a conduction-band electron and a hole residing in an intragap state associated with a native defect.
Article
Nanometer-scale islands that form spontaneously on a semiconductor substrate have atomlike properties and potential applications in optical and optoelectronic devices, quantum computing, and information storage.
Article
Transient nutations of the resultant nuclear magnetic moment vector are set up by applying radiofrequency power in the form of pulses in the neighborhood of resonance (omega=gammaH0). The nutations have an initial amplitude depending on the state of magnetization at the start of a pulse and on the proximity to resonance, and are damped by spin-spin and spin-lattice interaction. The thermal relaxation time can be directly found by observing the dependence of initial amplitude on the time between pulses. The spin-spin time constant T2 can be found from the rate of decay even in the presence of normally disturbing inhomogeneity in magnetic field. Sensitivity is in many cases comparable to that obtained in the modulation method with narrow band amplifiers. The fast response due to the relatively wide band widths used can be applied to a rapid search for unknown resonances. The effects observed are in qualitative accord with predictions based on the Bloch theory.
Article
Modern semiconductor technology makes it possible to fabricate particles of metals or pools of electrons in a semiconductor that are only a few hundred angstroms in size. Artificial atoms have a unique and spectacular property: the current through such an atom or the capacitance between its leads can vary by many orders of magnitude when its charge is changed by a single electron. Why this is so, and how this property can be used to measure the level spectrum of an artificial atom, is the subject of this paper. The various type of atoms are described schematically. 19 refs., 6 figs.
Article
Over a twenty-year period, condensed matter physicists and physical chemists have elucidated a series of scaling laws which successfully describe the size dependence of solid state properties [1,2]. Often the experiments were performed under somewhat exotic conditions, for instance on mass-selected clusters isolated in molecular beams or on quantum dots grown by molecular beam epitaxy and interrogated at low temperatures and in high magnetic fields. As a result, we now have an understanding of how thermodynamic, optical, electrical, and magnetic properties evolve from the atomic to the solid state limit. This area of research is presently undergoing a remarkable transformation. The scaling laws, previously the direct subject of research, now provide a tool for the design of advanced new materials. In the case of colloidal quantum dots, or semiconductor nanocrystals, these new insights are poised to have impact in disciplines remote from solid state physics [3].
Article
A new method of measuring nuclear or other magnetic moment is described. The method, which consists essentially in the measurement of a Larmor frequency in known magnetic fields, is of very general application and capable of the highest precision in absolute and relative measurements. The apparatus consists of two magnets in succession which produce inhomogeneous magnetic fields of oppositely directed gradients. A molecular beam of the substance to be studied possesses a sigmoid path in these magnets and is focused on a suitable detector. A third magnet which produces a homogeneous field is placed in the region between the two deflecting magnets. In this strong homogeneous field the nuclear moments are decoupled from other nuclear moments and from rotational moments of a molecule in a 1Sigma state, and precess with their Larmor frequency nu=muHhI. An oscillating field perpendicular to the homogeneous field produces transitions to other states of space quantization when the frequency of this field is close to nu. If such transitions take place the molecule is no longer focused on to the detector by the subsequent inhomogeneous field and the observed intensity diminishes. The application of the method to the molecules LiCl, LiF, NaF and Li2 is described. The nuclear moments of Li7, Li6 and F19 were found to be 3.250, 0.820 and 2.622 nuclear magnetons, respectively.
Article
We observe multiple excitonic optical Rabi oscillations in a semiconductor quantum well. Up to eight oscillation periods of the heavy-hole exciton density on a subpicosecond time scale are observed. An approximate linear dependence of the oscillation frequency on the light field amplitude is established. The experiment is based on a two-color detection scheme which allows for the observation of the heavy-hole exciton density via transmission changes at the light-hole exciton. The observations are in good agreement with theoretical computations based on multiband semiconductor Bloch equations.
Article
In sharp contrast to molecules, electronic states of clusters with an excited intermediate-shell electron can efficiently decay via an intermolecular Coulombic mechanism. Explicit examples are presented using large scale ab initio propagator calculations. The mechanism is illustrated and its generality is stressed.
Article
The optical nutation effect has been observed on a rotational-vibrational transition in gaseous SF6 at 10.6 μ.
Article
Propagation of an intense ultrafast pulse (100 fs) of Gaussian shape with its central frequency above band gap in a GaAs halfspace is investigated theoretically. Nonequilibrium occupation of one particle states, memory effects in the polarization, Coulomb interaction, LO-phonon Scattering, and spatial inhomogeneities, i. e. the deformation of the pulse shape are taken into account in a strict microscopic approach. The numerical analysis allows to follow the various mechanisms involved on a fs-time scale, e. g. (1) saturation, spectral hole burning, and relaxation of the one particle distribution, (2) memory effects, dephasing processes, and Coulomb-enhancement of the polarization and (3) the interplay between that during pulse propagation. The resulting behaviour cannot be simulated by phenomenologically introduced dampings and/or relaxations, thus underlining the importance of accounting for the above mentioned effects in a microscopic way.Es wird die Ausbreitung eines intensiven ultrakurzen Impulses (100 fs) mit Gaußform und einer Zentralfrequenz oberhalb der Bandlücke in einem GaAs-Halbraum theoretisch untersucht. Nichtgleich-gewichtsbesetzung von Einteilchenzuständen, Gedächtniseffekte der Polarisation, Coulomb-Wechsel-wirkung, LO-Phonon-Streuung und rämliche Inhomogenitäten, d. h. die Deformation der Impulsform werden in strikter mikroskopischer Behandlung berücksichtigt. Die numerische Analyse erlaubt, die verschiedenen beteiligten Mechanismen im fs-Zeitbereich zu verfolgen: (1) Sättigung, Spektralloch-„Einbrennen” und Relaxation der Einteilchenverteilung, (2) Gedächtniseffekte, phasenzerstörende Prozesse und Coulomb-Anhebung der Polarisation, und (3) das Wechselspiel dazwischen während der Impulsausbreitung. Das resulttierende Verhalten kann nicht simuliert werden durch phänomenologisch eingeführte Dämpfung und/oder Relaxation, was die Bedeutung der Berücksichtigung der oben erwähnten Effekte auf mikroskopischem Wege unterstreicht.
Article
In this paper we investigated the dynamics of an electron in the presence of a time-dependent laser field in a model potential for a two-level single-electron semiconductor quantum dot (QD) that is capable of undergoing interatomic Coulombic decay (ICD) together with an electron bound to a neighboring QD. We demonstrate that ICD can be initiated by coupling the two-level QD to either a continuous or a pulsed moderate to strong laser and we obtain the total and partial decay widths of the resonance excited state in agreement with that from the solely decay of the resonance [A. Bande, K. Gokhberg, and L. S. Cederbaum, J. Chem. Phys. 135, 144112 (2011)]. A detailed discussion of the effects of direct ionization by the laser in single- or multi-photon process as well as Rabi oscillations is furthermore presented.
Article
The theory of resonant Auger decay of atoms in a high-intensity coherent x-ray pulse is presented. The theory includes the coupling between the ground state and the resonance due to an intense x-ray pulse, taking into account the decay of the resonance and the direct photoionization of the ground state, both populating the final ionic states coherently. The theory also considers the impact of the direct photoionization of the resonance state itself which typically populates highly excited ionic states. The combined action of the resonant decay and of the direct ionization of the ground state in the field induces a non-Hermitian time-dependent coupling between the ground and the “dressed” resonance stats. The impact of these competing processes on the total electron yield and on the 2s22p4(1D)3p 2P spectator and 2s12p6 2S participator Auger decay spectra of the Ne 1s→3p resonance is investigated. The role of the direct photoionization of the ground state and of the resonance increases dramatically with the field intensity. This results in strong interference effects with distinct patterns in the electron spectra, which differ for the participator and spectator final states.
Article
Electron relaxation in quantum dot (QD) systems a has significant impact on QD optoelectronic devices such as lasers, photodetectors, and solar cells. Several different fundamental mechanisms are known. In this Brief Report we propose another possible relaxation mechanism which is based on the interatomic Coulombic decay (ICD) mechanism first predicted by Cederbaum and his coworkers in 1997 and has been recently observed in atomic van der Waals and in hydrogen-bonded molecular clusters. We show that the electron relaxation in a quantum dot dimer due to the ICD mechanism is on a picoseconds timescale. This mechanism enables us to design IR photodetectors which are extremely efficient for ultraweak radiation with a specific wavelength.
Article
Absorbing boundary conditions in the form of a complex absorbing potential are routinely introduced in the Schrödinger equation to limit the computational domain or to study reactive scattering events using the multiconfigurational time-dependent Hartree (MCTDH) method. However, it is known that a pure wave-function description does not allow the modeling and propagation of the remnants of a system of which some parts are removed by the absorbing boundary. It was recently shown [ S. Selstø and S. Kvaal J. Phys. B: At. Mol. Opt. Phys. 43 065004 (2010)] that a master equation of Lindblad form was necessary for such a description. We formulate a MCTDH method for this master equation, usable for any quantum system composed of any mixture of species. The formulation is a strict generalization of pure-state propagation using standard MCTDH for identical particles and mixtures. We demonstrate the formulation with a numerical experiment.
Article
The time-dependent theory of an atomic cascade decay process is discussed. We show that it is useful to measure the spectra of the emitted particles (electrons or photons) as a function of time and introduce time-dependent spectra and time-dependent coincidence spectra. The analysis reveals that there are several timescales involved in the atomic cascades and in the respective spectra. The work prepares the ground for the discussion of molecules, clusters, and solids, where the nuclear dynamics strongly participate in the cascade decay process making it much more complex.
Article
Self-organized InAs quantum dashes grown on In0.53Ga0.23Al0.24As/InP have been investigated by chemically sensitive scanning transmission electron microscopy. The quantum dashes, which consist of pure InAs, exhibit a triangular cross section. Most important, the quantum dash size depends linearly on the nominal InAs layer thickness and can be varied by a factor of 3 without changing the height∕width ratio. Thus, the emission wavelength can be controlled between 1.37 and 1.9 μm without modifying shape and composition of the quantum dashes by adjusting a single growth parameter.
Article
Quantum dots, often referred to as artificial atoms, open the field of quantum resolved spectroscopy to semiconductor physics. The current article is designed to review the field of interband optical spectroscopy on single semiconductor quantum dots. © 2000 American Institute of Physics.
Article
The gain and related characteristics of quantum dash structures are modeled and analyzed using a parabolic effective-mass theory and the density-matrix theory. Size fluctuation is included in the model and its effects are analyzed. Comparison of performance in terms of these characteristics has been made with quantum well (QW) and quantum wire structures. Owing to size fluctuation, quantum dashes have broad gain spectra, which allow wavelength tuning without significant increase in the injected carrier density. Quantum dashes have smaller differential gain than quantum wells, while the linewidth enhancement factors are similar for these two structures. Due to their broad gain profiles, quantum dashes have much smaller large-signal chirp (change of refractive index). These characteristics can be further improved by blueshifting the emission energy without significant change in the injected carrier density. After detuning, the differential gain can be improved only to half of quantum well’s values and the large-signal chirp can be reduced to one-tenth of the QW’s value.
Article
Intraband spectra of CdSe nanocrystal colloids are observed. The photoexcited nanocrystals, of average diameters 31.5, 38, and 43 Å, exhibit an isolated and strong infrared-induced absorption with peak energy between 0.5 and 0.3 eV. The energies and cross sections of the resonance are consistent with a one-electron transition between the delocalized 1Se and 1Pe states of the strongly confined quantum dots. © 1998 American Institute of Physics.
Article
Semiconductor quantum dot molecules (QDMs) are systems composed of two or more closely spaced and interacting QDs. QDMs are receiving much attention both as playground for studying coupling and energy transfer processes between “artificial atoms” and as new systems, which substantially extend the range of possible applications of QDs. QDMs can be conveniently fabricated by self-assembly either through chemical synthesis or epitaxial growth. Although QDMs relying on the random occurrence of nearby QDs can be used for fundamental studies, special fabrication protocols must be used to create QDMs with well-defined properties. In this article, we focus on self-assembled QDMs obtained by epitaxial growth and embedded in a semiconductor matrix, which are appealing for the possible realization of quantum gates based on two-level systems defined in QDs. We provide a comprehensive overview of the development and current stage of the research on QDMs composed of vertically (in the growth direction) or laterally (in the growth plane) aligned QDs. The review highlights some recent milestone works and points out the challenges and future directions in the field.
Article
Reflections or wraparound from boundaries of numerical grids have always presented a difficulty in applying discrete methods to simulate physical phenomena. This study presents a systematic derivation of absorbing boundary conditions which can be used in a wide class of wave equations. The derivation is applied to the Schrödinger equation and to the acoustic equation in one and two dimensions. The effectiveness of the absorbing boundary conditions can be evaluated apriori on the basis of analytic solutions.
Article
A new multi-configurational approach to the time-dependent Schrödinger equation is proposed. This approach leads to working equations which are particularly simple and transparent. It can be used for n degrees of freedom and for any choice of the number of configurations. The new approach is tested on a model of coupled oscillators showing fast convergence towards the exact results as the number of configurations is increased.
Article
The nonadiabatic transitions which a system with angular momentum J makes in a magnetic field which is rotating about an axis inclined with respect to the field are calculated. It is shown that the effects depend on the sign of the magnetic moment of the system. We therefore have an absolute method for measuring the sign and magnitude of the moment of any system. Applications to the magnetic moment of the neutron, the rotational moment of molecules, and the nuclear moment of atoms with no extra-nuclear angular momentum are discussed.
Article
In this work we demonstrate that the interatomic Coulombic decay (ICD), an ultrafast electron relaxation process known for atoms and molecules, is possible in general binding potentials. We used the multiconfiguration time-dependent Hartree method for fermions to study ICD in real time in a two-electron model system of two potential wells. Two decay channels were identified and analyzed by using the box stabilization analysis as well as by evaluating the autocorrelation function and measuring the outgoing electron flux during time-propagations. The total and partial ICD widths of an excited state localized in one potential well as a function of the distance between the two potentials was obtained. Finally, we discuss the results with a view to a possible application of ICD in quantum dot technology.
Article
Several classes of semiconductor quantum dots (QD), including groups II-VI, III-V, IV-VI, IV, and their alloys as well as various intergroup and intragroup core-shell configurations, and nanocrystal shapes have been synthesized. One approach to enhance efficiency in QD-based PV cells compared to conventional bulk semiconductor-based PV is to create efficient multiple exciton generation from a large fraction of the photons in the solar spectrum. Three generic types of QD solar cells that could utilize MEG to enhance conversion efficiency can be defined. They include photoelectrodes composed of QD arrays that form either Schottky junctions with a metal layer, a hetero p-n junction with a second NC semiconductor layer, or the i-region of a p-i-n device, QD-sensitized nanocrystalline TiO2 films, and QDs dispersed into a multiphase mixture of electron- and hole-conducting matrices, such as C60 and hole conducting polymers.