Excitons In Semiconductor Quantum Dots Research Articles

Optical Activity and Excitonic Characteristics of Chiral CdSe Quantum Dots.

Introduction of chirality to colloidal semiconductor quantum dots (QDs) triggers a chiroptical effect. However, there remains a knowledge gap in the mechanism of chirality transfer and amplification from molecules to QDs. By time-dependent density functional theory calculations combined with a correlated electron-hole picture, we explored the chiroptical activity of CdSe QDs decorated with different chiral monocarboxylic acids from an excitonic perspective. Our calculations showed strong circular dichroism (CD) signals in the visible region for the chiral CdSe QDs. The excitonic states with large CD originate from QDs, while the chiral molecules break the orthogonality between electric and magnetic transition dipoles, which synergistically facilitates the prominent dissymmetric effect. The considered monocarboxylic acid chiral molecules all favor the bidentate adsorption configuration of the carboxyl group on the CdSe surface, endowing an identical CD signature but distinct excitonic characteristics. These findings are crucial for the regulation of chirality and excitons in semiconductor QDs to develop excitonic devices.

Floquet engineering of excitons in semiconductor quantum dots

Within the Floquet theory of periodically driven quantum systems, we demonstrate that a high-frequency electromagnetic field can be used as an effective tool to control excitonic properties of semiconductor quantum dots (QDs). It is shown, particularly, that the field both decreases the exciton binding energy and dynamically stabilizes the exciton, increasing its radiative lifetime. The developed theory can serve as a basis for the ultrafast method to tune spectral characteristics of the QD-based photon emitters by a high-frequency field.

Engineering Long-Lived Blue Photoluminescence from InP Quantum Dots Using Isomers of Naphthoic Acid.

Leveraging triplet excitons in semiconductor quantum dots (QDs) in concert with surface-anchored molecules to produce long-lifetime thermally activated delayed photoluminescence (TADPL) continues to emerge as a promising technology in diverse areas including photochemical catalysis and light generation. All QDs presently used to generate TADPL in QD/molecule constructs contain toxic metals including Cd(II) and Pb(II), ultimately limiting potential real-world applications. Here, we report newly conceived blue-emitting TADPL-producing nanomaterials featuring InP QDs interfaced with 1- and 2-naphthoic acid (1-NA and 2-NA) ligands. These constitutional isomers feature similar triplet energies but disparate triplet lifetimes, translating into InP-based TADPL processes displaying two distinct average lifetime ranges upon cooling from 293 to 193 K. The time constants fall between 4.4 and 59.2 μs in the 2-NA-decorated InP QDs while further expanding between 84.2 and 733.2 μs in the corresponding 1-NA-ligated InP materials, representing a 167-fold time window. The resulting long-lived excited states enabled facile bimolecular triplet sensitization of 1O2 phosphorescence in the near-IR and promoted sensitized triplet-triplet annihilation photochemistry in 2,5-diphenyloxazole. We speculate that the discovery of new nanomaterials exhibiting TADPL lies on the horizon as myriad QDs can be readily derivatized using isomers of numerous classes of surface-anchoring chromophores yielding precisely regulated photophysical properties.

Strongly Coupled Exciton-Surface Lattice Resonances Engineer Long-Range Energy Propagation.

Achieving propagation lengths in hybrid plasmonic systems beyond typical values of tens of micrometers is important for quantum plasmonics applications. We report long-range optical energy propagation due to excitons in semiconductor quantum dots (SQDs) being strongly coupled to surface lattice resonance (SLRs) in silver nanoparticle arrays. Photoluminescence (PL) measurements provide evidence of an exciton-SLR (ESLR) mode extending at least 600 μm from the excitation region. We also observe additional energy propagation with range well beyond the ESLR mode and with dependency on the coupling strength, g, between SQDs and SLR. Cavity quantum electrodynamics calculations capture the nature of the PL spectra for consistent g values, while coupled dipole calculations show a SQD number-dependent electric field decay profile consistent with the experimental spatial PL profile. Our results suggest an exciting direction wherein SLRs mediate long-range interactions between SQDs, having possible applications in optoelectronics, sensing, and quantum information science.

Fano Effect and Quantum Entanglement in Hybrid Semiconductor Quantum Dot-Metal Nanoparticle System.

In this paper, we review the investigation for the light-matter interaction between surface plasmon field in metal nanoparticle (MNP) and the excitons in semiconductor quantum dots (SQDs) in hybrid SQD-MNP system under the full quantum description. The exciton-plasmon interaction gives rise to the modified decay rate and the exciton energy shift which are related to the exciton energy by using a quantum transformation method. We illustrate the responses of the hybrid SQD-MNP system to external field, and reveal Fano effect shown in the absorption spectrum. We demonstrate quantum entanglement between two SQD mediated by surface plasmon field. In the absence of a laser field, concurrence of quantum entanglement will disappear after a few ns. If the laser field is present, the steady states appear, so that quantum entanglement produced will reach a steady-state entanglement. Because one of all optical pathways to induce Fano effect refers to the generation of quantum entangled states, It is shown that the concurrence of quantum entanglement can be obtained by observation for Fano effect. In a hybrid system including two MNP and a SQD, because the two Fano quantum interference processes share a segment of all optical pathways, there is correlation between the Fano effects of the two MNP. The investigations for the light-matter interaction in hybrid SQD-MNP system can pave the way for the development of the optical processing devices and quantum information based on the exciton-plasmon interaction.

Direct optical state preparation of the dark exciton in a quantum dot

Because of their weak coupling to the electromagnetic field dark excitons in semiconductor quantum dots possess extremely long lifetimes, which makes them attractive candidates for quantum information processing. On the other hand, preparation and manipulation of dark states is challenging, because commonly used optical excitation mechanisms are not applicable. We propose a new, efficient mechanism for the deterministic preparation of the dark exciton exploiting the application of a tilted magnetic field and the optical excitation with a chirped, i.e., frequency modulated, laser pulse.

Tailoring the ultrafast control of quantum dot excitons using optical pulse shaping

AbstractFemtosecond pulse shaping provides a means to tailor the interaction of light with matter, enabling the optimization of optical control processes and the achievement of a target final quantum state of the matter system. While this approach has found widespread application in the control of atomic and molecular systems, its use for solid state quantum systems remains in its infancy. This review covers our application of this approach to the manipulation of the quantum states of excitons in semiconductor quantum dots (QDs). The achievement of simultaneous π, 2π rotations on excitons in two different QDs using a single engineered infrared pulse illustrates the flexibility of the pulse shaping approach for solid state quantum systems. This versatility is further explored through simulations that show the feasibility of arbitrary SU(2) control of several quantum dots. Shaping of femtosecond infrared control pulses enables the demonstration of adiabatic rapid passage in a single QD on a subpicosecond time scale, representing a substantial speedup and an important step towards the realization of dynamical decoupling. The dependence of the exciton inversion efficiency on the sign of pulse chirp confirms the role of (and ability to control) phonon‐related dephasing. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Temperature dependence of photoluminescence of semiconductor quantum dots upon indirect excitation in a SiO2 dielectric matrix

The processes of excitation and relaxation of confined excitons in semiconductor quantum dots upon indirect high-energy excitation have been considered. The temperature behavior of photoluminescence of quantum dots in a SiO2 dielectric matrix has been described using a model accounting for the process of population of quantum-dot triplet states upon excitation transfer through mobile excitons of the matrix. Analytical expressions that take into account the two-stage and three-stage schemes of relaxation transitions have been obtained. The applicability of these expressions for analyzing fluorescence properties of semiconductor quantum dots has been demonstrated using the example of silicon and carbon nanoparticles in the thin-film SiO2 matrix. It has been shown that the complex character of the temperature dependences of the photoluminescence upon indirect excitation can be an indication of a multistage relaxation of excited states of the matrix and quantum dots. The model concepts developed in this study allow one to predict the form of temperature dependences of the photoluminescence for different schemes of indirect excitation of quantum dots.

Analytical temperature dependence of the photoluminescence of semiconductor quantum dots

The excitation and relaxation of spatially confined excitons in semiconductor quantum dots have been considered. The temperature dependence of the luminescence of quantum dots in dielectric matrices is described by the model taking into account the singlet-triplet intercombination conversion of spatially confined excitons. The analytical expression describing the temperature dependence of photoluminescence is derived and the physical meaning of the constants involved in this expression is determined. The applicability of the expression to the analysis of the luminescent properties of the quantum dots is demonstrated by the example of silicon nanoclusters in a thin-film SiO2 matrix.

Mollow quintuplets from coherently excited quantum dots

Charge-neutral excitons in semiconductor quantum dots (QDs) have a small finite energy separation caused by the anisotropic exchange splitting. Coherent excitation of neutral excitons will generally excite both exciton components, unless the excitation is parallel to one of the dipole axes. We present a polaron master equation model to describe two-exciton pumping using a coherent continuous wave pump field in the presence of a realistic anisotropic exchange splitting. We predict a five-peak incoherent spectrum, namely a Mollow quintuplet under general excitation conditions. We experimentally confirm such spectral quintuplets for In(Ga)As QDs and obtain very good agreement with theory.

Exciton Dynamics in Semiconductor Nanocrystals

This review article provides an overview of recent advances in the study and understanding of dynamics of excitons in semiconductor nanocrystals (NCs) or quantum dots (QDs). Emphasis is placed on the relationship between exciton dynamics and optical properties, both linear and nonlinear. We also focus on the unique aspects of exciton dynamics in semiconductor NCs as compared to those in bulk crystals. Various experimental techniques for probing exciton dynamics, particularly time-resolved laser methods, are reviewed. Relevant models and computational studies are also briefly presented. By comparing different materials systems, a unifying picture is proposed to account for the major dynamic features of excitons in semiconductor QDs. While the specific dynamic processes involved are material-dependent, key processes can be identified for all the materials that include electronic dephasing, intraband relaxation, trapping, and interband recombination of free and trapped charge carriers (electron and hole). Exciton dynamics play a critical role in the fundamental properties and functionalities of nanomaterials of interest for a variety of applications including optical detectors, solar energy conversion, lasers, and sensors. A better understanding of exciton dynamics in nanomaterials is thus important both fundamentally and technologically.

Creation of entangled states in coupled quantum dots via adiabatic rapid passage

Quantum state preparation through external control is fundamental to established methods in quantum information processing and in studies of dynamics. In this respect, excitons in semiconductor quantum dots are of particular interest, since their coupling to light allows them to be driven into a specified state using the coherent interaction with a tuned optical field, such as an external laser pulse. We propose a protocol, based on adiabatic rapid passage, for the creation of entangled states in an ensemble of pairwise coupled two-level systems, such as an ensemble of coupled quantum dots. We show by quantitative analysis using realistic parameters for semiconductor quantum dots that this method is feasible where other approaches are unavailable. Furthermore, this scheme can be generically transferred to some other physical systems, including circuit QED, nuclear and electron spins in solid-state environments, and photonic coupled cavities.

Charged exciton creation with two-color optical excitation method and analysis of initialization process of electron spin qubit in quantum dots

An optical pumping method for creating charged excitons in semiconductor quantum dots is studied using a micro-photoluminescence (PL) technique with a two-color excitation method. This method employs two laser sources whose energies are in the resonant and non-resonant barrier excitation conditions, respectively; one laser creates excitons, the other creates electrons. The two-color excitation exploits an asymmetric energy band structure for conduction and valence bands, which induces negatively charged excitons in single quantum dots. The PL spectra of a single quantum dot clearly vary from exciton-originated PL to charged exciton-originated PL as the excitation conditions are changed. This excitation method is applied to the initialization process of an electron spin qubit without a magnetic field. Experimental results are analyzed theoretically based on the use of a density matrix method for the initialization. The analysis reveals that the electron spin in quantum dots is sufficiently polarized for subsequent quantum computation processes. These results provide a simple and effective way of implementing quantum computing with spin qubits.

Strong coupling among semiconductor quantum dots induced by a metal nanoparticle

Based on cavity quantum electrodynamics (QED), we investigate the light-matter interaction between surface plasmon polaritons (SPP) in a metal nanoparticle (MNP) and the excitons in semiconductor quantum dots (SQDs) in an SQD-MNP coupled system. We propose a quantum transformation method to strongly reveal the exciton energy shift and the modified decay rate of SQD as well as the coupling among SQDs. To obtain these parameters, a simple system composed of an SQD, an MNP, and a weak signal light is designed. Furthermore, we consider a model to demonstrate the coupling of two SQDs mediated by SPP field under two cases. It is shown that two SQDs can be entangled in the presence of MNP. A high concurrence can be achieved, which is the best evidence that the coupling among SQDs induced by SPP field in MNP. This scheme may have the potential applications in all-optical plasmon-enhanced nanoscale devices.

Ultrafast dynamics of excitons in semiconductor quantum dots on a plasmonically activenano-structured silver film

The excited state dynamics of core–shell type semiconductor quantum dots (QDs) ofvarious sizes in close contact with a plasmonically active silver thin film has beendemonstrated by using picosecond resolved fluorescence spectroscopy. The non-radiativeenergy transfer from the QDs to the metal surface is found to be of Förster resonance energytransfer (FRET) type rather than the widely expected nano-surface energy transfer(NSET) type. The slower rate of energy transfer processes compared to that of the electrontransfer from the excited QDs to an organic molecule benzoquinone reveals aninsignificant possibility of charge migration from the QDs to the metallic film.

How quantum bound states bounce and the structure it reveals

We investigate how quantum bound states bounce from a hard surface. Our analysis has applications to ab initio calculations of nuclear structure and elastic deformation, energy levels of excitons in semiconductor quantum dots and wells, and cold atomic few-body systems on optical lattices with sharp boundaries. We develop the general theory of elastic reflection for a composite body from a hard wall. On the numerical side we present ab initio calculations for the compression of alpha particles and universal results for two-body states. On the analytical side we derive a universal effective potential that gives the reflection scattering length for shallow two-body states.

Creation of charged excitons with two-color excitation method and initialization of electron spin qubit in quantum dots

An optical pumping method for creating charged excitons in semiconductor quantum dots is demonstrated using a two-color excitation method. This method employs two laser sources whose energies are in the resonant and barrier excitation conditions, respectively. The photoluminescence (PL) spectra of a single quantum dot clearly vary from exciton-originated PL to charged exciton-originated PL as the excitation conditions are changed. This method is applied to the initialization process of an electron spin qubit without a magnetic field, which provides a simple and effective way of implementing quantum computing with spin qubits.

Unraveling the Structure and Dynamics of Excitons in Semiconductor Quantum Dots

The quantum dot, one of the central materials in nanoscience, is a semiconductor crystal with a physical size on the nanometer length scale. It is often called an "artificial atom" because researchers can create nanostructures which yield properties similar to those of real atoms. By virtue of having a size in between molecules and solids, the quantum dot offers a rich palette for exploring new science and developing novel technologies. Although the physical structure of quantum dots is well known, a clear understanding of the resultant electronic structure and dynamics has remained elusive. However, because the electronic structure and dynamics of the dot, the "excitonics", confer its function in devices such as solar cells, lasers, LEDs, and nonclassical photon sources, a more complete understanding of these properties is critical for device development. In this Account, we use colloidal CdSe dots as a test bed upon which to explore four select issues in excitonic processes in quantum dots. We have developed a state-resolved spectroscopic approach which has yielded precise measurements of the electronic structural dynamics of quantum dots and has made inroads toward creating a unified picture of many of the key dynamic processes in these materials. We focus on four main topics of longstanding interest and controversy: (i) hot exciton relaxation dynamics, (ii) multiexcitons, (iii) optical gain, and (iv) exciton-phonon coupling. Using this state-resolved approach, we reconcile long standing controversies related to phenomena such as exciton cooling and exciton-phonon coupling and make surprising new observations related to optical gain and multiexcitons.

Transient interband light absorption by quantum dots: Degenerate pump-probe spectroscopy

The optical pump-probe method, which makes it possible to determine the energy relaxation rate for excited electron-hole pairs and excitons in semiconductor quantum dots (QDs), is theoretically described. A scheme in which the carrier frequencies of optical pump and probe pulses are close to resonance with the same interband transition in the QD electron subsystem (degenerate case) is considered. The pump-induced probe energy absorption is analyzed as a function of the delay time between the pump and probe pulses. It is shown that under certain conditions this dependence is reduced to monoexponential, whose exponent is proportional to the energy relaxation rate for the considered state of electron-hole pairs and excitons. The size dependence of the energy relaxation rate of the electron-hole pair states is modeled by the example of PbSe-based QDs, whose electron subsystem is in the strong-confinement regime.

Coulomb correlations of charged excitons in semiconductor quantum dots

In this study, we present a systematic investigation of the emission spectra of excitonic complexes confined in QDs when an external magnetic field is applied in a direction perpendicular to the optical wave vector Voigt configuration. The effective g factors of an electron and a hole in individual QDs are obtained on the basis of a consistent analysis of the Zeeman splittings in many QDs. Our results reveal a large uniformity of the electron g factors but a broad variation in hole g factors when analyzing QDs emitting at nearly the same energy. By comparing the polarized emission spectra from the neutral and charged excitons in the same QD, we evidence a significant modification of the carrier g factors that we attribute to Coulomb correlations within the few-particle excitonic complexes. We use pyramidal In0.1Ga0.9As/Al0.3Ga0.7As QDs that allow us to observe simultaneously the positively and the negatively charged excitons in the same photoluminescence