Processes In Quantum Dots Research Articles

Multicolor Luminescent Patterning via Photoregulation of Electron and Energy Transfer Processes in Quantum Dots.

Ability to create high-contrast multicolor luminescent patterns is essential to realize the full potential of quantum dots (QDs) in display technologies. The idea of using a nonemissive state is adopted in the present work to enhance the color-contrast of QD-based photopatterns. This is achieved at a multicolor level by the photoregulation of electron and energy transfer processes in a single QD nanohybrid film, composed of one QD donor and two dye acceptors. The dominance of photoinduced electron transfer over the energy transfer process generates a nonluminescent QD nanohybrid film, which provides the black background for multicolor patterning. The superior photostability of QDs over dyes is used for the photoregulation of electron and energy transfer processes. Selective photodegradation of electron acceptor dye triggered the onset of the energy transfer process, thereby imparting a luminescent color to the QD nanohybrid film. Further, a controlled photoregulation of energy transfer process paved the way for multicolor patterning.

Pressure-Induced Tunable Electron Transfer and Auger Recombination Rates in CdSe/ZnS Quantum Dot-Anthraquinone Complexes.

Electron transfer (ET) and Auger recombination (AR) processes in quantum dots (QDs) are key mechanisms for the advance of QD-based devices. However, it still remains a challenge to promote ET and suppress AR simultaneously. Here, we use in situ high-pressure ultrafast transient absorption spectroscopy to explore the impact of pressure on the ET between CdSe/ZnS and anthraquinone (AQ) and AR dissolved in cyclohexane. Remarkably, under compression, ET lifetimes are shorten, while suppression of AR lifetimes is present. The promotion of ET is attributed to the shortened distance between CdSe/ZnS and AQ induced by pressure. We rationalize that for the AR suppression, pressure may enhance the formation of an alloy layer at the core/shell interface. These findings indicate that compression is an effective approach to promote ET and suppress AR simultaneously. This study highlights a brand-new approach for modulating ET and AR and provides new routes toward QD-based applications.

Quantum memory on a charge qubit in an optical microresonator

A quantum-memory unit scheme on the base of a semiconductor structure with quantum dots is proposed. The unit includes a microresonator with single and double quantum dots performing frequencyconverter and charge-qubit functions, respectively. The writing process is carried out in several stages and it is controlled by optical fields of the resonator and laser. It is shown that, to achieve high writing probability, it is necessary to use high-Q resonators and to be able to suppress relaxation processes in quantum dots.

Three-dimensional model of quantum dots' self-assembly under the action of laser radiation

This study considered a process of quantum dots' self-assembly into nanostructure arrays with predefined geometry, which proceeds in the external resonant laser field. We considered the simplest case of assembling a stable structure of two particles. The problem was solved numerically using a three-dimensional model of Brownian dynamics. The idea of the method is that the attraction of the dots occurs due to the interaction of resonantly induced dipole moments, with the dots being then captured by the Van der Waals force. Finally, a three-dimensional model was considered; the average nanoparticle aggregation time as a function of the laser radiation wavelength was calculated; the probability of such structures' being formed was estimated for the calculated average aggregation time and for the laser pulse duration used in the experiment. The wavelength of the maximum probability was found to be shifted from the single particle resonance wavelength of 525 nm to the red area of 535 nm, which is in qualitative agreement with the redshift of the resonance wavelength of interacting particles.

Three-dimensional model of quantum dots' self-assembly under the action of laser radiation

This study considered a process of quantum dots' self-assembly into nanostructure arrays with predefined geometry, which proceeds in the external resonant laser field. We considered the simplest case of assembling a stable structure of two particles. The problem was solved numerically using a three-dimensional model of Brownian dynamics. The idea of the method is that the attraction of the dots occurs due to the interaction of resonantly induced dipole moments, with the dots being then captured by the Van der Waals force. Finally, a three-dimensional model was considered; the average nanoparticle aggregation time as a function of the laser radiation wavelength was calculated; the probability of such structures' being formed was estimated for the calculated average aggregation time and for the laser pulse duration used in the experiment. The wavelength of the maximum probability was found to be shifted from the single particle resonance wavelength of 525 nm to the red area of 535 nm, which is in qualitative agreement with the redshift of the resonance wavelength of interacting particles.

Electron capture processes in quantum dots due to one-and two-phonon assisted transitions: The role of optical phonon confinement

Electron capture induced by carrier heating in spherical quantum dot-quantum well (QD-QW) structure is studied theoretically. The capture rate (CR) in one- and two-polar- optical-phonon-mediated capture processes has been studied by taking into account the phonon confinement. We have derived the analytic expressions for carrier CRs which can be conveniently applied to practical calculations for the spherical quantum dot systems. The numerical results of the CR as function of dot radius, lattice temperature and electron density in GaAs/AlAs/vacuum and CdSe/ZnS/H2O QD systems are obtained and discussed.

Quantitative optical measurement of chemical potentials in intermediate band solar cells

Having shown some demonstration of two photon absorption and multiband emission processes in quantum dots (QD), multiquantum wells (MQW), and highly mismatched alloys, intermediate band solar cells are currently the subject of numerous studies. To better understand the underlying mechanisms, our objective is to experimentally probe the multiband operation of this device. We used photoluminescence recorded with a calibrated hyperspectral imager which provides spectrally resolved images with a spatial resolution of 2 μm and spectral resolution of 2 nm on proof of concept QD and MQW solar cells samples. Device emission can be described with the generalized Planck’s law from which the quasi-Fermi level splitting of the three bands can be determined. The advantage of the technique is that it can be used to investigate the intermediate band material without the need to make contacts or a full device structure. We also discuss the usefulness of a dual-beam method.

Effect of phonon confinement on one- and two-polar optical phonon capture processes in quantum dots

The electron capture in spherical quantum dot–quantum well structure is studied theoretically. The capture rate in one- and two-polar-optical-phonon-mediated capture processes has been studied by taking into account the phonon confinement. We have derived the explicit analytic expressions for carrier capture rates which can be conveniently applied to practical calculations for the spherical quantum dot systems. The numerical results of the capture rate as function of dot radius, lattice temperature and electron density in GaAs/AlAs QD systems are obtained and discussed. The dependence of the carrier capture rate for a fixed dot radius shows a maximum as a function of carrier density. It is shown that the capture rate of an electron from the barrier region to the quantum dot ground-state, via emission of one and two phonons, exhibits the existence of the bands of dot radii where capture is energetically allowed. We found that the height of the capture rate peaks obtained for one-phonon assisted processes is lowered as QD radius decreases when the phonon confinement is taken into account. The capture rates due to emission single and two optical phonon modes are compared. Carrier capture is shown to proceed with rates as high as 1010s−1 at temperatureT>100K. A short capture time is also achieved for a low carrier density.

Spin noise in a quantum dot ensemble: From a quantum mechanical to a semi-classical description

Spin noise spectroscopy is a promising technique for revealing the microscopic nature of spin dephasing processes in quantum dots. We compare the spin-noise in an ensemble of singly charged quantum dots calculated by two complementary approaches. The Chebyshev polynomial expansion technique (CET) accounts for the full quantum mechanical fluctuation of the nuclear spin bath and a semi-classical approach (SCA) is based on the averaging the electron spin dynamics over all different static Overhauser field configurations. We observe a remarkable agreement between both methods in the high-frequency part of the spectra, while the low-frequency part is determined by the long time fluctuations of the Overhauser field. We find small differences in the spectra depending on the distribution of hyperfine couplings. The spin-noise spectra in strong enough magnetic fields where the nuclear dynamics is quenched calculated by two complimentary approaches are in perfect agreement.

Boosting the Efficiency of Quantum Dot Sensitized Solar Cells through Modulation of Interfacial Charge Transfer

The demand for clean energy will require the design of nanostructure-based light-harvesting assemblies for the conversion of solar energy into chemical energy (solar fuels) and electrical energy (solar cells). Semiconductor nanocrystals serve as the building blocks for designing next generation solar cells, and metal chalcogenides (e.g., CdS, CdSe, PbS, and PbSe) are particularly useful for harnessing size-dependent optical and electronic properties in these nanostructures. This Account focuses on photoinduced electron transfer processes in quantum dot sensitized solar cells (QDSCs) and discusses strategies to overcome the limitations of various interfacial electron transfer processes. The heterojunction of two semiconductor nanocrystals with matched band energies (e.g., TiO(2) and CdSe) facilitates charge separation. The rate at which these separated charge carriers are driven toward opposing electrodes is a major factor that dictates the overall photocurrent generation efficiency. The hole transfer at the semiconductor remains a major bottleneck in QDSCs. For example, the rate constant for hole transfer is 2-3 orders of magnitude lower than the electron injection from excited CdSe into oxide (e.g., TiO(2)) semiconductor. Disparity between the electron and hole scavenging rate leads to further accumulation of holes within the CdSe QD and increases the rate of electron-hole recombination. To overcome the losses due to charge recombination processes at the interface, researchers need to accelerate electron and hole transport. The power conversion efficiency for liquid junction and solid state quantum dot solar cells, which is in the range of 5-6%, represents a significant advance toward effective utilization of nanomaterials for solar cells. The design of new semiconductor architectures could address many of the issues related to modulation of various charge transfer steps. With the resolution of those problems, the efficiencies of QDSCs could approach those of dye sensitized solar cells (DSSC) and organic photovoltaics.

Enhancement of two photon processes in quantum dots embedded in subwavelength metallic gratings

We show a large enhancement of two-photon absorption processes in nanocrystal quantum dots and of light upconversion efficiency from the IR to the near-IR spectral regime, using a hybrid optical device in which near-IR emitting InAs quantum dots were embedded on top a metallic nanoslit array. The resonant enhancement of these nonlinear optical processes is due to the strong local electromagnetic field enhancements inside the nanoslit array structure at the extraordinary transmission resonances. A maximal two-photon absorption enhancement of more than 20 was inferred. Different high field regions were identified for different polarizations, which can be used for designing and optimizing efficient nonlinear processes in such hybrid structures. Combining nanocrystal quantum dots with subwavelength metallic nanostructures is therefore a promising way for a range of possible nonlinear optical devices.

Interface roughness, valley-orbit coupling, and valley manipulation in quantum dots

We present a systematic study of interface roughness and its effect on coherent dynamical processes in quantum dots. The potential due to a sharp, flat interface lifts the degeneracy of the lowest energy valleys and yields a set of valley eigenstates. Interface roughness is characterized by fluctuations in the location of the interface and in the magnitude of the potential step. Variations in the position of the interface, which are expected to occur on the length scale of the lattice constant, reduce the magnitude of the valley-orbit coupling. Variations in the size of the interface potential step alter the magnitude of the valley-orbit coupling and induce transitions between different valley eigenstates in dynamics involving two (or more) dots. Such transitions can be studied experimentally by manipulating the bias between two dots and can be detected by charge sensing. However, if the random variable characterizing the position of the interface is correlated over distances of the order of a quantum dot, which is unlikely but possible, the phase of the valley-orbit coupling may be different in adjacent dots. In this case tunneling between like and opposite valley eigenstates is in effect a random variable and cannot be controlled. We suggest a resonant tunneling experiment that can identify the matrix elements for tunneling between like and opposite valley eigenstates.

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.

Quantum capture of electrons and intra-dot relaxation by means of Auger processes in quantum dots

A theoretical analysis of LO phonon-assisted electron capture and of intra-dot Auger relaxation in quantum dots is presented. The electron capture efficiency is investigated as a function of the lattice temperature and quantum dot radius for different injected electron concentration taking into account the confinement effect for the polar optical phonons in spherical quantum dots. Exact numerical calculations for the phonon-assisted electron capture rate as well as for the Auger scattering rate have been carried out.

Spin control by ultra short laser pulses in a Mn doped quantum dot

AbstractOptically induced spin flip processes in quantum dots doped with a single Mn atom are discussed. On the femtosecond time scale the exciton can be controlled by ultra short laser pulses. Via the exchange interaction the Mn spin can flip on a picosecond time scale by transferring its angular momentum to the exciton. For a heavy hole system this flipping process can be described in terms of exchange‐induced Rabi oscillations between the bright and dark exciton state accompanied by a flip of the Mn spin. Because of the energy splitting between bright and dark excitons the Rabi oscillations are off‐resonant and a series of laser pulses is required to achieve a complete flip. When a light hole exciton is resonantly excited in addition to the heavy hole exciton we show that a flip of the Mn spin by two can be achieved by a series of light pulses while the exciton system returns to its initial state. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Theory of two-photon processes in quantum dots: Coherent evolution and phonon-induced dephasing

The paper discusses coherent control of charge and spin states of a biexciton system in a quantum dot via coherent two-photon transitions. Rabi oscillations between the ground state of a quantum dot and the biexciton state, as well as oscillations between the two single-exciton states induced by laser pulses with different circular or linear polarizations are studied. The effect of phonon-induced decoherence on these processes is described. System properties and driving conditions that lead to optimal coherent control are identified. It is shown that proper optimization allows one to control the two-qubit biexciton system via two-photon transitions with a high fidelity.

Spin and angular momentum transitions in few-electron quantum dots and rings

The studies on quantum dots and rings containing $4\ensuremath{\sim}6$ electrons reveal that the shapes of the systems have great impact on the spin transitions of the ground states. For five-electron case, the different shapes lead to the different transition processes in quantum dots and rings. For six-electron case, the ring confinement forbids the spin transition, which really exists in dot confinement. The investigations on the angular momentum transitions in magnetic fields show that the sensibility of the transitions to the size of the system depends remarkably on the particle number. The four-electron case is almost size insensitive but the five-electron case has obvious size dependence. It is also found that the behaviors of the few-electron entanglement in quantum dots and rings are quite different. The entanglement entropies in quantum dots increase with the increase in the magnetic field but exhibit Aharonov-Bohm oscillation patterns in narrow rings. The studies of the shape, size, and particle number effects are important for understanding the liquid-to-crystal transitions, and may be helpful for the manipulation of electronic states in quantum dots and rings.

Two‐photon processes in quantum dots: biexciton Rabi oscillations and exciton polarization flipping

AbstractThe paper discusses two‐photon Rabi oscillations between the ground state of a quantum dot and the biexciton state, as well as two‐photon oscillations between the two single‐exciton states with different circular polarizations. The effect of phonon‐induced decoherence on these processes is also described. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

1.3 μm量子点半导体光放大器的非线性特性

The dynamics of nonlinear processes in quantum dot (QD) semiconductor optical amplifiers (SOAs) are investigated. Using small-signal measurements, the suitabilities of cross-gain and cross-phase modulation as well as four wave mixing (FWM) for wavelength conversion are examined. The cross-gain modulation is found to be suitable for wavelength conversion up to a frequency of 40 GHz.

Electronic spectral densities in quantum dots

The spectral line profiles of the luminescence transitions in individual quantum dots have been studied both theoretically and experimentally in the recent years. In these works the spectral line profiles have been shown to be clearly different from the simple form of a broadened delta function of frequency variable. In the present work we turn attention to these earlier measurements and calculations and compare them with the electronic spectral line profiles in individual quantum dots calculated under the assumption of quantum dot electrons multiple scattering on the longitudinal optical phonons. The calculated luminescence spectral line profiles of the optical transition will be expressed with help of the electronic spectral densities dependent on the electronic statistical distribution, as it develops in the course of the relaxation processes or other kinetic processes in quantum dots.