Exciton States In Quantum Dots Research Articles

Nanowire-mediated photon entanglement between orthogonal quantum-dot exciton states

Nanowire-mediated photon entanglement between orthogonal quantum-dot exciton states

Enhanced Exciton Quantum Coherence in Single CsPbBr3 Perovskite Quantum Dots using Femtosecond Two-Photon Near-Field Scanning Optical Microscopy.

Cesium-halide perovskite quantum dots (QDs) have gained tremendous interest as quantum emitters in quantum information processing applications due to their optical and photophysical properties. However, engineering excitonic states in quantum dots requires a deep knowledge of the coherent dynamics of their excitons at a single-particle level. Here, we use femtosecond time-resolved two-photon near-field scanning optical microscopy (NSOM) to reveal coherences involving a single cesium lead bromide perovskite QD (CsPbBr3) at room temperature. We show that, compared to other nonperovskite nanoparticles, the electronic coherence on a single perovskite QD has a relatively long lifetime of ca. 150 fs, whereas CdSe QDs have exciton coherence times shorter than 75 fs at room temperature. One possible explanation for the longer coherence time observed for the CsPbBr3 perovskite system is related to the exciton fine structure of these perovskite QDs compared to other nanoparticles. These perovskite QDs exhibit interesting optical properties that differ from those of the traditional QDs including bright triplet exciton states. In fact, due to the small amplitude of the energy gap fluctuations of dipole-allowed triplet states in perovskite QDs, the coherent superposition could be preserved for longer times. Furthermore, single-particle excitation approach implemented in this work allows us to remove effects of heterogeneity that are usually present in ensemble averaging experiments at room temperature. The realization of quantum-mechanical phase-coherence of a charge carrier that can operate at room temperature is an issue of great importance for the potential application of coherent electronic phenomena in electronic and optoelectronic devices. These interesting findings provide further evidence of the great potential of these perovskite QDs as candidates for quantum computing and information processing applications.

Spectroscopy of Single AlInAs Quantum Dots

A system of quantum dots based on Al x In1−xAs/Al y Ga1−yAs solid solutions is investigated. The use of Al x In1−xAs wide-gap solid solutions as the basis of quantum dots substantially extends the spectral emission range to the short-wavelength region, including the wavelength region near 770 nm, which is of interest for the development of aerospace systems of quantum cryptography. The optical characteristics of Al x In1−xAs single quantum dots grown by the Stranski–Krastanov mechanism were studied by cryogenic microphotoluminescence. The statistics of the emission of single quantum dot excitons was studied using a Hanbury Brown–Twiss interferometer. The pair photon correlation function indicates the sub-Poissonian nature of the emission statistics, which directly confirms the possibility of developing single-photon emitters based on Al x In1−xAs quantum dots. The fine structure of quantum dot exciton states was investigated at wavelengths near 770 nm. The splitting of the exciton states is found to be similar to the natural width of exciton lines, which is of great interest for the development of entangled photon pair emitters based on Al x In1−xAs quantum dots.

Spontaneous brightening of dark excitons in GaAs/AlGaAs quantum dots near a cleaved facet

Dark excitons (DEs) confined in epitaxial quantum dots (QDs) are interesting because of their long lifetime compared to bright excitons (BEs). For the same reason they are usually difficult to access in optical experiments. Here we report on the observation of vertically polarized light emission from DEs confined in high-quality epitaxial GaAs/AlGaAs QDs located in proximity of a cleaved facet of the QD specimen. Calculations based on the eight-band k\ifmmode\cdot\else\textperiodcentered\fi{}p method and configuration interaction allow us to attribute the brightening of the DE to the anisotropic strain present at the sample edge, which breaks the symmetry of the system and enhances valence-band mixing. The mechanism of DE brightening is discussed in detail by inspecting both the Bloch and envelope wave functions of the involved hole states. In addition, by investigating experimentally and theoretically QDs with different sizes, we find that the energy separation between DE and BEs tends to decrease with increasing QD height. Finally, the presence of a cleaved facet is found also to enhance the BE fine structure splitting. This work provides a simple method to optically probe dark excitonic states in QDs and shows that the properties of QDs can be significantly affected by the presence of nearby edges.

Anisotropy-Induced Quantum Interference and Population Trapping between Orthogonal Quantum Dot Exciton States in Semiconductor Cavity Systems.

We describe how quantum dot semiconductor cavity systems can be engineered to realize anisotropy-induced dipole-dipole coupling between orthogonal dipole states in a single quantum dot. Quantum dots in single-mode cavity structures as well as photonic crystal waveguides coupled to spin states or linearly polarized excitons are considered. We demonstrate how the dipole-dipole coupling can control the radiative decay rate of excitons and form pure entangled states in the long time limit. We investigate both field-free entanglement evolution and coherently pumped exciton regimes, and show how a double-field pumping scenario can completely eliminate the decay of coherent Rabi oscillations and lead to population trapping. In the Mollow triplet regime, we explore the emitted spectra from the driven dipoles and show how a nonpumped dipole can take on the form of a spectral triplet, quintuplet, or a singlet, which has applications for producing subnatural linewidth single photons and more easily accessing regimes of high-field quantum optics and cavity-QED.

AlInAs quantum dots

A system of quantum dots on the basis of AlxIn1-xAs/AlyGa1-y As solid solutions has been studied. The usage of broadband AlxIn1-x solid solutions as the basis of quantum dots makes it possible to expand considerably the spectral emission range into the short-wave region, including the wavelength region near 770 nm being of interest for the design of aerospace systems of quantum cryptography. The optical characteristics of single AlxIn1-xAs quantum dots grown according to the Stranski–Krastanov mechanism are studied by the cryogenic microphotoluminescence method. The fine structure of exciton states of quantum dots is studied in the wavelength region near 770 nm. It is shown that the splitting of exciton states is comparable with the natural width of exciton lines, which is of great interest for the design of emitters of pairs of entangled photons on the basis of AlxAs1-x quantum dots.

Delayed Exciton Emission and Its Relation to Blinking in CdSe Quantum Dots.

The efficiency and stability of emission from semiconductor nanocrystal quantum dots (QDs) is negatively affected by "blinking" on the single-nanocrystal level, that is, random alternation of bright and dark periods. The time scales of these fluctuations can be as long as many seconds, orders of magnitude longer than typical lifetimes of exciton states in QDs. In this work, we investigate photoluminescence from QDs delayed over microseconds to milliseconds. Our results prove the existence of long-lived charge-separated states in QDs. We study the properties of delayed emission as a direct way to learn about charge carrier separation and recovery of the exciton state. A new microscopic model is developed to connect delayed emission to exciton recombination and blinking from which we conclude that bright periods in blinking are in fact not characterized by uninterrupted optical cycling as often assumed.

Direct Quantitative Electrical Measurement of Many-Body Interactions in Exciton Complexes in InAs Quantum Dots

We present capacitance-voltage spectra for the conduction band states of InAs quantum dots obtained under continuous illumination. The illumination leads to the appearance of additional charging peaks that we attribute to the charging of electrons into quantum dots containing a variable number of illumination-induced holes. By this we demonstrate an electrical measurement of excitonic states in quantum dots. Magnetocapacitance-voltage spectroscopy reveals that the electron always tunnels into the lowest electronic state. This allows us to directly extract, from the highly correlated many-body states, the correlation energy. The results are compared quantitatively to state of the art atomistic configuration interaction calculations, showing very good agreement for a lower level of excitations and also limitations of the approach for an increasing number of particles. Our experiments offer a rare benchmark to many-body theoretical calculations.

Coherently-enabled environmental control of optics and energy transfer pathways of hybrid quantum dot-metallic nanoparticle systems

It is well-known that optical properties of semiconductor quantum dots can be controlled using optical cavities or near fields of localized surface plasmon resonances (LSPRs) of metallic nanoparticles. In this paper we study the optics, energy transfer pathways, and exciton states of quantum dots when they are influenced by the near fields associated with plasmonic meta-resonances. Such resonances are formed via coherent coupling of excitons and LSPRs when the quantum dots are close to metallic nanorods and driven by a laser beam. Our results suggest an unprecedented sensitivity to the refractive index of the environment, causing significant spectral changes in the Förster resonance energy transfer from the quantum dots to the nanorods and in exciton transition energies. We demonstrate that when a quantum dot-metallic nanorod system is close to its plasmonic meta-resonance, we can adjust the refractive index to: (i) control the frequency range where the energy transfer from the quantum dot to the metallic nanorod is inhibited, (ii) manipulate the exciton transition energy shift of the quantum dot, and (iii) disengage the quantum dot from the metallic nanoparticle and laser field. Our results show that near meta-resonances the spectral forms of energy transfer and exciton energy shifts are strongly correlated to each other.

Genuine entanglement among coherent excitonic states of three quantum dots located individually in separated coupled QED cavities

New scheme for generating genuine three-partite entanglement among three quantum dots (QDs) is proposed. The QDs are trapped in an one-dimensional (1D) array of three equidistance single-mode coupled cavities. Photon hopping is considered to be responsible for coupling between the cavities. The effective dynamics of the system leads to generate genuine three-partite entangled coherent excitonic states in QDs. The entanglement of these states, after encoding as three-qubit system, can be detected by entanglement witnesses (EWs) based on GHZ-states. It is shown that the generated entangled states can be arbitrarily very close to the GHZ-states.

The effect of scattering on single photon transmission of optical angular momentum

Schemes for the communication and registration of optical angular momentumdepend on the fidelity of transmission between optical system components. Itis known that electron spin can be faithfully relayed between exciton states inquantum dots; it has also been shown by several theoretical and experimental studiesthat the use of beams conveying orbital angular momentum can significantlyextend the density and efficiency of such information transfer. However, it remainsunclear to what extent the operation of such a concept at the single photon level ispracticable—especially where this involves optical propagation through a material system,in which forward scattering events can intervene. The possibility of transmitting anddecoding angular momentum over nanoscale distances itself raises other importantissues associated with near-field interrogation. This paper provides a framework toaddress these and related issues. A quantum electrodynamical representation isconstructed and used to pursue the consequences of individual photons, froma Laguerre–Gaussian beam, undergoing single and multiple scattering eventsin the course of propagation. In this context, issues concerning orbital angularmomentum conservation, and its possible compromise, are tackled by identifying therelevant components of the electromagnetic scattering and coupling tensors, using anirreducible Cartesian basis. The physical interpretation broadly supports the fidelity ofquantum information transmission, but it also identifies potential limitations ofprinciple.

Coherent superposition of exciton states in quantum dots induced by surface plasmons

We present an experimental demonstration of strong optical coupling between CdSequantum dots of different sizes which is induced by a surface plasmon propagating on a planar silver thin film. Attenuated total reflection measurements demonstrate the hybridization of exciton states, characterized by the observation of two avoided crossings in the energy dispersion measured for the interacting system.

On the conveyance of angular momentum in electronic energy transfer

When electronic excitation transfer occurs, it is of considerable interest to establish whether angular momentum can also be conveyed in the process. The question is prompted by a consideration that when the participating chromophores are atoms, ions, or molecular systems having high local symmetry, the electronic excited states that are involved are generally characterized not only by energy, but by angular momentum properties. Moreover, it is known that electron spin can be communicated between quantum dot exciton states. Resolving the general issue entails an electrodynamic representation exploiting irreducible tensor methods, the analysis being illustrated by application to energy transfer associated with a variety of multipolar transitions. The results exhibit novel connections between an angular momentum content of the electromagnetic coupling and a strongly varying distance dependence. It is concluded that the communication of angular momentum does not in general map unambiguously between a donor and energy acceptor.

Role of Cooper pairs for the generation of entangled photon pairs from single quantum dots

Generation of entangled photon pairs from semiconductor quantum dots (QDs) is highly desirable for realizing practical solid-state photon sources for quantum information processing and quantum cryptography. However, the energy splitting of exciton states in QDs almost prevent the generation of entangled photon pairs. This paper discusses the new possibility with the injection of electron as well as hole Cooper pairs into QDs.

Control of Photon Polarization in GaAs/AlAs Single Quantum Dot Emission

Polarization encoded single photons and photon pairs are expected to play a major role in future quantum communication schemes [1]. Therefore, control of polarization properties of single quantum dot emission is of a great value. We study magnetic field induced transition between anisotropy controlled and Zeeman controlled emission from individual GaAs/AlAs quantum dot (QD). We demonstrate the utility of these studies, involving polarized photon correlation measurements, for determination of the anisotropic exchange splitting of excitonic states in quantum dots. The sample was mounted directly on a microscope objective [2] and placed inside a pumped helium cryostat. Continuous wave excitation was performed with use of tunable Ti:Al2O3 laser at wavelength of 718 nm assuring excitation below the energy gap of the barrier material. The typical power of excitation beam was 100μW over the spot of a diameter of about 1μm. Temperature was kept constant at 1.8K. Photoluminescence was excited and collected through the same microscope objective. Correlation measurements were performed in a Hanbury-Brown and Twiss setup. The signal after polarization and spectral filtering was detected using two avalanche photodiodes. Polarization resolved crosscorrelations between biexciton (XX) and exciton (X) photons emitted subsequently in a radiative cascade were measured in magnetic field ranging from 0 to 0.5 T, applied in Faraday configuration. Polarization correlation of photon pairs in parallel linear polarizations appeared as a bunching peak in histograms of correlated counts in the absence of magnetic field. No polarization correlation in circular basis was observed at B = 0T. This is expected for QDs exhibiting in-plane anisotropy, since intermediate X state of the cascade is split by energy of electron-hole exchange interaction into two components emitting in orthogonal linear polarizations. Application of magnetic field transforms Anisotropic Exchange Splitting (AES) into Zeeman splitting of the X level, reducing thus the influence of the anisotropy. In a sufficiently high magnetic field pure exciton eigenstates of angular momentum M = +/1 are observed [3]. Expected conversion of linearly polarized excitonic states to circularly polarized ones is demonstrated by increase of the degree of circular polarization correlation with increasing magnetic field. We apply a simple model proposed by Besombes et al. [4] to describe the progressive change of the polarization correlation with increasing field. The model allows calculating of the degree of polarization correlation as a function of Zeeman splitting to AES ratio. By fitting the model to the experimental data, we were able to determine the anisotropic exchange splitting AES value of order of tens of μeV. This method of the AES determination can be used even in cases when direct measurement by polarization resolved microphotoluminescence is not sensitive enough.

Determination of energetics and kinetics from single-particle intermittency and ensemble-averaged fluorescence intensity decay of quantum dots

Quantification of energetics and kinetics for the band-edge exciton states of quantum dots and the long-lived dark state is important for better understanding of the underlying mechanism for single-particle intermittency and ensemble fluorescence intensity decay. Based on a multistate diffusion-reaction model by extending our previous studies, we analyze experimental data from ensemble measurements and fluorescence intermittency of single quantum dots and determine important molecular-based quantities such as Stokes shift, free energy gap, activation energy, reorganization energy, and other kinetic parameters.

Exciton spin relaxation in quantum dots measured using ultrafast transient polarization grating spectroscopy

On the basis of quantum dot exciton states and selection rules for their excitation, a microscopic picture of a nonlinear optical spectroscopy that provides a direct probe of spin relaxation among quantum dot exciton states is described. Equations of motion which govern the evolution of the third order exciton population density are solved numerically to simulate the measured signals. It is shown how cross linearly-polarized pulse sequences in three-pulse transient grating experiments form a polarization grating that monitors the history of the bright exciton $(F=\ifmmode\pm\else\textpm\fi{}1)$ spin states. Spin flips among those states lead to a decay of the grating, and consequently the diffracted probe signal. In the microscopic picture elucidated from the simulations, destructive interference between the third-order polarizations radiated by populations of excitons with flipped and conserved spin states causes the signal decay. The experiment permits the direct observation of the kinetics of exciton spin state flips in an isotropic ensemble of quantum dots. Such measurements are demonstrated for colloidal CdSe quantum dots at room temperature, and compared with results for a control experiment. The relationship between this experiment and a difference measurement based on circularly-polarized pump and probe pulses is established.

Phonon-Induced Dephasing of Optically Driven Exciton States in Quantum Dots: Spectral Interpretation

In recent years it has become possible to control the charge state of a single quantum dot (QD) far beyond the linear absorption regime. A convincing example of this experimental progress are pulse area-dependent Rabi oscillations of the exciton occupation, i.e., coherent transitions from the ground crystal state to the singleor bi-exciton state and back, driven by optical pulses of variable intensity [1]. These achievements show that the atomic-like properties of charge systems confined in a QD may allow one to transfer quantum control schemes developed in the quantum-optical context to the semiconductor systems. However, charges in QDs, unlike those in natural atoms, undergo strong interactions with their solid-state environment resulting in strong decoherence of their quantum states. In self-assembled dots, an important contribution to decoherence is that of pure dephasing due to carrier-phonon interactions [2, 3]. These phonon-related effects may be analyzed for arbitrary driving pulses within the theoretical approach developed recently [4, 5], which allows one to treat phonon effects perturbatively, while unperturbed system evolution, including external driving, is included exactly.

Dark and bright excitonic states in nitride quantum dots

Formation of excitonic states in quantum dots, of nitride based III-V semiconductors GaN and AlN, including Coulombic interaction, exchange interaction, and dielectric effects, are investigated. Dark exciton formation is found to occur for both GaN quantum dots (QD's) with wurtzite structure having positive crystal field splitting and GaN and AlN QD's with zinc-blende structure having zero crystal field splitting. The transition from dark to bright exciton occurs between radii range $20--40\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ depending on the amount of dielectric mismatch between the dot and the surroundings. In wurtzite AlN QD's with negative crystal field splitting, the splitting between the dark and bright excitonic states is very small and vanishes at about $15\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$.

Spin flip from dark to bright states in InP quantum dots

We report measurements of the time for spin flip from dark (nonlight emitting) exciton states in quantum dots to bright (light emitting) exciton states in InP quantum dots. Dark excitons are created by two-photon excitation by an ultrafast laser. The time for spin flip between dark and bright states is found to be at most $200\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$, independent of density and temperature, below $70\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. This is much shorter than observed in other quantum dot systems.