Ensemble Of Dots Research Articles

Quantum emission of light with densely packed driven dipoles

Collective excitations in regular structures of strongly interacting driven dipoles have been found to lead to strongly antibunched statistics of the emitted photons $[{g}^{(2)}(0)\ensuremath{\ll}0.5]$, but precise control of the number of emitters and their spatial distribution is in general a complex experimental task. We overcome this limitation by showing quantum light emission from densely packed driven dipoles does not require their arrangement in any specific structure, pointing strongly towards possible single-photon sources based on hot thermal vapors confined within nanocells, or on irregular quantum dot ensembles.

Cross-correlation spectra in interacting quantum dot systems

Two-color spin-noise spectroscopy of interacting electron spins in singly charged semiconductor quantum dots provides information on the interquantum dot interactions. We investigate the spin cross-correlation function in a quantum dot ensemble employing a modified semiclassical approach. Spin-correlation functions are calculated using a Hamilton quaternion approach that maintains local quantum mechanical properties of the spins. This method takes into account the effects of the nuclear-electric quadrupolar interactions, the randomness of the coupling constants, and the variation of the electron $g$ factor on the spin-noise power spectra. We demonstrate that the quantum dot ensemble can be mapped on an effective two-quantum dot problem and discuss how the characteristic length scale of the interdot interaction modifies the low-frequency cross-correlation spectrum. We argue that details on the interaction strength distribution can be extracted from the cross-correlation spectrum when applying a longitudinal or a transversal external magnetic field.

Optically controlled dual-band quantum dot infrared photodetector

We present the design for a novel type of dual-band photodetector in the thermal infrared spectral range, the Optically Controlled Dual-band quantum dot Infrared Photodetector (OCDIP). This concept is based on a quantum dot ensemble with a unimodal size distribution, whose absorption spectrum can be controlled by optically injected carriers. An external pumping laser varies the electron density in the QDs, permitting to control the available electronic transitions and thus the absorption spectrum. We grew a test sample which we studied by AFM and photoluminescence. Based on the experimental data, we simulated the infrared absorption spectrum of the sample, which showed two absorption bands at 5.85 μm and 8.98 μm depending on the excitation power.

Photocurrent in plasmonic nanofibers

We developed a theory of photoresponse and photocurrent in photonic nanofibers. Photonic nanofiber is a compound system doped with an ensemble of quantum dots and metallic nanoparticles, where they interact with each other via the dipole–dipole interaction. The bound states of the confined probe photons in the nanofiber hybrid are calculated using the transfer matrix method based on Maxwell’s equations. It is found that the density of states of photons in the nanofiber depends on the dipole–dipole interaction coupling. The nonradiative decay rate due to dipole–dipole interaction rates is calculated using the quantum mechanical perturbation theory. An analytical expression of the photoresponse coefficient and the photocurrent is calculated using the density matrix method. We predicted that the quenching in photocurrent is due to the dipole–dipole interaction. Furthermore, we have shown that the photoluminescence quenching increases as the strength of the dipole–dipole coupling increases. We also compared our theory with the experimental results of the photocurrent in a nanofiber doped with Al metallic nanoparticle nanodisks and Ge/Si quantum dots. A good agreement between theory and experiment is found. Our analytical expressions can be used by experimentalists to perform new types of experiments and for inventing new types of nanosensors and nanoswitches.

Polaritonic crystal formed of a tunnel-coupled microcavity array and an ensemble of quantum dots

Propagation of polariton excitations in a defect-containing one-dimensional lattice of microcavities with embedded ultracold atomic nanoclusters (quantum dots) is being considered. The virtual crystal approximation is used to study the properties of electromagnetic excitation spectrum resulting from random variations of the atomic subsystem composition and positions of micropores, as well as from a homogeneous elastic deformation of the considered one-dimensional structure. The group velocity dependence of polariton excitations on structural defect concentration and on deformation parameter is being numerically modeled.

Homogeneous optical anisotropy in an ensemble of InGaAs quantum dots induced by strong enhancement of the heavy-hole band Landé parameter q

The authors report on a mechanism of strong enhancement of the band Land\'e parameter $q$ due to in-plane confinement of holes and the valence-band warping. This explains the surprisingly large in-plane hole $g$ factor in symmetric self-assembled (In,Ga)As/GaAs quantum dots with $D2d$ symmetry as revealed by coherent optical spectroscopy. The proposed mechanism results in uniform magnetic field induced optical anisotropy for the entire quantum dot ensemble, which is a prerequisite for the realization of spin quantum memories and spin-photon entanglement in the ensemble.

Room-temperature coherent revival in an ensemble of quantum dots

We report a demonstration of the hallmark concept of quantum optics: periodic collapse and revival of quantum coherence (QCR) in a room temperature ensemble of quantum dots (QD). Control over quantum states, inherent to QCR, together with the dynamical QD properties present an opportunity for practical room temperature building blocks of quantum information processing. The amplitude decay of QCR is dictated by the QD homogeneous linewidth, thus, enabling its extraction in a double-pulse Ramsey-type experiment. The more common photon echo technique was also invoked and yielded the same linewidth. Measured electrical bias and temperature dependencies of the transverse relaxation times enable to determine the two main decoherence mechanisms: carrier-carrier and carrier-phonon scatterings.

Exciton-Related Raman Scattering, Interband Absorption and Photoluminescence in Colloidal CdSe/CdS Core/Shell Quantum Dots Ensemble.

By using the numerical discretization method within the effective-mass approximation, we have theoretically investigated the exciton-related Raman scattering, interband absorption and photoluminescence in colloidal CdSe/CdS core/shell quantum dots ensemble. The interband optical absorption and photoluminescence spectra have been revealed for CdSe/CdS quantum dots, taking into account the size dispersion of the ensemble. Numerical calculation of the differential cross section has been presented for the exciton-related Stokes–Raman scattering in CdSe/CdS quantum dots ensemble with different mean sizes.

Suppression of nuclear spin fluctuations in an InGaAs quantum dot ensemble by GHz-pulsed optical excitation

The coherent electron spin dynamics of an ensemble of singly charged (In,Ga)As/GaAs quantum dots in a transverse magnetic field is driven by periodic optical excitation at 1 GHz repetition frequency. Despite the strong inhomogeneity of the electron g factor, the spectral spread of optical transitions, and the broad distribution of nuclear spin fluctuations, we are able to push the whole ensemble of excited spins into a single Larmor precession mode that is commensurate with the laser repetition frequency. Furthermore, we demonstrate that an optical detuning of the pump pulses from the probed optical transitions induces a directed dynamic nuclear polarization and leads to a discretization of the total magnetic field acting on the electron ensemble. Finally, we show that the highly periodic optical excitation can be used as universal tool for strongly reducing the nuclear spin fluctuations and preparation of a robust nuclear environment for subsequent manipulation of the electron spins, also at varying operation frequencies.

Nuclear magnetic resonance spectroscopy of nonequilibrium steady states in quantum dots

The optically induced polarization of localized electron spins in an ensemble of quantum dots (QDs) dephases due to the interaction with the surrounding nuclear spins. Despite this dephasing, the spins in the QDs can be controlled to respond coherently by applying periodic laser pulses, leading to a revival of the spin polarization before each pulse. This effect, known as spin mode locking, strongly depends on an emerging selection of certain polarizations of the nuclear spin bath which is driven to a steady state far from equilibrium. We investigate the influence of the nuclear composition in In x GaAs QDs on this nonequilibrium behavior and demonstrate that nuclear magnetic resonances (NMR) appear as very sharp minima in the magnetic field dependence of the revival amplitude. This suggests a novel kind of NMR spectroscopy.

Modeling Quantum Dot Systems as Random Geometric Graphs with Probability Amplitude-Based Weighted Links.

This paper focuses on modeling a disorder ensemble of quantum dots (QDs) as a special kind of Random Geometric Graphs (RGG) with weighted links. We compute any link weight as the overlap integral (or electron probability amplitude) between the QDs (=nodes) involved. This naturally leads to a weighted adjacency matrix, a Laplacian matrix, and a time evolution operator that have meaning in Quantum Mechanics. The model prohibits the existence of long-range links (shortcuts) between distant nodes because the electron cannot tunnel between two QDs that are too far away in the array. The spatial network generated by the proposed model captures inner properties of the QD system, which cannot be deduced from the simple interactions of their isolated components. It predicts the system quantum state, its time evolution, and the emergence of quantum transport when the network becomes connected.

Optical quantum yield in plasmonic nanowaveguide

We have developed a theory of the quantum yield for plasmonic nanowaveguide where the cladding layer is made of an ensemble of quantum dots and the core layer consists of an ensemble of metallic nanoparticles. The bound states of the confined probe photons in the plasmonic nanowaveguide are calculated using the transfer matrix method based on the Maxwell equations. It is shown that the number of bound states in the nanowaveguide depends on the dielectric properties of the core and cladding layers. The surface plasmon polaritons (SPPs) produced by the metallic nanoparticles interacts with the excitons of the quantum dots. The radiative and nonradiative linewidths of excitons in the quantum yield are calculated using the quantum mechanical perturbation theory. We have found that the quantum yield decreases as the dipole–dipole interaction between metallic nanoparticles increases. We have also calculated the photoluminescence and found that the enhancement in photoluminescence is due to the SPPs coupling. On the other hand, the quenching in the photoluminescence is due to the quantum yield. We compared our theory with experiments of a nanowaveguide where the core is fabricated from Ag- nanoparticles and the cladding is fabricated from the perovskite quantum dots. A good agreement between theory and experiments is found. Our analytical expressions of the quantum yield and photoluminescence can be used by experimentalists to proforma new types of experiments and for inventing new types of nanosensors and nanoswitches.

Spontaneous Emission of Vector Vortex Beams

Harnessing the spontaneous emission of incoherent quantum emitters is one of the hallmarks of nano-optics. Yet, an enduring challenge remains-making them emit vector beams, which are complex forms of light associated with fruitful developments in fluorescence imaging, optical trapping and high-speed telecommunications. Vector beams are characterized by spatially varying polarization states whose construction requires coherence properties that are typically possessed by lasers-but not by photons produced by spontaneous emission. Here, we show a route to weave the spontaneous emission of an ensemble of colloidal quantum dots into vector beams. To this end, we use holographic nanostructures that impart the necessary spatial coherence, polarization and topological properties to the light originating from the emitters. We focus our demonstration on vector vortex beams, which are chiral vector beams carrying non-zero orbital angular momentum, and argue that our approach can be extended to other forms of vectorial light.

Spontaneous emission in nanofibers doped with an ensemble of quantum dots and quantum emitters

We studied the effect of the spontaneous emission on the photoluminescence (PL) of photonic nanofibers doped with an ensemble of quantum dots (QDs) and quantum emitters. Quantum emitters can be molecular dyes or DNA molecules. Bound photonic states of the nanofiber hybrid are calculated using the transfer-matrix method based on the Maxwell equations. It is shown that the number of bound states in the nanofiber hybrid depends on the size and shape of the nanofiber along with the concentration of quantum dots and quantum emitters including their dielectric constants. The bound photon electric field induces dipoles in quantum dots and quantum emitters, and they interact with each other via the dipole-dipole interaction (DDI). We found that excited excitons decay spontaneously due to the interaction between excitons and the DDI field. It is found that the decay linewidth is enhanced when the bound photon energy is close to the exciton energy. On the other hand, we predicted that in the weak DDI coupling limit and when the bound photon energy is far away from the exciton energy the decay linewidths are suppressed (quenched). An analytical expression of the photoluminescence is found using the density-matrix method in the presence of the DDI coupling. We have predicted that in the strong DDI coupling limit the peak in the photoluminescence spectrum splits into two peaks when the bound photon energy is far away from the exciton energy. Furthermore, we have shown that when the bound photon energy is close to the exciton energy, two peaks merge into one peak. We have also compared our theory with PL experiments of a nanofiber doped with an ensemble of the CdSe QDs and sulforhodamine 101 dye molecules. A good agreement between theory and experiments is found. Our theory can be used by experimentalists to perform alternative types of experiments and for fabricating types of nanosensors and nanoswitches.

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

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

Effect of dipole–dipole interactions on the one-photon and two-photon photoluminescence in an ensemble of quantum dots doped in a polymer matrix

We have developed a theory for the one-photon and two-photon fluorescence of a nanohybrid composite comprised of an ensemble of quantum dots encased in a matrix of polymer. The quantum dots are comprised of four-level quantum emitters encased within a coating of dielectric material. The fluorescence of the quantum dot system is calculated using the master equation method where dipole–dipole interaction (DDI) coupling is present. The application of a probe field to the nanohybrid composite material in conjunction with the DDI between the quantum dots generates an enhancement of both the single-photon and double-photon fluorescence, and in certain conditions causes spectrum splitting. With a strong DDI field, the two-photon emission spectrum experiences slight quenching and shifts from one peak to two peaks. The dressed states are a result of strong coupling effects between the DDI field and the quantum dots. We generated an analytical expression for the fluorescence spectrum and intensity, and quantitatively compared it to experimental data in which ZnSe quantum dots were embedded within a P3HT polymer film, with our theory strongly correlating with the experiment. We also quantitatively compared our theory with experimental data that demonstrated the single-photon emission spectrum of colloidal quantum dots comprised of a CdZnS/ZnS gradient structure in varying probe field strengths. We found that our theory predicted the spectrum peak splitting that the experiment produced.

Acceleration techniques for semiclassical Maxwell–Bloch systems: An application to discrete quantum dot ensembles

The solution to Maxwell–Bloch systems using an integral-equation-based framework has proven effective at capturing collective features of laser-driven and radiation-coupled quantum dots, such as light localization and modifications of Rabi oscillations. Importantly, it enables observation of the dynamics of each quantum dot in large ensembles in a rigorous, error-controlled, and self-consistent way without resorting to spatial averaging. Indeed, this approach has demonstrated convergence in ensembles containing up to 104 interacting quantum dots (Glosser et al., 2017). Scaling beyond 104 quantum dots tests the limit of computational horsepower, however, due to the O(NtNs2) scaling (where Nt and Ns denote the number of temporal and spatial degrees of freedom). In this work, we present an algorithm that reduces the cost of analysis to O(NtNslog2Ns). While the foundations of this approach rely on well-known particle–particle/particle–mesh and adaptive integral methods, we add refinements specific to transient systems and systems with multiple spatial and temporal derivatives. Accordingly, we offer numerical results that validate the accuracy, effectiveness and utility of this approach in analyzing the dynamics of large ensembles of quantum dots.

Electron tunneling dynamics between two-dimensional and zero-dimensional quantum systems: Contributions of momentum matching, higher subbands, and phonon-assisted processes

We investigate tunneling dynamics of electrons from an ensemble of self-assembled InAs quantum dots into the subbands of a two-dimensional electron gas (2DEG). LO-phonon-assisted tunneling processes and tunneling into higher subbands of the 2DEG electronic structure cause distinct resonances in the evolution of the tunneling rate as a function of the energy detuning between quantum dot and 2DEG ground state. By devising a semiquantitative model, we identify the momentum mismatch between the quantum dot and 2DEG wave function as the crucial quantity governing the evolution of the tunneling rate. In particular, we demonstrate that this mechanism along with the availability of tunneling into the second 2DEG subband allows for tuning of the tunneling rate by more than two orders of magnitude.

Semiclassical modeling of coupled quantum-dot–cavity systems: From polaritonlike dynamics to Rabi oscillations

Semiconductor quantum dots in photonic cavities are strongly coupled light-matter systems with prospective applications in optoelectronic devices and quantum information processing. Here we present a theoretical study of the coupled exciton--light field dynamics of a planar quantum dot ensemble, treated as two-level systems, embedded in a photonic cavity modeled by Maxwell's equations. When excited by coupling an external short laser pulse into the cavity, we find an exciton-polariton-like behavior for weak excitation and Rabi oscillations for strong excitation with a sharp transition between these regimes. In the transition region we find highly non-linear dynamics involving high harmonics of the fundamental oscillation. We perform a numerical study based on the Finite-Difference-Time-Domain method for the solution of Maxwell's equations coupled to Bloch equations for the quantum dots and also derive an analytical model to describe the coupled cavity-quantum dot system, which allows us to describe the light field dynamics in terms of a Newton-like dynamics in an effective anharmonic potential. From the shape of this potential combined with the initial conditions the transition can be well understood. The model is then extended to a broadened ensemble of quantum dots. For weak excitation the polariton spectrum broadens and the lines slightly shift, however, the sharp transition to the Rabi oscillation regime is still present. Furthermore, we find a second, lower threshold with additional lines in the spectra which can be traced back to Rabi oscillations driven by the polariton modes. Our approach provides new insights in the dynamics of both quantum dot and light field in the photonic structure.

Resolution-enhanced imaging using interferenceless coded aperture correlation holography with sparse point response

Interferenceless coded aperture correlation holography (I-COACH) is a non-scanning, motionless, incoherent digital holography technique. In this study we use a special type of I-COACH in which its point spread hologram (PSH) is ensemble of sparse dots. With this PSH an imaging resolution beyond the classic diffraction limit is demonstrated. This resolution improvement is achieved due to the position of the coded aperture between the object and the lens-based imaging system. The coded aperture scatters part of the light, that otherwise is blocked by the system aperture, into the optical system, and by doing that, extends the effective numerical aperture of the system. The use of sparse PSH increases the signal-to-noise ratio of the entire imaging system. A lateral resolution enhancement by a factor of about 1.6 was noted in the case of I-COACH compared to direct imaging.