Ensemble Of Quantum Dots Research Articles

Effectiveness of AlGaAs barrier layers as a redistribution channel of photoexcited carriers on anomalous temperature dependence of photoluminescence properties of GaAs quantum dots

Photoluminescence (PL) measurements at a wide temperature range, up to room temperature, for high-quality and high-density GaAs/AlGaAs quantum dots (QDs) fabricated by droplet epitaxy were carried out to investigate the anomalous temperature dependence of the PL peak energy from the QD ensemble. In addition to a reported redshift that deviated from the so-called Varshni's curve of the PL peak energy in the low temperature region, a new blueshift was observed above 200 K. We analyzed the experimental results using a steady-state rate equation model and observed a good agreement. The distribution of the QD sizes and the presence of the AlGaAs barrier layer as a carrier coupling channel were considered in this model. This means that the wetting layer proposed thus far is not a necessary condition for explaining the anomalous temperature behavior of the PL properties. In addition, it was found that the anomalous temperature behavior was smeared out by the insertion of a GaAs height adjustment layer in order to homogenize the apparent QD size. We found that sufficient control of the QD size is a necessary factor for high temperature stability of QD devices.

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.

Efficient Quantum Dot Light‐Emitting Diodes Based on Well‐Type Thick‐Shell CdxZn1−xS/CdSe/CdyZn1−yS Quantum Dots

AbstractDevice grade quantum dots (QDs) require QDs ensembles to retain their original superior optical properties as in solution. QDs with thick shells are proven effective in suppressing the inter‐dot interaction and preserving the emission properties for QDs solids. However, lattice strain–induced defects may form as the shell grows thicker, resulting in a notable photoluminescence quenching. Herein, a well‐type CdxZn1−xS/CdSe/CdyZn1−yS QDs is proposed, where ternary alloys CdZnS are adopted to match the lattice parameter of intermediate CdSe by separately adjusting the x and y parameters. The resultant thick‐shell Cd0.5Zn0.5S/CdSe/Cd0.73Zn0.27S QDs reveal nonblinking properties with a high PL QY of 99% in solution and 87% in film. The optimized quantum dot light‐emitting diodes (QLEDs) exhibit a luminance of 31547.5 cd m−2 at the external quantum efficiency maximum of 21.2% under a bias of 4.0 V. The shell thickness shows great impact on the degradation of the devices. The T50 lifetime of the QLEDs with 11.2 nm QDs reaches 251 493 h, which is much higher than that of 6.5 and 8.4 nm QDs counterparts. The performances of the well‐type thick‐shell QLEDs are comparable to state‐of‐the‐art devices, suggesting that this type of QDs is a promising candidate for efficient optoelectronic devices.

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.

Resonant and Nonresonant Tunneling Injection Processes in Quantum Dot Optical Gain Media

Resonant and non-resonant tunneling injection of charge carriers in a quantum dot optical gain medium in the form of an optical amplifier operating in the telecom wavelength range were identified using multi wavelength pump probe measurements. Since the inhomogeneously broadened ensemble of quantum dots exhibits a very wide spectrum of transition energies, both tunneling injection processes take place simultaneously at different spectral locations and thus can be identified. These findings highlight the fact that at least one of the tunneling processes must occur at a wavelength at or near the gain peak, where laser oscillations take place. An energetic misalignment leads to reduced laser performance since in that case, the tunneling process becomes a carrier loss mechanism.

Photocatalytic properties of hybrid structures based on Titania nanoparticles and semiconductor quantum dots

Titania nanoparticles (NPs) demonstrate the highest photocatalytic activity among metal oxides nanoparticles. A combination of Titania nanoparticles with brightly luminescent semiconductor quantum dots (QDs) makes it possible to obtain highly efficient photocatalytic systems activated by visible light due to photoinduced electron transfer. Multilayered Titania NPs/QDs hybrid structures have been formed using Langmuir–Blodgett technique. Photoluminescent properties of the structures have been analyzed taking into account multiexponential decay of QDs’ photoluminescence and their nonuniform distribution in the structures. It has been shown that luminescent fractions of QDs are strong electron donors for Titania NPs with electron transfer rate $$k_{ET}\ge 4\cdot 10^{10}\, \hbox{s}^{-1}$$. It allows for electron transfer with 85% efficiency. At the same time, it was observed that average electron transfer efficiency in the structures is $$55\pm 5\%$$ because of presence of a dark fraction in QDs ensemble. Our results clearly demonstrate that Titania NPs/QDs hybrid structures are prospective generator of reactive oxygen species under visible light and reducing QDs’ dark fraction should increase average electron transfer efficiency in the structures.

Cooperative excitonic quantum ensemble in perovskite-assembly superlattice microcavities

Perovskites—compounds with the CaTiO3-type crystal structure—show outstanding performance in photovoltaics and multiparameter optical emitters due to their large oscillator strength, strong solar absorption, and excellent charge-transport properties. However, the ability to realize and control many-body quantum states in perovskites, which would extend their application from classical optoelectronic materials to ultrafast quantum operation, remains an open research topic. Here, we generate a cooperative quantum state of excitons in a quantum dot ensemble based on a lead halide perovskite, and we control the ultrafast radiation of excitonic quantum ensembles by introducing optical microcavites. The stimulated radiation of excitonic quantum ensemble in a superlattice microcavity is demonstrated to not be limited by the classical population-inversion condition, leading to a picosecond radiative duration time to dissipate all of the in-phase dipoles. Such a perovskite-assembly superlattice microcavity with a tunable radiation rate promises potential applications in ultrafast, photoelectric-compatible quantum processors.

Спиновая динамика отрицательно заряженных экситонов в квантовых точках InP/(In,Ga)P в магнитном поле

The dynamics of the photoluminescence negative circular polarization of the InP/(In,Ga)P quantum dots ensemble was studied. We find that in the time-resolved dependences of the polarization there are no oscillations in Voigt magnetic field. Also, with increasing field the polarization declines to zero. Such behavior is attributed to the peculiarities of the negatively charged exciton spin dynamics, particularly, to the fact that in the negatively charged exciton ground state the spin dynamics is governed by the heavy hole. We show that magnetic field depolarization of the photoluminescence occurs once the field of dynamically polarized nuclear spins acting on electron spins is surpassed.

Ultra-small aqueous glutathione-capped Ag-In-Se quantum dots: luminescence and vibrational properties.

We introduce a direct aqueous synthesis of luminescent 2–3 nm Ag–In–Se (AISe) quantum dots (QDs) capped by glutathione (GSH) complexes, where sodium selenosulfate Na2SeSO3 is used as a stable Se2− precursor. A series of size-selected AISe QDs with distinctly different positions of absorption and PL bands can be separated from the original QD ensembles by using anti-solvent-induced size-selective precipitation. The AISe–GSH QDs emit broadband PL with the band maximum varying from 1.65 eV (750 nm) to 1.90 eV (650 nm) depending on the average QD size and composition. The PL quantum yield varies strongly with basic synthesis parameters (ratios of constituents, Zn addition, duration of thermal treatment, etc.) reaching 4% for “core” AISe and 12% for “core/shell” AISe/ZnS QDs. The shape and position of PL bands is interpreted in terms of the model of radiative recombination of a self-trapped exciton. The AISe–GSH QDs reveal phonon Raman spectra characteristic for small and Ag-deficient tetragonal Ag–In–Se QDs. The ability of ultra-small AISe QDs to support such “bulk-like” vibrations can be used for future deeper insights into structural and optical properties of this relatively new sort of QDs.

Internal quantum efficiencies of AlGaN quantum dots grown by molecular beam epitaxy and emitting in the UVA to UVC ranges

AlyGa1−yN quantum dots (QDs) have been grown by molecular beam epitaxy on AlxGa1−xN (0001) using a 2-dimensional–3-dimensional growth mode transition that leads to the formation of QDs. QDs have been grown for Al compositions y varying between 10% and 40%. The influence of the active region design [composition y, QD height, and bandgap difference (ΔEg) between the AlxGa1−xN cladding layer and the AlyGa1−yN QDs] is discussed based on microscopy, continuous wave photoluminescence (PL), and time-resolved PL (TRPL) measurements. In particular, increasing y leads to a shift of the QD emission toward shorter wavelengths, allowing covering a spectral range in the UV from 332 nm (UVA) to 276 nm (UVC) at room temperature (RT). The low-temperature (LT) internal quantum efficiency of the QD ensembles was estimated from TRPL experiments at 8 K and values between 11% and 66% were deduced. The highest internal quantum efficiency (IQE)-LT is found for the QDs with higher Al content y. Then, the PL spectrally integrated intensity ratios between RT and LT were measured to estimate the IQE of the samples at RT. The PL ratio is higher for larger ΔEg, for QDs with y of 0.1 or 0.2, and high PL intensity ratios up to 30% were also measured for QDs with larger y of 0.3 and 0.4. RT IQE values between 5% and 20% are deduced for AlyGa1−yN QDs emitting in the 276–308 nm range.

Emission spectrum of broadband quantum dot superluminescent diodes

We present a microscopic theory of the amplified spontaneous emission of a spectrally broadband quantum dot superluminescent diode with tilted end facets within the quantum white noise limit. From this multimode quantum theory, we have the ability to obtain all orders of temporal correlation functions. In particular, we derive rate equations for the optical power densities, the level occupation of inhomogeneous ensemble of quantum dots within the diode, as well as the emitted optical spectra. As the main result, we find the external power spectrum as a convolution of the intra-diode photon spectrum with a Lorentzian response. Assuming a Gaussian light-matter coupling results in a similar shaped Gaussian output spectrum, which agrees very well with available experimental data.

Features of the Simultaneous Generation of Low-Q and High-Q Modes in Heterolasers Based on Quantum Dots with a Long Incoherent Relaxation Time of Optical Dipole Oscillations

The features of the multimode steady-state generation of superradiant heterolasers, in which quantum dots with a long incoherent relaxation time serve as an active medium, while low-Q combined Fabry–Perot cavities with a distributed feedback of counter-propagating waves serve as cavities, are studied. It is shown that due to the quantum-coherent dynamics of optical dipole oscillations and population inversion of the working levels in a quantum-dot ensemble with strong inhomogeneous broadening of the spectral line, the simultaneous lasing of modes with various degrees of phasing and/or correlation and with qualitatively different dynamic behavior, notably, quasi-stationary, metastable, self-modulation, pulse-periodic, and quasi-chaotic is possible.

Simultaneous Existence of Confined and Delocalized Vibrational Modes in Colloidal Quantum Dots

Coupling to phonon modes is a primary mechanism of excitonic dephasing and energy loss in semiconductors. However, low-energy phonons in colloidal quantum dots and their coupling to excitons are poorly understood because their experimental signatures are weak and usually obscured by the unavoidable inhomogeneous broadening of colloidal dot ensembles. We use multidimensional coherent spectroscopy at cryogenic temperatures to extract the homogeneous nonlinear optical response of excitons in a CdSe/CdZnS core/shell colloidal quantum dot ensemble. A comparison to the simulation provides evidence that the observed lineshapes arise from the coexistence of confined and delocalized vibrational modes, both of which couple strongly to excitons in CdSe/CdZnS colloidal quantum dots.

Spin-dependent directional emission from a quantum dot ensemble embedded in an asymmetric waveguide.

In this study, we examine a photonic wire waveguide embedded with an ensemble of quantum dots (QDs) that directionally emits into the waveguide depending on the spin state of the ensemble. The directional emission is facilitated by the spin-orbit interaction of light. The waveguide has a two-step stair-like cross section and QDs are embedded only in the upper step, such that the circular polarization of emission from the spin-polarized QDs controls the direction of the radiation. We numerically verify that more than 70% of the radiation from the ensemble emitter is toward a specific direction in the waveguide. We also examine a microdisk resonator with a stair-like edge, which supports selective coupling of the QD ensemble radiation into a whispering gallery mode that rotates unidirectionally. Our study provides a foundation for spin-dependent optoelectronic devices.

Optical properties demonstrating strong coupling of compactly arranged Ge quantum dots.

Coupled quantum dots (QDs) have been extensively investigated for their unique collective properties and potential applications in optoelectronic devices. Herein, dense Ge QDs with a compact arrangement in-plane are readily obtained. Systematic studies on power-dependent photoluminescence (PL) from the QD ensemble demonstrate a PL peak with a superior intensity, a constant peak energy and width as a function of the excitation power. Moreover, the temperature-dependent PL spectra exhibit a pronounced red-shift and a rapid PL quenching with increasing temperature. Such PL properties are attributed to the formation of miniband and the delocalization of holes in the QD ensemble due to the strong coupling between closely adjacent QDs.

A hot-carrier assisted InAs/AlGaAs quantum-dot intermediate-band solar cell

The intermediate-band assisted hot-carrier solar cell (IB-HCSC) concept has been proposed in order to assist the extraction of hot carriers from an absorber that has an intermediate band (IB). In this study, we used a heterostructure based on ten layers of In(Ga)As quantum dots (QDs) embedded in an Al0.2Ga0.8As single-junction solar cell designed for an intermediate band solar cell (IBSC). QD-IBSCs have proven to be limited by the thermal escape of photo-carriers from QDs at room temperature. In such cases, fundamental improvements in conversion efficiency are only possible if carriers in the IB are not in thermal equilibrium with either conduction or valence bands. Additionally, the IB-HCSC concept provides a high-efficiency limit and enables us to work with all relaxation mechanisms (thermalization, carrier-carrier scattering, and thermal radiation). Under high irradiation, we confirmed the emergence of a hot carrier population in the QDs, limited mostly by thermionic emission, which assists the IBSC by providing a thermoelectric gain in voltage without hindering the possibility of sequential two-photon absorption. Absolute intensity calibrated photoluminescence spectroscopy indicated that the triggering mechanism happens when the QD ensemble is estimated to have a high carrier concentration that behaves as a metal-like IB. Experimental results suggested that the hot carrier effect also occurred in other solar cells based on quantum heterostructures and directions for improvements of hot-carrier assisted QD-IBSCs are proposed for further efficiency gain in optimized device architectures.

Inhomogeneous Broadening of the Exciton Band in Optical Absorption Spectra of InP/ZnS Nanocrystals.

In this work, we have simulated the processes of broadening the first exciton band in optical absorption spectra (OA) for InP/ZnS ensembles of colloidal quantum dots (QDs). A phenomenological model has been proposed that takes into account the effects of the exciton–phonon interaction, and allows one to analyze the influence of the static and dynamic types of atomic disorder on the temperature changes in the spectral characteristics in question. To vary the degree of static disorder in the model system, we have used a parameter δ, which characterizes the QD dispersion in size over the ensemble. We have also calculated the temperature shifts of the maxima and changes in the half-width for the exciton peaks in single nanocrystals (δ = 0), as well as for the integrated OA bands in the QD ensembles with different values of δ = 0.6–17%. The simulation results and the OA spectra data measured for InP/ZnS nanocrystals of 2.1 nm (δ = 11.1%) and 2.3 nm (δ = 17.3%), are in good mutual agreement in the temperature range of 6.5 K–RT. It has been shown that the contribution of static disorder to the observed inhomogeneous broadening of the OA bands for the QDs at room temperature exceeds 90%. The computational experiments performed indicate that the temperature shift of the maximum for the integrated OA band coincides with that for the exciton peak in a single nanocrystal. In this case, a reliable estimate of the parameters of the fundamental exciton–phonon interaction can be made. Simultaneously, the values of the specified parameters, calculated from the temperature broadening of the OA spectra, can be significantly different from the true ones due to the effects of static atomic disorder in real QD ensembles.

A tunable external cavity laser operating at excited states of bimodal-sized quantum-dot

In this paper, a tunable external cavity diode laser (ECDL) based on the first excited states of two independent quantum dot (QD) ensemble has been demonstrated. The device exhibits stabilized lasing with high-power of 120 mW. We attribute the excellent output characteristics of the QD ECDL to carrier radiation recombination in high energy states, which has higher degeneracy than low energy states and can accommodate more carriers. The linewidth of ECDL is less than 0.2 nm, which is also much narrower than most of QD ECDLs as reported. Preliminary discussion attributes the spectra feature to the lower QD density and independent carrier transitions. The tunable range is 28.9 nm (970.1–999 nm) and the wavelength shift with the injection current is 0.7 nm A−1.

Photoluminescence of Ag-In-S/ZnS quantum dots: Excitation energy dependence and low-energy electronic structure

Cd-free I-III-VI group semiconductor quantum dots (QDs) like Ag-In-S and Cu-In-S show unstructured absorption spectra with a pronounced Urbach tail, rendering the determination of their band gap energy (Eg) and the energy structure of the exciton difficult. Additionally, the origin of the broad photoluminescence (PL) band with lifetimes of several hundred nanoseconds is still debated. This encouraged us to study the excitation energy dependence (EED) of the PL maxima, PL spectral band widths, quantum yields (QYs), and decay kinetics of AIS/ZnS QDs of different size, composition, and surface capping ligands. These results were then correlated with the second derivatives of the corresponding absorption spectra. The excellent match between the onset of changes in PL band position and spectral width with the minima found for the second derivatives of the absorption spectra underlines the potential of the EED approach for deriving Eg values of these ternary QDs from PL data. The PL QY is, however, independent of excitation energy in the energy range studied. From the EED of the PL features of the AIS/ZnS QDs we could also derive a mechanism of the formation of the low-energy electronic structure. This was additionally confirmed by a comparison of the EED of PL data of as-synthesized and size-selected QD ensembles and the comparison of these PL data with PL spectra of single QDs. These results indicate a strong contribution of intrinsic inhomogeneous PL broadening to the overall emission features of AIS/ZnS QDs originating from radiative transitions from a set of energy states of defects localized at different positions within the quantum dot volume, in addition to contributions from dimensional and chemical broadening. This mechanism was confirmed by numerically modelling the absorption and PL energies with a simple mass approximation for spherical QDs and a modified donor-acceptor model, thereby utilizing the advantages of previously proposed PL mechanisms of ternary QDs. These findings will pave the road to a deeper understanding of the nature of PL in quantum confined I-III-VI group semiconductor nanomaterials.