Dot States Research Articles

Electrical control of spins and giant g-factors in ring-like coupled quantum dots

Emerging theoretical concepts for quantum technologies have driven a continuous search for structures where a quantum state, such as spin, can be manipulated efficiently. Central to many concepts is the ability to control a system by electric and magnetic fields, relying on strong spin-orbit interaction and a large g-factor. Here, we present a mechanism for spin and orbital manipulation using small electric and magnetic fields. By hybridizing specific quantum dot states at two points inside InAs nanowires, nearly perfect quantum rings form. Large and highly anisotropic effective g-factors are observed, explained by a strong orbital contribution. Importantly, we find that the orbital contributions can be efficiently quenched by simply detuning the individual quantum dot levels with an electric field. In this way, we demonstrate not only control of the effective g-factor from 80 to almost 0 for the same charge state, but also electrostatic change of the ground state spin.

Prediction of the spin triplet two-electron quantum dots in Si: Towards controlled quantum simulations of magnetic systems

Ground state of two-electron quantum dots in single-valley materials like GaAs is always a spin singlet regardless of what the potential and interactions are. This statement cannot be generalized to the multi-valley materials like $n$-doped Si. Here we calculate numerically the spectrum of a two-electron Si quantum dot and show that the dot with the lateral size of several nm can have the spin triplet ground state which is impossible in the single-valley materials. Predicted singlet-triplet level crossing in two-electron Si quantum dots can potentially establish the platform for quantum simulation of magnetic many body systems based on quantum dots. We suggest several examples of such systems that open a way to controlled quantum simulations within the condensed matter setting.

Quantum-enhanced tunable spin-valley dependent excitonic second harmonic generation in molybdenum disulfide quantum dots

Developing a robust system with a strong tunable nonlinear second harmonic generation (SHG) is desirable. In this work, we calculate SHG arising from the excitonic states of monolayer molybdenum disulfide quantum dots (MoS2 QDs) with a parabolic confinement potential within massive Dirac fermion model using the density matrix formalism. We theoretically demonstrate that MoS2 QDs can exhibit a giant and tunable Spin-Valley dependent excitonic SHG, which can be tuned from 0 to ∼107 pm V−1 and realized by biasing nanostructured gates or by position-dependent doping. Remarkably, the strength of SHG response is more than two orders of magnitude higher than that in monolayer MoS2. Furthermore, robust excitonic effects together with strong spin-valley coupling in monolayer transition metal dichalcogenides quantum dots (TMDC QDs), which are tunable depending on the strength of the quantum confinement, make them as a promising candidate for ultrathin nonlinear optical materials with large nonlinearities. We believe that this study could spark interest in the nonlinear optical properties of TMDC QDs and open up a variety of new avenues for versatile novel 2D nonlinear photonics and optoelectronic nanodevices.

Graphene Quantum Dots Band Structure Tuned by Size for Efficient Organic Solar Cells

The electronic states of graphene quantum dots (GQDs) can be tuned by varying the lateral size and edge structure, which further influence their optoelectronic properties and the applications. Herein, three kinds of GQDs with different lateral size are prepared by photon‐Fenton reaction and separated through gel column chromatography, and their effects on the photovoltaic performances of inverted organic solar cells based on the poly(3‐hexylthiophene) (P3HT) and poly(3‐hexylthiophene)/(6,6)‐phenyl‐C61 butyric acid methylester (PCBM) blend films are studied systematically. In comparison with P3HT:PCBM cells, the power conversion efficiency of the P3HT:PCBM solar cells containing 0.8% of GQDs‐blue, GQDs‐green, and GQDs‐orange can be increased from 3.06% to 3.54%, 4.43%, and 3.73% with a short‐circuit current density of 10.3, 13.34, and 11.19 mA cm−2. It is illustrated also that the band structures (electronic energy states) tuned mainly by their lateral size of GQDs are the crucial factor to dominate their photovoltaic preferences as additives in conventional P3HT:PCBM solar cells.

Tunable quantum dot lasers grown directly on silicon

Tunable semiconductor lasers are often listed in critical technology road maps for future dense-wavelength-division-multiplexing (DWDM) systems and high-performance computing systems, and they are increasingly demanded in long-haul, metropolitan, and access networks. The capability to produce such lasers directly on silicon (Si) could boost the use of Si photonics and facilitate the adoption of optical data transmission even at the chip scale. Moreover, just the use of Si as a cheap and large-diameter substrate for device production is very advantageous, as the fabrication can take advantage of the highly optimized processing techniques and economy of scale enabled by decades of development in Si microelectronics. Here, we report a tunable single-wavelength quantum dot (QD) laser directly grown on Si. The high carrier confinement and a real dot density of QDs provide reduced sensitivity to crystalline defects, which allows for exceptional lasing performance even in lattice-mismatched material systems. The discrete density of states of dots yields unique gain properties that show promise for improved device performance and new functionalities relative to the quantum well counterparts, including high temperature stability, low threshold operation, reduced sidewall recombination, and isolator-free stability. We implement a simple, integrable architecture to achieve over 45 dB side-mode-suppression-ratio without involving regrowth steps or subwavelength grating lithography. Under continuous-wave electrical injection at room temperature, we achieved a 16 nm tuning range with output powers of over 2.7 mW per tuning wavelength. This work represents a step towards using III–V/Si epitaxy to form efficient, easily manufacturable on-chip Si light sources for not only DWDM networks, but also spectroscopy, biosensors, and many other emerging applications.

Cover Feature: Manipulating the Optical Properties of Carbon Dots by Fine‐Tuning their Structural Features (ChemSusChem 19/2019)

The Cover Feature shows emission in oxidized and reduced carbon dots. The interaction between oxidized carbon dots (upper left) with UV light results in green emission whereas in the case of reduced carbon dots (lower right) mainly photo-assisted thermal relaxation is detected. Such different phenomena occurring within the exited states of hydrothermal carbon dots are controlled by different structural features, with oxidized carbon dots containing more oxygen functionalities and reduced carbon dots having mainly conjugated domains. More information can be found in the Full Paper by H. Luo et al. on page 4432 in Issue 19, 2019 (DOI: 10.1002/cssc.201901795).

Excitable interplay between lasing quantum dot states

The optically injected semiconductor laser system has proven to be an excellent source of experimental nonlinear dynamics, particularly regarding the generation of excitable pulses. Typically for low-injection strengths, these pulses are the result of a small above-threshold perturbation of a stable steady state, the underlying physics is well described by the Adler phase equation, and each laser intensity pulse is accompanied by a 2π phase rotation. In this article, we show how, with a dual-state quantum dot laser, a variation of type I excitability is possible that cannot be described by the Adler model. The laser is operated so that emission is from the excited state only. The ground state can be activated and phase locked to the master laser via optical injection while the excited state is completely suppressed. Close to the phase-locking boundary, a region of ground-state emission dropouts correlated to excited-state pulses can be observed. We show that the phase of the ground state undergoes bounded rotations due to interactions with the excited state. We analyze the system both experimentally and numerically and find excellent agreement. Particular attention is devoted to the bifurcation conditions needed for an excitable pulse as well as its time evolution.

Emission-Frequency Separated High Quality Single-Photon Sources Enabled by Phonons.

We demonstrate theoretically that the single-photon purity of photons emitted from a quantum dot exciton prepared by phonon-assisted off-resonant excitation can be significantly higher in a wide range of parameters than that obtained by resonant preparation for otherwise identical conditions. Despite the off-resonant excitation, the brightness stays on a high level. These surprising findings exploit the fact that the phonon-assisted preparation is a two-step process where phonons first lead to a relaxation between laser-dressed states while high exciton occupations are reached only with a delay to the laser pulse maximum by adiabatically undressing the dot states. Due to this delay, possible subsequent processes, in particular multiphoton excitations, appear at a time when the laser pulse is almost gone. The resulting suppression of reexcitation processes increases the single-photon purity. Due to the spectral separation of the signal photons from the laser frequencies this enables the emission of high quality single photons not disturbed by a laser background while taking advantage of the robustness of the phonon assisted scheme.

Ultrahigh Gain from Plasmonic Quantum Dot Nanolaser

This work studies the gain from quantum dot plasmonic nanolaser. A metal/semiconductor/metal structure was considered to attain plasmonic nanocavity with active region contains: quantum dot, wetting layer and barrier layers. Band alignment between layers was used to predict their parameters. Momentum matrix element for transverse magnetic mode in quantum dot structure was formulated. Waveguide Fermi energy was introduced and formulated, for the first time, in this work to cover the waveguide contribution (Ag metal layer) in addition to the active region. The gain obtained here overcomes the electron scattering losses which promises in high gain, high power and high speed applications. The waveguide Fermi energy goes deep in the valence band which explains the high gain, where it is shown that covering the structure by a metal makes valence band quantum dot states fully occupied which refers to an efficient hole contribution.

Single-particle energy – and optical absorption – spectra of multilayer graphene quantum dots

The energy spectra of quantum dots formed by stacked disks of graphene are calculated as a function of the disk shape and size, the number of disk layers and the edge type (zigzag /armchair) within a tight-binding approach in the single-particle picture. Stacks formed by up to seven disks of hexagonal, rhombic and triangular shape are considered. For both edge types, we find a monotonously decreasing energy gap as the number of stacked disks or the disk size is increased, with the exception of triangular dots with zigzag edges, which have a metallic character. In some cases, simple scaling relations for the energy gaps can be formulated. Furthermore, the optical absorption spectra within the dipole approximation are calculated. The results are interpreted with the help of selected HOMO wave functions that form inside the stacks. • The densities of states of multilayer graphene quantum dots of different shapes (GQDs) are calculated. • Graphene quantum dots with up to seven layers are considered. • The corresponding optical absorption spectra are computed. • Zigzag and armchair multilayer dots show strong differences in the energy gaps. • The limit of infinitely many layers is computed for both types of edges.

Discrete energy states induced broadband emission from self-assembly InGaN quantum dots

Light-emission contributed from discrete energy levels of self-assembly InGaN quantum dots is explored to fabricate broadband light-emitting diodes (LEDs). A series of InGaN quantum dots samples are self-assembly grown on sapphire substrate by metal-organic vapor phase epitaxy. Emission from the discrete energy states of the InGaN quantum dots is detected by micro-zone photoluminescence under cryogenic temperature. The peaks assignments are also verified by theoretical simulation. A green LED incorporating InGaN multi-quantum dots as active medium is fabricated, demonstrating a broader emission than the referred multi-quantum wells LED. It suggests quantum dots be of great potential to improve the color-rendering index of white LEDs.

Quantum dot state initialization by control of tunneling rates

We study the loading of electrons into a quantum dot with dynamically controlled tunnel barriers. We introduce a method to measure tunneling rates for individual discrete states and to identify their relaxation paths. Exponential selectivity of the tunnel coupling enables loading into specific quantum dot states by tuning independently energy and rates. While for the single-electron case orbital relaxation leads to fast transition into the ground state, for electron pairs triplet-to-singlet relaxation is suppressed by long spin-flip times. This enables the fast gate-controlled initialization of either a singlet or a triplet electron pair state in a quantum dot with broad potential applications in quantum technologies.

Multiphoton Excitation of CsPbBr3 Perovskite Quantum Dots (PQDs): How Many Electrons Can One PQD Donate to Multiple Molecular Acceptors?

Metastable multiexcitonic states (MESs) of semiconductor quantum dots can be involved in multielectron transfer reactions, which opens new perspectives in nanomaterials-based optoelectronic applications. Herein, we demonstrate the generation of a MES in CsPbBr3 perovskite quantum dots (PQDs) and its dissociation dynamics through multiple electron transfers to molecular electron acceptors, anthraquinones (AQs), bound to the PQD surface by a carboxylic anchor. As many as 14 excitons are produced at an excitation density of roughly 220 μJ cm–2 without detectable PQD degradation. Addition of AQ results in the formation of PQD–AQ hybrids with excess of AQs (PQD:AQ ≈ 1:20), which opens the possibility of multielectron transfer acts from MES to AQs. We found that the electron transfer saturates after roughly five transfer acts and that the first electron transfer (ET) time constant is as short as 1 ps. However, each ET increases the Coulomb potential barrier for the next ET, which decreases the rate of ET, resulting in a saturation after five ETs.

Effect of magnetic field and impurities in InAs/GaAs and GaN/AlN self-assembled quantum dots

A theoeritical study on the effect of a magnetic field or impurities on the carries states of self-assembled quantum dots is presented. The magnetic field is applied along the growth direction of the dots, and for comparison two systems are considered, InAs embeded in GaAs, and GaN in AlN. The electronic states and energy are calculated in the framework of the k.p theory in 8 bands including the strain and piezoelectric effects. Zeeman splitting and anticrossings are observed in InAs/GaAs, while the field introduces small changes in the nitrides. It is also included a study about hidrogen-like impurities, which may be negative or positive. It is noted that depending on the type of impurity, the confinement energy of carriers is changed, and the distribution of the probability density of the carriers is affected too.

Fidelity benchmarks for two-qubit gates in silicon.

Universal quantum computation will require qubit technology based on a scalable platform1, together with quantum error correction protocols that place strict limits on the maximum infidelities for one- and two-qubit gate operations2,3. Although various qubit systems have shown high fidelities at the one-qubit level4-10, the only solid-state qubits manufactured using standard lithographic techniques that have demonstrated two-qubit fidelities near the fault-tolerance threshold6 have been in superconductor systems. Silicon-based quantum dot qubits are also amenable to large-scale fabrication and can achieve high single-qubit gate fidelities (exceeding 99.9 per cent) using isotopically enriched silicon11,12. Two-qubit gates have now been demonstrated in a number of systems13-15, but as yet an accurate assessment of their fidelities using Clifford-based randomized benchmarking, which uses sequences of randomly chosen gates to measure the error, has not been achieved. Here, for qubits encoded on the electron spin states of gate-defined quantum dots, we demonstrate Bell state tomography with fidelities ranging from 80 to 89 per cent, and two-qubit randomized benchmarking with an average Clifford gate fidelity of 94.7 per cent and anaverage controlled-rotation fidelity of 98 per cent. These fidelities are found to be limited by the relatively long gate times used here compared with the decoherence times of the qubits. Silicon qubit designs employing fast gate operations with high Rabi frequencies16,17, together with advanced pulsing techniques18, should therefore enable much higher fidelities in the near future.

Temperature-Dependence of Solvent-Induced Stokes Shift and Fluorescence Tunability in Carbon Nanodots

We carried out a cryogenic investigation on the optical properties of carbon dots, aiming to better understand their emission mechanism and the role of the solvent. The solvatochromic Stokes shift is quantified by a low temperature approach which allows freezing of the photo-excited state of carbon dots, preventing any solvation relaxation. Moreover, the reduction in temperature helps to identify the dynamical inhomogeneous contribution to the broadening of the emission band; therefore, disentangling the role of solvent from other types of broadening, such as the homogeneous and the static inhomogeneous contributions.

Engineering Auger recombination in colloidal quantum dots via dielectric screening

Auger recombination is the main non-radiative decay pathway for multi-carrier states of colloidal quantum dots, which affects performance of most of their optical and optoelectronic applications. Outstanding single-exciton properties of CdSe/CdS core/shell quantum dots enable us to simultaneously study the two basic types of Auger recombination channels—negative trion and positive trion channels. Though Auger rates of positive trion are regarded to be much faster than that of negative trion for II-VI quantum dots in literature, our experiments find the two rates can be inverted for certain core/shell geometries. This is confirmed by theoretical calculations as a result of geometry-dependent dielectric screening. By varying the core/shell geometry, both types of Auger rates can be independently tuned for ~ 1 order of magnitude. Experimental and theoretical findings shed new light on designing quantum dots with necessary Auger recombination characteristics for high-power light-emitting-diodes, lasers, single-molecular tracking, super-resolution microscope, and advanced quantum light sources.

Dynamics of Broadband Lasing Cascade from a Single Dot-in-well InGaAs Microdisk

The development of a fast semiconductor laser is required for the realization of next-generation telecommunication applications. Since lasers operating on quantum dot ground state transitions exhibit only limited gain due to the saturation effect, we investigate lasing from excited states and compare its corresponding static and dynamic behavior to the one from the ground state. InAs quantum dots (QDs) grown in dot-in-well (DWELL) structures allowed to obtain light emission from ground and three excited states in a spectral range of 1.0–1.3 μm. This emission was coupled to whispering gallery modes (WGMs) of a 6 μm microdisk resonator and studied at room temperature by steady-state and time-resolved micro-photoluminescence. We demonstrate a cascade development of lasing arising from the ladder of quantum dot states, and compare the lasing behavior of ground and excited state emission. While the lasing threshold is being increased from the ground state to the highest excited state, the dynamic behavior is improved: turn-on times and lifetimes of WGMs become shorter paving the way towards high frequency direct driven microlasers.

A Review of High-Performance Quantum Dot Lasers on Silicon

Laser gain regions using quantum dots have numerous improvements over quantum wells for photonic integration. Their atom-like density of states gives them unique gain properties that can be finely tuned by changing growth conditions. The gain bandwidth can be engineered to be broad or narrow and to emit at a wide range of wavelengths throughout the near infrared. The large energy level separation of the dot states from the surrounding material results in excellent high-temperature performance and gain recovery at sub-picosecond timescales. The fact that the quantum dots are isolated from each other and act independently at inhomogeneously broadened wavelengths results in ultralow linewidth enhancement factors, highly stable broadband mode-locked lasers, single-section mode locking, and the possibility of reduced crosstalk between amplified signals at low signal injection and enhanced four-wave mixing at high signal injection. The high carrier confinement and areal dot density provide reduced sensitivity to crystalline defects allowing for long device lifetimes even when epitaxially grown on silicon at high dislocation densities.

Modeling optical coupling of plasmons and inhomogeneously broadened emitters.

Optically coupling quantum emitters to nanoparticles provides the foundation for many plasmonic applications. Including quantum mechanical effects within the calculations can be crucial for designing new devices, but classical approximations are sometimes sufficient. Comprehending how the classical and quantum mechanical descriptions of quantum emitters alter their calculated optical response will lead to a better understanding of how to design devices. Here, we describe how the semiclassical Maxwell-Liouville method can be used to calculate the optical response from inhomogeneously broadened states. After describing the Maxwell-Liouville algorithm, we use the method to study the photon echoes from quantum dots and compare the results against analytical models. We then modify the quantum dot's state distribution to match a PbS 850 nm quantum dot's absorption spectra to see how the complete quasi-band structure affects their coupling to gold nanoislands. Finally, we compare the results with previously published work to demonstrate where the complete quantum dot description is necessary.