Transitions In Quantum Dots Research Articles

Orbital angular momentum light interacted with double quantum dot-metal nanoparticle hybrid structure under spontaneous coherence

This work studies the generation of the orbital angular momentum (OAM) beam in the double quantum dot-metal nanoparticle (DQD-MNP) system under the application of the OAM beam. First, an analytical model is derived to attain the relations of probe and generated fields as a distance function in the DQD-MNP system under OAM applied field and spontaneously generated coherence (SGC) components. The calculation here is of material property; it differs from others by calculating energy states of the DQDs and the computation of the transition momenta between quantum dot (QD)-QD and QD-wetting layer (WL) transitions. The orthogonalized plane wave (OPW) calculates QD-WL transitions and their momenta. The momentum calculation is essential to specify the Rabi frequency of the input field. Such characteristics are not used in earlier models. The results show that SGC is vital in increasing the generated field. The signal field generated in the DQD-MNP system doubles that from the DQD system alone. So, the DQD-MNP system is preferred to the DQD system. The generated field in the DQD-MNP for the strong coupling DQD-MNP system is higher than that for the weak coupling. Increasing the distance separating the DQD-MNP reduces the generated field. Higher OAM number reduce the generated field at a long distance in the device. The model is then extended to study the effect of incoherent pumping () and the relations are modified to cover this part. The results show that reduces the generated field. While the results that compare the weak and strong coupling appear for the first, others compare well to the literature.

A Study of Halide Ion Exchange-Induced Phase Transition in CsPbBr3 Perovskite Quantum Dots for Detecting Chlorinated Volatile Compounds.

The unique optical properties of perovskite quantum dots (PQDs), particularly the tunable photoluminescence (PL) across the visible spectrum, make them a promising tool for chlorinated detection. However, the correlation between the fluorescence emission shift behavior and the interface of phase transformation in PQDs has not been thoroughly explored. In this study, we synthesized CsPbBr3 PQDs via the hot-injection method and demonstrated their ability to detect chlorinated volatile compounds such as HCl and NaOCl through a halide exchange process between the PQDs' solid thin film and the chlorinated vapor phase. This exchange process, which occurs alongside chloride (Cl) and bromine (Br) ion exchange and halide atom rearrangement, leads to sequential structural changes: the initial CsPbBr3 cubic Pm3̅m phase transitions to the CsPb2BrxCl5-x tetragonal I4/mcm phase, which subsequently transforms into the CsPbBrxCl3-x orthorhombic Pnma phase. The detailed exploration of this proposed mechanism during chlorinated vapor detection with CsPbBr3 PQDs thin films, supported by X-ray diffraction (XRD) analysis and PL spectrum over time, revealed high sensitivity to HCl vapor. The limit of detection (LOD) for HCl vapor was determined to be 0.02 ppm in visual recognition and 0.005 ppm via PL spectra. Additionally, the LOD for NaOCl was established at 0.50 ppm, facilitated by the photolysis reaction accelerating the conversion of NaOCl to HCl vapor under UV light irradiation. These insights have enriched our understanding of the mechanisms involved and broadened the potential use of CsPbBr3 PQDs as PL detection probes for chloride ions.

Highly Efficient and Stable CsPbI3 Perovskite Quantum Dots Light-Emitting Diodes Through Synergistic Effect of Halide-Rich Modulation and Lattice Repair.

Currently, CsPbI3 quantum dots (QDs) based light-emitting diodes (LEDs) are not well suited for achieving high efficiency and operational stability due to the binary-precursor method and purification process, which often results in the nonstoichiometric ratio of Cs/Pb/I. This imbalance leads to amounts of iodine vacancies, inducing severe non-radiative recombination processes and phase transitions of QDs. Herein, red-emitting CsPbI3 QDs are reported with excellent optoelectronic properties and stability based on the synergistic effects of halide-rich modulation passivation and lattice repair. First, a ternary-precursor method is employed to better control the feed ratio of Cs/Pb/I and create a halide-rich environment. Second a solvent-free solid-liquid reaction employing a multifunctional guanidinium iodide (GAI) additive is used after purification to repair iodine vacancies and partially replace surface Cs atoms, thereby effectively modifying its tolerance factor. Additionally, this short-chain GA+ can be used as the surface ligand to improve the conductivity of the QDs and suppress trap-assisted non-radiative Auger recombination. Consequently, PeLEDs based on GAI-QDs exhibit a great maximum external quantum efficiency (EQE) of 27.1% and an operational half-lifetime (T50) of 1001.1min at an initial luminance of 100cdm-2.

Tailoring intraband transition via composition in self-doped Ag2SxSey alloy nanocrystals

This study demonstrates that intraband transitions in Ag2SxSey alloy colloidal quantum dots can be modulated by altering their composition. This capability expands material options, offering new prospects for infrared optoelectronic device design.

InAs/GaAs SK quantum dots stacking: Impact of spacer layer on optical properties

In this paper, we report the optical properties of vertically stacked multilayers of self-organized InAs/GaAs quantum dots QDs with different GaAs spacer layer thicknesses of 10 and 15 nm, using photoluminescence PL measurement. The 10 K PL spectrum exhibits double-emission peaks ambiguously identified in PL spectra where the excitation power dependence reveals that these emission peaks are attributed to fundamental ground state GS of large quantum dots in the low energy side, and first excited state 1ES transitions of large quantum dots in coincidence with fundamental ground state GS of smaller ones, in the high energy side. Both the excitation power dependence and the temperature dependence measurement results exhibited competition between the latter's optical transitions in the QDs, which becomes more prevalent at higher excitation powers. The main PL peak is quenched above 190 K, giving rise to “a 1D miniband”, ascribed to the electronic coupling between QDs along the stacking direction and tuning the emission wavelength around 1.3 μm.Our results emphasize the Key role of the vertically stacked InAs/GaAs QDs structures with thin GaAs spacer layers in optimizing design for long-wavelength devices.

Various nonlinear transitions of a self-assembled quantum dot (SA-QD) semiconductor laser subject to negative optoelectronic feedback

Various nonlinear transitions of a self-assembled quantum dot (SA-QD) semiconductor laser subject to negative optoelectronic feedback

Fluorescence Enhancement of CdS:Ag Quantum Dots Co-Assembled with Au Nanoparticles in a Hollow Nanosphere Form.

Colloidal quantum dots (QDs) have exceptional fluorescence properties. Overcoming aggregation-induced quenching and enhancing the fluorescence of colloidal QDs have remained a challenging issue in this field. In this study, composite hollow nanospheres composed of Au nanoparticles (NPs) and CdS:Ag-doped QDs were successfully constructed through controlled microemulsion-based cooperative assembly. This method harnessed the localized surface plasmon resonance (LSPR) effect of Au NPs nearby doped QDs, resulting in enhanced doped QD fluorescence and the observation of the Purcell effect. The composite hollow nanospheres show a fluorescence enhancement compared to that of the pure CdS:Ag QDs. The enhanced fluorescence was demonstrated to come from the synergetic enhancement of the absorption and emission transition of the doped QDs. This approach provides a feasible technological pathway to address the challenge of improving the fluorescence performance of the doped QDs.

Influence of external magnetic field on intraband transitions in lens-shaped quantum dot

Intraband linear and nonlinear optical absorption in a strongly oblate lens-shaped Ge/Si quantum dot in the presence of an axial magnetic field was theoretically studied. Quantum transitions are considered in the heavy hole subband, when the scalar effective mass approximation is correct. The linear and nonlinear absorption coefficients, refractive index changes and the second harmonic generation coefficient were determined. The influence of the effects of temperature, size quantization and magnetic field on the behavior of the above parameters was revealed.

Multimodule microwave assembly for fast readout and charge-noise characterization of silicon quantum dots

Fast measurements of quantum devices is useful in areas such as quantum sensing, quantum computing, and nanodevice quality analysis. Here, we develop a superconductor-semiconductor multimodule microwave assembly to demonstrate charge-state readout at the state of the art. The assembly consist of a superconducting readout resonator interfaced to a silicon-on-insulator (SOI) chiplet containing quantum dots (QDs) in a high-κ nanowire transistor. The superconducting chiplet contains resonant and coupling elements as well as LC filters that, when interfaced with the silicon chip, result in a resonant frequency f=2.12 GHz, a loaded quality factor Q=850, and a resonator impedance Z=470Ω. Combined with the large gate lever arms of SOI technology, we achieve a minimum integration time for single and double QD transitions of 2.77 and 13.5 ns, respectively. We utilize the assembly to measure charge noise over 9 decades of frequency up to 500 kHz and find a 1/f dependence across the whole frequency spectrum as well as a charge-noise level of 4μeV/Hz at 1 Hz. The modular microwave circuitry presented here can be directly utilized in conjunction with other quantum devices to improve the readout performance as well as enable large bandwidth noise spectroscopy, all without the complexity of superconductor-semiconductor monolithic fabrication. Published by the American Physical Society 2024

Polarization-Selective Enhancement of Telecom Wavelength Quantum Dot Transitions in an Elliptical Bullseye Resonator.

Semiconductor quantum dots are promising candidates for the generation of nonclassical light. Coupling a quantum dot to a device capable of providing polarization-selective enhancement of optical transitions is highly beneficial for advanced functionalities, such as efficient resonant driving schemes or applications based on optical cyclicity. Here, we demonstrate broadband polarization-selective enhancement by coupling a quantum dot emitting in the telecom O-band to an elliptical bullseye resonator. We report bright single-photon emission with a degree of linear polarization of 96%, Purcell factor of 3.9 ± 0.6, and count rates up to 3 MHz. Furthermore, we present a measurement of two-photon interference without any external polarization filtering. Finally, we demonstrate compatibility with compact Stirling cryocoolers by operating the device at temperatures up to 40 K. These results represent an important step toward practical integration of optimal quantum dot photon sources in deployment-ready setups.

Reversible hydrochromic CsPbBr3/Cs4PbBr6@MCM-41 to enable organic solvent polarity detection

Hydrochromic materials have received much attention in anticounterfeiting due to their reversible photoemission properties toward water. However, existing hydrochromic materials usually suffer from slow hydrochromic response and limited stability, repeatability, and color tunability. This paper found for the first time that CsPbBr3/Cs4PbBr6 composites with quantum-limited domain effects had reversible hydrochromism. CsPbBr3/Cs4PbBr6 composites were successfully synthesized in molecular sieve MCM-41 using an improved mechanochemical method. The composites underwent a reversible phase transition from Cs4PbBr6/CsPbBr3 to CsPbBr3 upon treatment with a trace amount of water, the photoluminescence colors underwent a change from blue to green, and reversion occurred due to heating. Mechanistic investigation revealed that quantum dots underwent soft agglomeration due to van der Waals forces in polar solvent environments, affecting the action of quantum domain-limiting effects. Therefore, organic solvents with different polarities can cause different degrees of reversible discoloration of the material. This study provides new ideas for understanding the phase transition and agglomeration of metal halide perovskite quantum dots in polar solvent environments. Additionally, it offered new options for aqueous color-changing materials and organic solvent polar detection materials.

Enhancement of Spontaneous Photon Emission in Inverse Photoemission Transitions in Semiconductor Quantum Dots.

Inverse photoemission (IPE) is a radiative electron capture process where an electron is transiently captured in the conduction band (CB) followed by intraband de-excitation and spontaneous photon emission. IPE in quantum dots (QDs) bypasses optical selection rules for populating the CB and provides insights into the capacity for electron capture in the CB, the propensity for spontaneous photon emission, intraband transition energies where both initial and final states are in the CB, and the generation of photons with frequencies lower than the bandgap. Here, we demonstrate using time-dependent perturbation theory that judicious application of electric fields can significantly enhance the IPE transition in QDs. For a series of CdSe, CdS, PbSe, and PbS QDs, we present evidence of field-induced enhancement of IPE intensities (188% for Cd54Se54), field-dependent control of emitted photon frequencies (Δω = 0.73 eV for Cd54Se54), and enhancement of light-matter interaction using directed Stark fields (103% for Cd54Se54).

Symmetry breaking via alloy disorder to explain radiative Auger transitions in self-assembled quantum dots

Symmetry breaking via alloy disorder to explain radiative Auger transitions in self-assembled quantum dots

On the prospect of identifying visible emissions from optical measurements of self-assembled QDs

The work presented here focuses on the optical properties of molecular beam epitaxy-grown self-organized InAs quantum dots (QDs). Under low excitation power densities, the photoluminescence (PL) spectrum of the structure under study exhibits a double-peak profile, which has been related to the optical emission from the dots. To properly argue for or against explaining a high energy peak as being the result of different-sized QDs or excited state emissions, a comparative study of the optical properties of a set of two grown QD samples was employed. The laser power dependence of the linearity of the PL intensities of the QD transitions was shown to be very useful in determining the possible origins of the PL sub-bands detectable in a double-peak emission from an ensemble of self-organized QDs. Indeed, the change in the linearity of the dot-luminescence intensity with the increase of the excitation density could give helpful information about the active confinement states from which the dots are emitting. On the other hand, despite the use of very low excitation power densities, the optical measurements of the studied structure have revealed an appearance of a PL signal relating to the radiative emission from the first excited state of the dots. A random pathway of carrier capture by the intrinsic levels of the QDs is thought to be the most likely cause of this aspect.

Controllable Black-to-Yellow Phase Transition by Tuning the Lattice Symmetry in Perovskite Quantum Dots.

The black-to-yellow phase transition in perovskite quantum dots (QDs) is more complex than in bulk perovskites, regarding the role of surface energy. Here, with the assistance of in situ grazing-incidence wide-angle and small-angle X-ray scattering (GIWAXS/GISAXS), distinct phase behaviors of cesium lead iodide(CsPbI3 ) QDfilms under two different temperature profiles-instant heating-up (IHU) and slow heating-up (SHU) is investigated. The IHU process can cause the phase transition from black phase to yellow phase, while under the SHU process, the majority remains in black phase. Detailed studies and structural refinement analysis reveal that the phase transition is triggered by the removal of surface ligands, which switches the energy landscape. The lattice symmetry determines the transition rate and the coexistence black-to-yellow phase ratio. The SHU process allows longer relaxation time for a more ordered QD packing, which helps sustain the lattice symmetry and stabilizes the black phase. Therefore, one can use the lattice symmetry as a general index to monitor the CsPbI3 QD phase transition and finetune the coexistence black-to-yellow phase ratio for niche applications.

Quantum dot transition rate modifying by coupling to lattice plasmon

Quantum dot transition rate modifying by coupling to lattice plasmon

Two-state lasing in a quantum dot racetrack microlaser.

The peculiarities of two-state lasing in a racetrack microlaser with an InAs/GaAs quantum dot active region are investigated by measuring the electroluminescence spectra at various injection currents and temperatures. Unlike edge-emitting and microdisk lasers, where two-state lasing involves the ground and first excited-state optical transitions of quantum dots, in racetrack microlasers, we observe lasing through the ground and second excited states. As a result, the spectral separation between lasing bands is doubled to more than 150 nm. A temperature dependence of threshold currents for lasing via ground and second excited states of quantum dots was also obtained.

Photoreduced graphene oxide recovers graphene hot electron cooling dynamics

Reduced graphene oxide (rGO) is a bulk-processable quasi-amorphous 2D material with broad spectral coverage and fast electronic response. rGO sheets are suspended in a polymer matrix and sequentially photoreduced while measuring the evolving optical spectra and ultrafast electron relaxation dynamics. Photoreduced rGO yields optical absorption spectra that fit with the same Fano lineshape parameters as monolayer graphene. With increasing photoreduction time, rGO transient absorption kinetics accelerate monotonically, reaching an optimal point that matches the hot electron cooling in graphene. All stages of rGO ultrafast kinetics are simulated with a hot-electron cooling model mediated by disorder-assisted supercollisions. While the rGO room temperature 0.31 ps$^{-1}$ electronic cooling rate matches monolayer graphene, subsequent photoreduction can rapidly increase the rate by ~10-12$\times$. Such accelerated supercollision rates imply a reduced mean-free scattering length caused by photoionized point-defects on the rGO sp$^2$ sub-lattice. For visible range excitations of rGO, photoreduction shows three increasing spectral peaks that match graphene quantum dot (GQD) transitions, while a broad peak from oxygenated defect edge states shrinks. These three confined GQD states donate their hot carriers to the graphene sub-lattice with a 0.17 ps rise-time that accelerates with photoreduction. Collectively, many desirable photophysical properties of 2D graphene are replicated through selectively reducing rGO scaffolded within a 3D bulk polymeric network.

Distributed Bragg Reflector–Mediated Excitation of InAs/InP Quantum Dots Emitting in the Telecom C‐Band

Herein, it is demonstrated that optical excitation of InAs quantum dots (QDs) embedded directly in an InP matrix can be mediated via states in a quaternary compound constituting an InP/InGaAlAs bottom distributed Bragg reflector (DBR) and native defects in the InP matrix. It does not only change the carrier relaxation in the structure but could also lead to the imbalanced occupation of QDs with charge carriers, because the band structure favors the transfer of holes. Thermal activation of carrier transfer can be observed as an increase in the emission intensity versus temperature for excitation powers below saturation on the level of both an inhomogeneously broadened QD ensemble and single QD transitions. That increase in the QD emission is accompanied by a decrease in the emission from the InGaAlAs layer at low temperatures. Finally, carrier transfer between the InGaAlAs layer of the DBR and the InAs/InP QDs is directly proven by the photoluminescence excitation spectrum of the QD ensemble. The reported carrier transfer can increase the relaxation time of carriers into the QDs and thus be detrimental to the coherence properties of single and entangled photons. It is important to take it into account while designing QD‐based devices.

Narrow homogeneous linewidths and slow cooling dynamics across infrared intra-band transitions in n-doped HgSe colloidal quantum dots.

Intra-band transitions in colloidal quantum dots (QDs) are promising for opto-electronic applications in the mid-IR spectral region. However, such intra-band transitions are typically very broad and spectrally overlapping, making the study of individual excited states and their ultrafast dynamics very challenging. Here, we present the first full spectrum two-dimensional continuum infrared (2D CIR) spectroscopy study of intrinsically n-doped HgSe QDs, which exhibit mid-infrared intra-band transitions in their ground state. The obtained 2D CIR spectra reveal that underneath the broad absorption line shape of ∼500cm-1, the transitions exhibit surprisingly narrow intrinsic linewidths with a homogeneous broadening of 175-250cm-1. Furthermore, the 2D IR spectra are remarkably invariant, with no signof spectral diffusion dynamics at waiting times up to 50ps. Accordingly, we attribute the large static inhomogeneous broadening to the distribution of size and doping level of the QDs. In addition, the two higher-lying P-states of the QDs can be clearly identified in the 2D IR spectra along the diagonal with a cross-peak. However, there is no indication of cross-peak dynamics indicating that, despite the strong spin-orbit coupling in HgSe, transitions between the P-states must be longer than our maximum waiting time of 50ps. This study illustrates a new frontier of 2D IR spectroscopy enabling the study of intra-band carrier dynamics in nanocrystalline materials across the entire mid-infrared spectrum.