Excited Quantum Dots Research Articles

3D-printed copolymer semi-cavity sensor with inner optical filter-effecting quantum dots photoluminescence for multiple drugs and amino acids detection

Here, we demonstrate a prototype for a novel optical sensor based on 3D-printed copolymer semi-cavity embedded with colloidal ZnCuInS (ZCIS) quantum dots (QDs) and utilizing the optical filter-effect in the QDs photoluminescence excitation pathway by analytes in the solution. First, in situ copolymerization of styrene and n-butyl methacrylate (P(S-BMA)) with colloidal ZCIS QDs allowed formation of solid P(S-BMA) matrix homogeneously impregnated with QDs. Then QDs@P(S-BMA) mixed with polylactic acid (PLA) has been processed into QDs@P(S-BMA)@PLA hybrid filaments using extruder. Then, a semi-cavity optical cell was 3D-printed out of QDs@P(S-BMA)@PLA filaments. Such 3D-printed semi-cavity cell can be used as the optical sensor on various molecules utilizing the optical absorption by analyte molecules through inner filter effect (IFE) of a properly selected QDs excitation beam. The multiple drug and amino acid molecules have been utilized having selective UV absorption to examine our proof of concept.

Dissipation-Induced Photon Blockade in the Anti-Jaynes–Cummings Model

Due to the fundamental differences between the quantum world and the classical world, some phenomena, such as entanglement and wave–particle duality, only exist in the quantum realm. These peculiar phenomena cannot be demonstrated by classical means: Quantum networks, quantum cryptography, and quantum precision measurements all require quantum sources. Photons are particularly well-suited as quantum sources owing to their minimal interaction with the environment, high flight speed, and ease of interaction with current typical quantum systems. Single-photon sources include pulsed excitation of quantum dots, spontaneous parametric down-conversion, and photon blockade. Herein, we propose that the anti-Jaynes–Cummings model can induce a pronounced photon antibunching effect when subjected to intense cavity dissipation. Similar to the photon blockade caused by strong photon–photon interaction, this antibunching effect is referred to as ’dissipation-induced blockade’. Our findings indicate that the minimum decay rate of a qubit, coupled with a high decay rate for photons, is conducive to achieving strong antibunching within the system. Notably, g(2)(0)<g(2)(τ), a characteristic of photon antibunching, is only valid under the optimal condition Δ=0. Conversely, g(2)(0)<1 is satisfied across all parameters, indicating that g(2)(0)<1 is not a prerequisite for antibunching in the anti-Jaynes–Cummings model. Moreover, under the optimal conditions of the antibunching effect, the average photon number attains its peak value. Consequently, the current anti-Jaynes–Cummings model is promising for developing single-photon sources characterized by excellent purity and average photon number.

Utilizing direct Zener tunneling in Germanium for cryogenic quantum applications

The temperature-dependent electroluminescent properties of Ge-Diodes, especially the Ge-Zener-Emitter, with tunnel transitions are investigated. The direct band-gap behavior of Germanium below a temperature of 140 K is demonstrated, facilitated by Zener tunneling. Pulsed excitation of the Ge-Zener-Emitter results in an optical output power density of 6 μW, which is sufficient to excite quantum dots for single-photon emission. The peak energy of 0.86 eV suits the non-resonant excitation of InGaAs quantum dots at cryogenic temperatures. This paper presents a potential optical pump source for a quantum photonic integrated circuit.

Temperature-Enhanced Exciton Emission from GaAs Cone-Shell Quantum Dots.

The temperature-dependent intensities of the exciton (X) and biexciton (XX) peaks from single GaAs cone-shell quantum dots (QDs) are studied with micro photoluminescence (PL) at varied excitation power and QD size. The QDs are fabricated by filling self-assembled nanoholes, which are drilled in an AlGaAs barrier by local droplet etching (LDE) during molecular beam epitaxy (MBE). This method allows the fabrication of strain-free QDs with sizes precisely controlled by the amount of material deposited for hole filling. Starting from the base temperature T = 3.2 K of the cryostat, single-dot PL measurements demonstrate a strong enhancement of the exciton emission up to a factor of five with increasing T. Both the maximum exciton intensity and the temperature Tx,max of the maximum intensity depend on excitation power and dot size. At an elevated excitation power, Tx,max becomes larger than 30 K. This allows an operation using an inexpensive and compact Stirling cryocooler. Above Tx,max, the exciton intensity decreases strongly until it disappears. The experimental data are quantitatively reproduced by a model which considers the competing processes of exciton generation, annihilation, and recombination. Exciton generation in the QDs is achieved by the sum of direct excitation in the dot, plus additional bulk excitons diffusing from the barrier layers into the dot. The thermally driven bulk-exciton diffusion from the barriers causes the temperature enhancement of the exciton emission. Above Tx,max, the intensity decreases due to exciton annihilation processes. In comparison to the exciton, the biexciton intensity shows only very weak enhancement, which is attributed to more efficient annihilation processes.

Quantum dots interacting with a non-classical field under Kerr-phase modulation as a resource for repeatable quantum algorithms

An analytical solution describing the dynamics of a hybrid system comprising an interacting quantum dot and a quantum electromagnetic field in the presence of Kerr nonlinearity is obtained. A completely new regime of interaction is found. It provides stable periodically repeating dynamics for the total bipartite system and for the quantum dot excitation, accompanied by full and pure revivals. This is even observed when the initial field state is in a squeezed vacuum. Periodical strong entanglement and full disentanglement between the interacting subsystems are demonstrated. The obtained regime forms the basis for the development of quantum information algorithms and photon–matter interfaces involving these hybrid systems. The formation of non-Gaussian field states is revealed. Methods to control and manage the specific features of the found pure and mixed states of the considered subsystems are developed.

Multi-focused electric and magnetic field sourcing from an azimuthally polarized vortex circular hyperbolic umbilic beam.

In this paper, one kind of multi-focusing electric and magnetic field which is sourced from an azimuthally polarized vortex circular hyperbolic umbilic beam (APVCHUB) is presented. After passing through a high NA objective, both the electric and magnetic fields of the APVCHUBs will focus multiple times, and a high-purity longitudinal magnetic field (p q =80%) will be generated. Besides, the mutual induction of the vortex phase and azimuthal polarization changes the electric and magnetic fields' vibration state and intensity distribution, making the longitudinal magnetic field carry an m-order concentric vortex. Our findings suggest that the APVCHUB could have potential applications in magnetic particle manipulation, extremely weak magnetic detection, data storage, semiconductor quantum dot excitation, etc.

Time-resolved absorption measurements quantify the competition of energy and electron transfer between quantum dots and cytochrome c

We applied transient absorption spectroscopy to study the early photodynamics in a system composed of CdTe quantum dots (QDs) and cytochrome c (Cyt c) protein. In the QDs and Cyt c mixtures, about 25 % of the excited QD electrons quickly relax (∼23 ps) to the ground state and roughly 75 % decay on slower time scale – mostly due to quenching by Cyt c. On the basis of the assumed model, we estimated the contribution of electron transfer and other mechanisms to this quenching. The primary quenching mechanism is probably energy transfer but electron transfer makes a significant contribution (∼8 %), resulting in photoreduction of Cyt c. The lifetime of one fraction of reduced Cyt c (35–90 %) is ∼ 1 ms and the lifetime of the remaining fraction was longer than the ∼ 50-ms time window of the experiment. We speculate that, in the former fraction, the back electron transfer from the reduced Cyt c to QDs occurs and the latter fraction of Cyt c is stably reduced.

Hollow structure molecularly imprinted ratiometric fluorescence sensor for the selective and sensitive detection of dopamine.

Developing hollow-structure quantum dot carriers to increase quantum luminous efficiency is a creative idea for designing a novel sensor. A ratiometric hollow CdTe@H-ZIF-8/CDs@MIPs sensor was developed for the sensitive and selective detection of dopamine (DA). CdTe QDs and CDs were used as the reference signal and recognition signal, respectively, and thus showed a visual effect. MIPs provided high selectivity toward DA. The TEM image demonstrated that the sensor was a hollow structure, which could have ample opportunity to excite quantum dots to emit light through multiple light scattering through holes. In the presence of DA, the fluorescence intensity of the optimum CdTe@H-ZIF-8/CDs@MIPs was remarkably quenched by DA, achieving a linear range of 0-600 nM and a limit of detection of 12.35 nM. The developed ratiometric fluorescence sensor showed an obvious and meaningful color change with a gradual increase in DA concentration under a UV lamp. Moreover, the optimum CdTe@H-ZIF-8/CDs@MIPs was remarkably sensitive and selective in detecting DA among various analogs and showed good anti-interference ability. The HPLC method also further confirmed that CdTe@H-ZIF-8/CDs@MIPs shows good practical application prospects.

Effective Blue Light-Absorbing AuAg Nanoparticles in InP Quantum Dots-Based Color Conversion.

In typical color-by-blue mode-based quantum dot (QD) display devices, only part of the blue excitation light is absorbed by QD emitters, thus it is accompanied by the leakage of blue light through the devices. To address this issue, we offer, for the first time, the applicability of AuAg alloy nanoparticles (NPs) as effective blue light absorbers in InP QD-based color-by-blue platforms. For this, high-quality fluorescent green and red InP QDs with a double shell scheme of ZnSe/ZnS were synthesized and embedded in a transparent polymer film. Separately, a series of Au/Ag ratio-varied AuAg NPs with tunable plasmonic absorption peaks were synthesized. Among them, AuAg NPs possessing the most appropriate absorption peak with respect to spectral overlap with blue emission are chosen for the subsequent preparation of AuAg NP polymeric films with varied NP concentrations. A stack of AuAg NP polymeric film on top of InP QD film is then placed remotely on a blue light-emitting diode, successfully resulting in systematically progressive suppression of blue light leakage with increasing AuAg NP concentration. Furthermore, the beneficial function of the AuAg NP polymeric overlayer in mitigating undesirable QD excitation upon exposure to ambient lights was further examined.

Effect of the Spherical Nanoparticle with a Metal Shell on Deactivation of the Excited Quantum Dot

Effect of the Spherical Nanoparticle with a Metal Shell on Deactivation of the Excited Quantum Dot

Ultrafast electron dynamics of graphene quantum dots: High harmonic generation

We study theoretically nonlinear optical properties of graphene quantum dots placed in a field of a short and strong linearly polarized optical pulse. We address the problem of high harmonic generation in quantum dots and how such nonlinear effect is affected by dephasing processes in a quantum dot. The dephasing makes the ultrafast electron dynamics more irreversible with a large residual population of the excited quantum dot levels. In relation to the high-harmonic spectrum, with increasing the dephasing time, the intensities of the low-frequency harmonics increase while the cutoff energy decreases. The dependence of the cutoff energy on the amplitude of the optical pulse is also sensitive to the frequency of the pulse. When the frequency of the optical pulse is much less than the quantum dot band gap, this dependence is almost linear, but when the frequency of the pulse is comparable to the band gap, the cutoff energy shows saturation behavior at large field amplitude, $>0.4$ V/\AA{}.

Enhancing thermal and optical properties for liquid quantum-dot-based self-loop system toward laser illumination by controlling the layer location

White light sources based on laser diodes are considered to be the next-generation light sources owing to their high luminous efficiency, high luminous flux, and excellent directivity. Quantum dots (QDs) are new fluorescent materials that have been widely used in lighting and displays. However, when used for high-power illumination, QDs excitation and emission are accompanied by obvious attenuation problems owing to their reabsorption effect and low stability in high temperature environment. To solve these problems, this study proposes three types of QD-based self-loop systems for laser illumination and analyzes their thermal and optical properties. White light is obtained by exciting red and green QDs using a blue laser. The three types of self-loop converters are green-QD-front type (structure I), red-QD-front type (structure II), and hybrid type (structure III). The average temperatures of the excitation region of structures I and II were maintained at approximately 37.3 °C and 37.0 °C, respectively, which were both lower than that of the hybrid structure (46.7 °C). The radiation flux of structure I only attenuated by 15.6% after laser irradiation for 120 min. Furthermore, the shift in color coordinates was the smallest. When the laser driving current was changed from 500 mA to 1500 mA, the correlated color temperature (CCT) drift was only 474 K. We believe that these results will provide guidelines for QD-based laser-driven illumination.

Triggered Single‐Photon Emission of Resonantly Excited Quantum Dots Grown on (111)B GaAs Substrate

Triggered Single‐Photon Emission of Resonantly Excited Quantum Dots Grown on (111)B GaAs Substrate

Triggered Single‐Photon Emission of Resonantly Excited Quantum Dots Grown on (111)B GaAs Substrate

In the recent decades, semiconductor quantum dots (QDs) have proven to be valuable candidates as sources for various photonic applications. Of particular interest are, due to their wide‐ranging applicability in quantum communication and computation, indistinguishable photons as well as entangled photon pairs, which can be emitted on demand. The latter require highly symmetric QDs with a small to vanishing excitonic fine‐structure splitting. Herein, quantum optical properties of QDs grown via molecular‐beam epitaxy (MBE) on (111)B‐GaAs substrate are reported to study their emission in terms of triggered single‐photon emission under quasi‐resonant p‐shell excitation, and under strict resonant s‐shell excitation. The investigations reveal very good emission properties of these types of QDs, especially under s‐shell resonant excitation. In fact, the results yield almost background‐free triggered single photons with excellent multiphoton suppression associated with g(2)(0) = (0.033 ± 0.027) and a degree of indistinguishability of (41 ± 10)%. The achieved results underline the high optical quality of (111)‐QDs and show their high potential for applications in photonic quantum technologies.

Quantum Dot-Induced Blue Shift of Surface Plasmon Spectroscopy.

We experimentally demonstrate the spectral blue shift of surface plasmon resonance through the resonant coupling between quantum dots (QDs) and surface plasmons, surprisingly in contrast to the conventionally observed red shift of plasmon spectroscopy. Multimode optical fibers are used for extended resonant coupling of surface plasmons with excited states of QDs adsorbed to the plasmonic metal surface. The long-lived nature of excited QDs permits QD-induced negative change in the local refractive index near the plasmonic metal surface to cause such a blue shift. The analysis utilizes the physical causality-driven optical dispersion relation, the Kramers–Kronig (KK) relation, attempting to understand the abnormal behavior of the QDs-induced index dispersion extracted from blue shift measurement. Properties of QDs’ gain spectrally resonating with plasmons can account for such blue shift, though their absorbance properties never allow the negative index change for the blue shift observed according to the KK relation. We also discuss the limited applicability of the KK relation and possible QDs gain saturation for the experiment–theory disagreement. This work may contribute to the understanding of the photophysical properties critical for plasmonic applications, such as plasmonic local index engineering required in analyte labeling QDs coupled with plasmons for biomedical imaging or assay.

Higher-Order Photon Statistics as a New Tool to Reveal Hidden Excited States in a Plasmonic Cavity

Among the best known quantities obtainable from photon correlation measurements are the g(m) correlation functions. Here, we introduce a new procedure to evaluate these correlation functions based on higher-order factorial cumulants CF,m that integrate over the time dependence of the correlation functions, that is, summarize the available information at different time spans. In a systematic manner, the information content of higher-order correlation functions as well as the distribution of photon waiting times is taken into account. Our procedure greatly enhances the sensitivity for probing correlations and, moreover, is robust against a limited counting efficiency and time resolution in experiment. It can be applied even in case g(m) is not accessible at short time spans. We use the new evaluation scheme to analyze the photon emission of a plasmonic cavity coupled to a single quantum dot. We derive criteria that must hold if the system can be described by a generic Jaynes–Cummings model. A violation of the criteria can be explained by the presence of an additional excited quantum dot state.

Quantitative analysis of nonlinear optical input/output of a quantum-dot network based on the echo state property.

The echo state property, which is related to the dynamics of a neural network excited by input driving signals, is one of the well-known fundamental properties of recurrent neural networks. During the echo state, the neural network reveals an internal memory function that enables it to remember past inputs. Due to the echo state property, the neural network will asymptotically update its condition from the initial condition and is expected to exhibit temporally nonlinear input/output. As a physical neural network, we fabricated a quantum-dot network that is driven by sequential optical-pulse inputs and reveals corresponding outputs, by random dispersion of quantum-dots as its components. In the network, the localized optical energy of excited quantum-dots is allowed to transfer to neighboring quantum-dots, and its stagnation time due to multi-step transfers corresponds to the hold time of the echo state of the network. From the experimental results of photon counting of the fluorescence outputs, we observed nonlinear optical input/output of the quantum-dot network due to its echo state property. Its nonlinearity was quantitatively verified by a correlation analysis. As a result, the relation between the nonlinear input/outputs and the individual compositions of the quantum-dot network was clarified.

In-plane resonant excitation of quantum dots in a dual-mode photonic-crystal waveguide with high β-factor

A high-quality quantum dot (QD) single-photon source is a key resource for quantum information processing. Exciting a QD emitter resonantly can greatly suppress decoherence processes and lead to highly indistinguishable single-photon generation. It has, however, remained a challenge to implement strict resonant excitation in a stable and scalable way, without compromising any of the key specs of the source (efficiency, purity, and indistinguishability). In this work, we propose a novel dual-mode photonic-crystal waveguide that realizes direct in-plane resonant excitation of the embedded QDs. The device relies on a two-mode waveguide design, which allows exploiting one mode for excitation of the QD and the other mode for collecting the emitted single photons with high efficiency. By proper engineering of the photonic bandstructure, we propose a design with single-photon collection efficiency of β > 0.95 together with a single-photon impurity of ϵ < 5 × 10−3 over a broad spectral and spatial range. The device has a compact footprint of and would enable stable and scalable excitation of multiple emitters for multi-photon quantum applications.

Enhanced fluorescence from semiconductor quantum dot-labelled cells excited at 280 nm

Semiconductor quantum dots (QDs) have significant advantages over more traditional fluorophores used in fluorescence microscopy including reduced photobleaching, long-term photostability and high quantum yields, but due to limitations in light sources and optics, are often excited far from their optimum excitation wavelengths in the deep-UV. Here, we present a quantitative comparison of the excitation of semiconductor QDs at a wavelength of 280 nm, compared to the longer wavelength of 365 nm, within a cellular environment. We report increased fluorescence intensity and enhanced image quality when using 280 nm excitation compared to 365 nm excitation for cell imaging across multiple datasets, with a highest average fluorescence intensity increase of 3.59-fold. We also find no significant photobleaching of QDs associated with 280 nm excitation and find that on average, ∼80% of cells can tolerate exposure to high-intensity 280 nm irradiation over a 6-hour period.

Far-field directional emission of fluorescence enhanced by dielectric microsphere and metallic planar nanolayers

Controlling the emission characteristics of fluorescent substances and increasing the intensity of fluorescence emission are crucial for fluorescence detecting technology in single-molecule detection, biomedicine, and sensing applications. Since fluorescence emission is isotropic in nature, the collected fluorescence is only accounted for a small fraction of the total emitted fluorescence. In this paper, a composite structure composed of dielectric microsphere and metallic planar nanolayers is proposed to enhance the fluorescence far-field directional emission intensity and improve the fluorescence collection efficiency. The excitation process and the emission process of quantum dots (QDs) located between the dielectric microspheres and the gold layer are investigated by the finite difference time domain (FDTD) method. In the emission process, the emission of QDs in a homogeneous medium is isotropic. Therefore, we usually select several special polarizations in theoretical analysis state for research. In this paper, we first study the effect of the structure on the fluorescence emission enhancement of QDs when the QDs are in the &lt;i&gt;x-&lt;/i&gt;, &lt;i&gt;y-&lt;/i&gt;, and&lt;i&gt; z-&lt;/i&gt;polarization state. Some results can be obtained as shown below. When the radiation direction of the QDs is perpendicular to the microsphere plane layered structure, the structure is coupled with the emitted fluorescence, thereby realizing the directional enhancement of the emitted fluorescence of the QDs, and the obvious fluorescence enhancement is obtained in the &lt;i&gt;x-&lt;/i&gt; and &lt;i&gt;y-&lt;/i&gt;polarization state. Therefore, in the research, we choose and investigate the dipole light source of &lt;i&gt;x&lt;/i&gt;-polarization state. We mainly study the influence of microsphere radius, refractive index, and QDs position on the fluorescence directional enhancement. The QDs as a fluorescent material are coated in polymethyl methacrylate (PMMA) to control the distance from the gold layer to tune the fluorescence enhancement. The structure is based on the synergistic effect among plasmon coupling, whispering gallery mode and photonic nanojet, which enhances the far-field fluorescence of QDs by a factor of 230, and the fluorescence collection efficiency is as high as 70%. Comparing with the enhanced fluorescence of the dielectric microspheres and the gold sphere dimer composite structure, the distance between the gold sphere dimers is not easy to control, and the QDs should be placed at specific positions between the gold spheres. The structure we propose is more convenient to implement. In this paper, not only the emission enhancement process of QDs is studied in detail, but also the excitation process of QDs is investigated. Our proposed dielectric microsphere metal planar nanolayered structure can enhance the excitation of QDs in most areas, proving that our designed structure can effectively realize the excitation enhancement of QDs. The above results have very important applications in the fluorescence biological detection, imaging, and light-emitting devices.