Quantum Dot Molecules Research Articles

New Conjugates Based on AIS/ZnS Quantum Dots and Aluminum Phthalocyanine Photosensitizer: Synthesis, Properties and Some Perspectives.

Today, fluorescent diagnostics and photodynamic therapy are promising methods for diagnosing and treating oncological diseases. The development of new photosensitizers (PS) is one of the most important tasks to improve the efficiency of both laser-induced diagnostics and therapy. In our study, we conjugated PS with AIS/ZnS triple quantum dots (QDs) to obtain non-aggregated complexes. It was shown that the conjugation of PS with QDs does not change the PS fluorescence lifetime, which is a marker of the preservation of PS photophysical properties. In particular, efficient resonant Förster energy transfer (FRET), from QDs to PS molecules in the conjugate, increases the PS luminescence response. The FRET from QD to PS molecules with different ratios of donor and acceptors are shown. It has been demonstrated that the average efficiency of FRET depends on the ratio of PS and QD and reaches a maximum value of 80% at a ratio of 6 PS molecules per 1 QD molecule. Thus, these studies could help to contribute to the development of new complexes based on QD and PS to improve the efficiency of phototheranostics.

Quantum coherence-enhanced optical properties and drag of photons and SPPs in semiconducting quantum dots and resonantly-coupled dot–nanoparticle plasmonic interfaces

Enhancing the optical response and Fresnel–Fizeau light dragging in relation to photon plus surface plasmons through various coupled atomic–metal plasmonic interfaces has prompted great interest recently. This is due to the many potential implications of optical properties and drag of light, both from fundamental and technological viewpoints. Herein, we report theoretical results on the quantum-coherence enhanced optical features and light dragging through a well-shaped ensemble of quantum dot molecules (QDMs) and plasmonically coupled dot–(metal)nanoparticle interface. To investigate and tune the required optical properties and light dragging in the superluminal and subluminal regimes, we employed interdot tunneling-driven quantum coherence via electromagnetically-induced transparency (EIT) and plasmonically-induced transparency (PIT). Specifically, in order to determine the required theoretical results, we employed a density-matrix formulation to calculate the complex susceptibility of probe/signal pulse through the proposed QDM. We interpreted the light drag and optical response of probe field and surface plasmons via dressed eigenstates of PIT/EIT and Fano-type quantum interference in the proposed QDMs and dot–metal nanoparticle plasmonic interfaces. Unlike bulk metals, to account for the reduced electron mean free path, hereby, we employed the size-dependent corrected dielectric constant of the Drude model. The maximum predicted value of the wave vector for surface plasmons was 2m−1, which led to greatly enhanced propagation length of the order of 0.25 m. The calculated wavelength was 162.91×10−3 m, corresponding to a quality factor of 9.64.

Hybridization and deconfinement in colloidal quantum dot molecules.

The structural, electronic, and optical properties of CdSe/CdS core-shell colloidal quantum dot molecules, a new class of coupled quantum dot dimers, are explored using atomistic approaches. Unlike the case of dimers grown by molecular beam epitaxy, simulated strain profile maps of free-standing colloidal dimers show negligible additional strain resulting from the attachment. The electronic properties of the relaxed dimers are described within a semiempirical pseudopotential model combined with the Bethe-Salpeter equation within the static screening approximation to account for electron-hole correlations. The interplay of strain, hybridization (tunneling splitting), quantum confinement, and electron-hole binding energies on the optical properties is analyzed and discussed. The effects of the dimensions of the neck connecting the two quantum dot building blocks, as well as the shell thickness, are studied.

Front Cover: Quantum Dot Molecule Devices with Optical Control of Charge Status and Electronic Control of Coupling (Adv. Quantum Technol. 10/2022)

Front Cover: Quantum Dot Molecule Devices with Optical Control of Charge Status and Electronic Control of Coupling (Adv. Quantum Technol. 10/2022)

Coherence limitations in the optical control of the singlet-triplet qubit in a quantum dot molecule

We analyze the optically driven dynamics of a qubit implemented on a singlet-triplet subspace of two-electron states in a self-assembled quantum dot molecule. We study two possible control schemes based on the coupling to an excited (four-particle) state either by two spectrally separated laser pulses or by a single spectrally broad pulse. We quantitatively characterize the imperfections of the qubit operation resulting from non-adiabatic evolution and from limited spectral selectivity in a real system, as compared to the ideal adiabatic Raman transfer of occupation in the $\Lambda$-system. Next, we study the effects of decoherence induced by the coupling to the phonons of the surrounding crystal lattice and by radiative recombination. As a result, we are able to identify the optimization trade-offs between different sources of errors and indicate the most favorable conditions for quantum control of the singlet-triplet qubit in the two optical control schemes.

Large-bandwidth Transduction Between an Optical Single Quantum Dot Molecule and a Superconducting Resonator

Quantum transduction between the microwave and optical domains is an outstanding challenge for long-distance quantum networks based on superconducting qubits. For all transducers realized to date, the generally weak light-matter coupling does not allow high transduction efficiency, large bandwidth, and low noise simultaneously. Here we show that a large electric dipole moment of an exciton in an optically active self-assembled quantum dot molecule (QDM) efficiently couples to a microwave resonator field at a single-photon level. This allows for transduction between microwave and optical photons without coherent optical pump fields to enhance the interaction. With an on-chip device, we demonstrate a sizeable single-photon coupling strength of 16 MHz. Thanks to the fast exciton decay rate in the QDM, the transduction bandwidth between an optical and microwave resonator photon reaches several 100s of MHz.

Adjusting amplitude of the stored optical solitons by inter-dot tunneling coupling in triple quantum dot molecules

We study the propagation properties of a probe field in an aligned asymmetric triple quantum dot molecule with both sides inter-dot tunneling coupling effect. It is shown that the probe field can form optical soliton due to the destructive quantum interference induced by the quantum inter-dot tunneling coupling effect. Interestingly, these optical solitons can be stored and retrieved by adjusting single or double inter-dot tunneling coupling effect, different from that light memory in the ultra-cold atom system. Furthermore, we also find that the amplitude of the stored optical soliton can be adjusted by the strength of the single or double inter-dot tunneling coupling. It is possible to improve the stability and the fidelity of the optical information in the process of the storage and retrieval in semiconductor quantum dots devices.

Photon correlations in colloidal quantum dot molecules controlled by the neck barrier

<h2>Summary</h2> The charge redistribution upon optical excitation of various necked homodimer colloidal quantum dot molecules (CQDMs) is investigated using single-particle emission spectroscopy. By tuning the hybridization of the electron wavefunctions at a fixed center-to-center distance through controlling the neck girth, we reveal two coupling limits: on one hand, a "connected-but-confined" situation where neighboring CQDs are weakly fused to each other, manifesting a weak-coupling regime, and on the other hand, a "connected-and-delocalized" situation where the neck is filled beyond the facet size, leading to a rod-like architecture manifesting strong coupling. The interplay between the radiative and non-radiative Auger decays of these states turns emitted photons from the CQDMs in the weak-coupling regime highly bunched unlike CQD monomers, while the antibunching is regained at the strong-coupling regime. This work sets an analogy for the artificial molecule CQDMs with regular molecules, where the two distinct regimes of weak and strong coupling correspond to ionic- or covalent-type bonding, respectively.

Pit rim decomposition into multiple quantum dots on surfaces of epitaxial thin films grown on pit-patterned substrates

We report results of dynamical simulations according to an experimentally validated surface morphological evolution model on the formation of regular arrays of quantum dot molecules (QDMs) consisting of 1D arrays of smaller interacting quantum dots (QDs). These QD arrays form along the sides of each pit rim on the surface of a coherently strained thin film epitaxially deposited on a semiconductor substrate, the surface of which consists of a periodic pattern of inverted prismatic pits with rectangular pit openings. We find that this complex QDM pattern results from the decomposition of the pit rim from a “quantum fortress” with four elongated QDs into four 1D arrays of multiple smaller QDs arranged along each side of the pit rim. Systematic parametric analysis indicates that varying the pit opening dimensions and the pit wall inclination directly impacts the number of QDs in the resulting QDM pattern, while varying the pit depth only affects the dimensions of the QDs in the nanostructure pattern. Therefore, the number, arrangement, and sizes of QDs in the resulting pattern of QDMs on the film surface can be engineered precisely by proper tuning of the pit design parameters. Our simulation results are supported by predictions of morphological stability analysis, which explains the pit rim decomposition into multiple QDs as the outcome of a tip-splitting instability and provides a fundamental characterization of the post-instability nanostructure pattern. Our theoretical findings can play a vital role in designing optimal semiconductor surface patterns toward enabling future nanofabrication technologies.

Quantum Dot Molecule Devices with Optical Control of Charge Status and Electronic Control of Coupling

AbstractTunnel‐coupled pairs of optically active quantum dots—quantum dot molecules (QDMs)—offer the possibility to combine excellent optical properties such as strong light‐matter coupling with two‐spin singlet–triplet () qubits having extended coherence times. The basis formed using two spins is inherently protected against electric and magnetic field noise. However, since a single gate voltage is typically used to stabilize the charge occupancy of the dots and control the inter‐dot orbital couplings, operation of the qubits under optimal conditions remains challenging. Here, an electric field tunable QDM that can be optically charged with one (1h) or two holes (2h) on demand is presented. A four‐phase optical and electric field control sequence facilitates the sequential preparation of the 2h charge state and subsequently allows flexible control of the inter‐dot coupling. Charges are loaded via optical pumping and electron tunnel ionization. One‐ and two‐hole charging efficiencies of (93.5 ± 0.8)% and (80.5 ± 1.3)% are achieved, respectively. Combining efficient charge state preparation and precise setting of inter‐dot coupling allows for the control of few‐spin qubits, as would be required for the on‐demand generation of 2D photonic cluster states or quantum transduction between microwaves and photons.

Realization of phase dependent optical bistability in a quantum dot molecule via three-wave mixing

In this paper, we have proposed a new model based on the electron tunneling effect for controlling the optical bistability (OB) and optical multistability (OM) in a defect slab structure with a quantum dot molecule (QDM). Due to the presence of the electron tunneling effect, an additional weak signal beam can be generated in the system which makes the medium becomes phase dependent. We have studied the refractive index properties of the QDM via optical susceptibility of the propagated probe light. We have discussed the relative phase effect on negative refractive index (NRI) of the QDM nanostructure due to presence of electron tunneling. Then, by using the transfer matrix method, we have studied the transmission coefficient for NRI of the QDM nanostructure. Then, we have investigated the properties of transmitted light versus incident light for realizing the OB and OM. We have considered the relative phase effect on OB and OM properties of the transmitted beam and find that the switching between them is possible when electron tunneling is present in the quantum dot. Moreover, the optical thickness of the defect layer is also investigated on OB and OM in resonance and non-resonance conditions.

Pauli spin blockade in a resonant triple quantum dot molecule

A Pauli spin blockade in quantum dot systems occurs when the charge transport is allowed only for some spin states, and it has been an efficient tool in spin-based qubit devices in semiconductors. We theoretically investigate a Pauli spin blockade in a triple quantum dot molecule consisting of three identical quantum dots in a semiconductor in the presence of an external magnetic field through the molecule. When the three-electron state is on resonance with two- or four-electron states, the Aharonov–Bohm oscillation and the Zeeman splitting lead to a periodic spin blockade effect. We focus on the spin blockade at a two- and three-electron resonance and show that we can tune the magnetic field to selectively allow only either a spin-singlet or spin-triplet state to add an additional electron from tunnel-coupled leads. This spin blockade maintains the three quantum dots at the optimal sweet spot against the charge noise, demonstrating its potential as an efficient readout scheme for the qubits in quantum dot systems.

Asymmetric diffraction grating via optical vortex light in a tunneling quantum dot molecule

In this letter, we have put out a fresh idea for managing the diffraction sample of optical vortex light that was transmitted from a four-level quantum dot molecule. We describe the creation of a new weak laser field via inter-dot tunneling, which causes the diffraction grating to be dependent on the orbital angular momentum (OAM) of the optical vortex light. We found that the relative phase between the implemented lights and the OAM number of the vortex light affect the intensity distribution of the asymmetric grating. Additionally, we discovered that the maximum amount of probe energy could be dispersed in higher orders of diffracted angles that were negative and positive by modifying the inter-dot tunneling and OAM number.

Enhanced solar photocurrent using a quantum-dot molecule

We present a detailed study on the influence of coherent tunneling on the photovoltaic properties of a semiconductor molecule driven by solar radiation. The connection between the power delivered by the quantum-dot molecule (QDM) and quantum coherence is not simply proportional but depends on interplay between the interdot coherent tunneling, the interaction of the system with thermal phonon reservoirs, and the resonance between the QDM and conduction bands. We explored numerically various parameter regimes and found that the maximum power delivered by the molecule is up to 30% greater than the power delivered by a single quantum-dot device. The calculated photovoltaic conversion efficiency is presented in terms of accessible experimental parameters and, as expected, is constrained by the second law.

Controllable storage and retrieval of optical solitons in triple quantum dot molecules by inter-dot tunneling coupling effect

We study the storage and retrieval of the optical field in an aligned asymmetric triple quantum dot molecule with both sides inter-dot tunneling coupling effect. It is shown that the optical solitons can be stored and retrieved by manipulating two inter-dot tunneling coupling effect, different from light memory in the ultra-cold atom system. Furthermore, the amplitude of the optical soliton emerges a trend of first decreasing and then increasing with the increasing strength of the two inter-dot tunneling coupling. It is possibility to improve the fidelity of the storage and retrieval of the optical information in semiconductor quantum dots devices.

Enhanced Spin Coherence of a Self-Assembled Quantum Dot Molecule at the Optimal Electrical Bias.

A pair of coupled dots with one electron in each dot can provide improvements in spin coherence, particularly at an electrical bias called the "sweet spot," but few measurements have been performed on self-assembled dots in this regime. Here, we directly measure the T_{2}^{*} coherence time of the singlet-triplet states in this system as a function of bias and magnetic field, obtaining a maximum T_{2}^{*} of 60ns, more than an order of magnitude higher than an electron spin in a single quantum dot. Our results uncover two main dephasing mechanisms: electrical noise away from the sweet spot, and a magnetic field dependent interaction with nuclear spins due to a difference in g factors.

Optical response in a double quantum dot molecule inside a nonlinear photonic crystal cavity

In this work, we theoretically analyze optical bistability and absorption in coupled double quantum dot (CDQDs) cavity system in the presence of a third-order Kerr nonlinear χ(3) medium. We find that the optical bistability can be tuned by the coupling strength between the two quantum dots, nonlinear medium strength and other system parameters. The possibility of making the system an efficient optical switch is also studied. We further show that the system parameters can also tune the absorption spectrum which shows both positive and negative absorption (emission). Such a study of the CDQDs system should be helpful for various applications in optical logic devices.

Controllable fast light in quantum dot molecules assisted hybrid optomechanical system

AbstractWe investigate the superluminal effect of transmitted probe field in three‐level quantum dot molecules (QDMs) assisted optomechanical system which consist of mechanical resonator. We show that the superluminal behavior of transmitted probe field can be controlled by changing the tunneling strength and number of QDMs inside the cavity. Furthermore, it is shown that in the absence of tunneling strength, the transmitted probe field shows the fast light effect and by increasing the number of QDMs, the enhancement in superluminal behavior is decreased and converts into slow light. While, in the presence of the tunneling strength, with the increase of the number of QDMs the superluminal behavior of transmitted field is obtained at smaller detuning frequency. The influence of tunneling strength and number of QDMs on superluminal part of the transmitted probe field is quite useful in optical memory, optical buffers, and quantum information processing.

Controllable asymmetric diffraction grating with PT symmetry in quantum dot molecules

Controllable asymmetric diffraction grating with PT symmetry in quantum dot molecules

Twisting Enabled Charge Transfer Excitons in Epitaxially Fused Quantum Dot Molecules.

A heterojunction with type-II band alignment has long been considered as a prerequisite to realize charge transfer (CT) excitons which are highly appealing for exploration of quantum many-body phenomena, such as excitonic Bose-Einstein condensation and superfluidity. Herein, we have shown CT excitons can be activated via twisting in epitaxially fused heterodimer quantum dot (QD) molecules with quasi type-II band alignment, and even in QD homodimer molecules, therefore breaking the constraint of band alignment. The enabling power of twisting has been revealed. It modulates the orbital spatial localization toward charge separation that is mandatory for CT excitons. Meanwhile, it manifests an effective band offset that counterbalances the distinct many-body effects felt by excitons of different nature, thus ensuring the successful generation of CT excitons. The present work extends the realm of twistroincs into zero-dimensional materials and opens a novel pathway of manipulating the properties of QD materials and closely related molecular systems.