Double Quantum Dot Molecule Research Articles

Tunneling induced transparency and slow light in an asymmetric double quantum dot molecule—Metal nanoparticle hybrid

We investigate the optical properties appearing in a nanostructure that is composed of an asymmetric double semiconductor quantum dot (SQD) molecule and a metal nanoparticle (MNP). The profile of the total linear absorption spectrum is proportional to the SQD contribution, while the MNP contribution is important. The profile of the doublet of resonances detected on the total linear absorption spectrum creates a transparency window. The doublet is asymmetric for small SQD-MNP distances and has a narrow peak and a wide peak. The width of the transparency window is increased, either with the enhancement of the rate at which the electron tunneling effect takes place within the double SQD molecule or with the decrease of the distance that separates the SQD molecule from the center of the MNP. The steep slope detected on the linear dispersion spectrum for frequencies laying within the transparency window owes its presence to the tunneling induced transparency and leads to slow light production. The corresponding value of the slow down factor is maximized for low values of the electron tunneling rate as well as for low center-to-center distances between the components of the hybrid nanostructure.

Optimal and robust control of population transfer in asymmetric quantum-dot molecules

We present an optimal and robust quantum control method for efficient population transfer in asymmetric double quantum-dot molecules. We derive a long-duration control scheme that allows for highly efficient population transfer by accurately controlling the amplitude of a narrow-bandwidth pulse. To overcome fluctuations in control field parameters, we employ a frequency-domain quantum optimal control theory method to optimize the spectral phase of a single pulse with broad bandwidth while preserving the spectral amplitude. It is shown that this spectral-phase-only optimization approach can successfully identify robust and optimal control fields, leading to efficient population transfer to the target state while concurrently suppressing population transfer to undesired states. The method demonstrates resilience to fluctuations in control field parameters, making it a promising approach for reliable and efficient population transfer in practical applications.

Dispersion-dependent superluminal propagation and photon drag in GaAs/AlGaAs quantum dot molecule

Controlling the basic properties of light such as polarization, angular momentum, and speed through various optical media leads to interesting phenomena in various fields such as quantum optics, photonics, plasmonics, and quantum information technology. Few of the phenomenon include, for instance, light storage, ultra-slow and ultra-fast propagation, optical transparency, and quantum entanglement via quantum coherence or superposition. The understanding of these phenomenon of milestone implications paved the way for the invention of novel classical and quantum technology. In this article, we present detailed theoretical and numerical demonstration on the dispersion-dependent phenomenon of slow and fast light, optically induced transparency, and photon drag. For this, we employ a three-level scheme of an ensemble of GaAs\\AlGaAs double quantum dot molecule in cascade configuration. In particular, by tuning the medium dispersion we predict interesting interplay between ultra-slow and ultra-fast light propagation with vanishing, infinite, ultra-positive and ultra-negative group velocities, and optical-assisted quantum transparency. The quantum coherence of the dressed states modifies considerably the optical properties of the system in view of dispersion, absorption, electron energy loss, transmission, and photon drag. An interesting interplay between ultra-slow to ultra-fast propagation is found by tuning various optical parameters. The calculated superluminal value of group velocity is m s−1. We also predict light propagation at vanishing and infinite group velocity. The calculated light drag is rad/m.

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.

Incoherent control of Goos–Hänchen shifts in a tunneling quantum dot molecule

In this letter, we propose a novel model for adjusting the Goos–Hänchen shifts (GH) from a fixed cavity. A three-level double quantum dot (QD) molecule is doped in the cavity, which interacts with a probe laser field and an incoherent pumping rate. We initially investigate the refraction index of double QDs utilizing the quantum mechanical density matrix approach in order to achieve negative and positive refractive indexes. The GH shifts of reflected and transmitted light beams in positive and negative refraction indices of the intracavity medium were then studied. The improved negative or positive GH shifts in reflected and transmitted light beams may be accomplished by changing the incoherent pumping rate and tunneling effect. Our findings reveal that the magnitude of the GH shifts is greatly influenced by the absorption of incoming probing light in double QDs. The enormous GH shifts for the perfect tuning cavity have been seen at a given incidence angle.

Quantum coherence-assisted optical properties and drag of SPPs on quantum dots and resonantly-coupled dots-metal plasmonic interfaces via interbands tunneling and Fano resonance

Owing to their peculiar properties of great fundamental and practical implications, surface plasmons and exciton-plasmons interaction have attracted immense interest in the recent decades. In this regard, quantum dots and dots-metal waveguides and interfaces are promising candidates. In this article, we report first theoretical demonstration on the detailed optical and optotransport properties and surface plasmon polaritons (SPPs) dragging via quantum dots interbands tunneling-induced quantum coherence. For the required properties of SPPs, we proposed a resonantly coupled dots-metal plasmonic interface, which comprises of ensemble of three-level double quantum dot molecules (DQDMs) placed on the flat surface of gold thin film. For the theoretical results, we first solved analytically the density-matrix equation for the eigenvalues of the DQDMs in the Λ− type configuration. In view of the dressed-states picture, we interpreted the results further with regard to EIT (electromagnetically induced transparency), PIT (plasmonic induced transparency) and Fano interference. In particular, we elaborate on the role of interplay of the quantum dots and the coupled plasmonic resonances on the optical, optical-cum transport properties and drag of SPPs. We predict ultrafast superluminal group velocity for SPPs of the order 1.5×1010 m/s, quality factor of 38.48, confinment factor 1.52 and the optimum drag for SPPs of order 0.2 micro meter/radian.

Photon-assisted spin transport characteristics of A-B interferometer embedded with three quantum dots

We design an A-B interferometer with a “single quantum dot atom” embedded in the upper arm and a “double quantum dot molecule” embedded in the lower arm. The transmission coefficient and average current characteristics are studied as the time-dependent external field is introduced. When the interdot coupling strength is weak, the change of Rashba spin orbit coupling strength has a strong influence on the spin transmission coefficient. When the interdot coupling strength and Rashba spin orbit coupling strength take appropriate values, the spin up and spin down transmission coefficients completely coincide, and a resonance band emerges at the Fermi level. The spin polarization can be close to +1 or −1 by adjusting the Zeeman magnetic field, the interdot coupling strength, and the gate voltage. In the presence of photon assistance, the spin polarization changes periodically in a certain proportion, so as to realize the spin-polarized transport with controllable time-dependent external field. These findings provide a theoretical basis for the spin polarized function of A-B interferometer.

Coherent effects in energy absorption in double quantum dot molecule – Metal nanoparticle hybrids

We theoretically study the optical properties of a hybrid structure consisting of a metal nanoparticle (MNP) and an asymmetric double semiconductor quantum dot (SQD) molecule, which are coupled together, via long-range Coulomb interaction. We derive and solve numerically the relevant density-matrix equations and use electromagnetic calculations to evaluate the energy absorption rate of the MNP and the asymmetric double SQD molecule, separately, as well as, the total absorption rate for the entire hybrid structure, as a function of the energy of an applied electromagnetic field , for two different applied field polarization directions. We also investigate the impact of the applied field intensity and the electron tunnelling coupling rate on the spectral profiles, for several values of the interparticle distance. The Autler-Townes splitting and double nonlinear Fano effects are obtained in the absorption spectra of the asymmetric double SQD molecule, the MNP and the total system.

Role of quantum-coherence dressed states on the Fresnel–Fizeau drag and optical properties of surface plasmon polaritons through asymmetric double quantum dot-metallic plasmonic interfaces

Owing to the exceptional structural-design flexibility and tunability of physical properties of quantum dots, the manipulation of light in view of surface plasmons via dots–based hybrid nanostructures and dots-metallic interfaces finds promising device applications. Hereby, we first demonstrate theoretically the role of intense laser-excited dressed states on the light propagation, optical properties, and lateral and rotary drag of surface plasmon polaritons (SPPs), via plasmon-assisted induced transparency (PIT). For this, we employed a scheme of plasmonic interface comprising an asymmetric double quantum dot molecule GaAs/AlGaAs in the Λ − type configuration embedded in gold metallic surface. In particular, using density-matrix formulation, we present a detailed analytical solution of the quantum dot system for the required eigenvalues in view of the dressed eigenstates. The dressed states played a vital role in the formation of PIT transparency and hence the manipulation of SPPs propagation, optical properties and SPPs drag, via quantum interference. The calculated theoretical results on the optical properties such as absorption and dispersion spectrum, group index, group velocity, group delay, phase delay, wave vector, propagation length and SPPs drag are further elaborated with aid of numerical simulation. The maximum calculated lateral and rotary SPPs drag was of order ± 6.0 × 10−8 m or rad, respectively. The value of SPPs propagation lengths calculated along and perpendicular to the dot-metallic interface are 3.3 × 10−7 m and 2.0 × 10−9 m, respectively.

Asymmetric, mixed-valence molecules for spectroscopic readout of quantum-dot cellular automata

Mixed-valence compounds may provide molecular devices for an energy-efficient, low-power, general-purpose computing paradigm known as quantum-dot cellular automata (QCA). Multiple redox centers on mixed-valence molecules provide a system of coupled quantum dots. The configuration of mobile charge on a double-quantum-dot (DQD) molecule encodes a bit of classical information robust at room temperature. When arranged in non-homogeneous patterns (circuits) on a substrate, local Coulomb coupling between molecules enables information processing. While single-electron transistors and single-electron boxes could provide low-temperature solutions for reading the state of a ∼1 nm scale molecule, we propose a room-temperature read-out scheme. Here, DQD molecules are designed with slightly dissimilar quantum dots. Ab initio calculations show that the binary device states of an asymmetric molecule have distinct Raman spectra. Additionally, the dots are similar enough that mobile charge is not trapped on either dot, allowing device switching driven by the charge configuration of a neighbor molecule. A technique such as tip-enhanced Raman spectroscopy could be used to detect the state of a circuit comprised of several QCA molecules.

Controlling the Pump-Probe Optical Response in Asymmetric Tunneling-Controlled Double Quantum Dot Molecule—Metal Nanoparticle Hybrids

In the present work, we investigate the modified nonlinear pump-probe optical properties due to the excitonic–plasmonic interaction of a double semiconductor quantum dot (SQD) molecule coupled to a metal nanoparticle (MNP). More specifically, we study the absorption and the dispersion spectra of a weak electromagnetic field in a hybrid structure with two counterparts, a molecule of two coupled SQDs, and a spherical MNP driven by a field of high intensity. We solve the relevant density matrix equations, calculate the first-order optical susceptibility of the probe field in the strong pumping regime, and investigate the way in which the distance between the two counterparts modifies the optical response, for a variety of values of the physical constants of the system, including the pump-field detuning, the tunnelling rate, and the energy separation gap associated with the excited states of the coupled SQDs.

Role of intense laser-excited dressed states via electromagnetically induced transparency on the Fresnel-Fizeau photon drag through an asymmetric double quantum dot molecule (GaAs/AlGaAs) in the Λ-type configuration

Hereby, we first report theoretical demonstration on the role of intense laser-excited dressed states on the superluminal-assisted propagation, optical properties, and lateral and rotary photon drag, via electromagnetically induced transparency (EIT). For this, we employed a scheme of an asymmetric double quantum dot molecule GaAs/AlGaAs in the Λ − type configuration. In particular, using the compact density-matrix formulism, we present a detailed analytical solution for the required eigenvalues of the system in view of the dressed states. The dressed-states played a vital role on the formation of EIT and hence the modification of dispersion and absorption spectra of the system via quantum interference. The calculated theoretical results on the optical properties such as absorption and dispersion spectrum, group index, group velocity , group delay, phase delay and photon drag are further elaborated with the aid of numerical simulation. Switching ON the control field, the OFF-field single ON-resonance absorption peak split into an OFF-resonance absorption doublet accompanied by an ON-resonance absorption-less dip in the form of EIT window. Interestingly, the absorption doublet split further into an absorption peak and a gain singlet well below and just above the resonance, at a small control field detuning. Further increase in the control-field detuning resulted in a near-resonance gain singlet. Alongside, by tuning various spectral parameters, we observed an interesting switching from luminal to subluminal and superluminal propagation. The optimal calculated values of positive and negative group velocity are 6 × 10 11 m/s and − 2.5 × 10 11 m/s, respectively. The predicted lateral and rotary photon drag are of order ± 6 × 10 − 8 rad. • Role of dressed sates in view of EIT coherence on the light propagation, optical properties and photon drag. • The nature of dressed states influence largely the optical properties such as group index, group velocity, group delay, and photon drag. • The optimal values of positive and negative group velocity are 6 × 10 11 m/s and − 2.5 × 10 11 m/s, respectively. • The calculated lateral and rotary photon drag are of order ± 6 × 10 − 8 rad.

Double versus single junction: From tunneling to contact regime in a double quantum dot molecule system

In this work a theoretical study of the charge transport through a nanostructure composed by a double quantum dot was made by treating the system in two different ways: as single junction (SJ) within the scanning tunneling microscopy theory and a double junction (DJ) within the usual non equilibrium Green's function theory approach. In the first case (SJ), the three blocks – contact-device-contact – is treated as a single junction by ‘breaking’ the nanostructure into two parts and considering each one as belonging to an ‘extended contact’. In the DJ case, the usual treatment with two contacts-related broadening and the exact Green's function describing the propagation inside the system was made. Proceeding in this way, for a minimum two-level model system, we show that the results obtained with the SJ and DJ treatment are identical despite they start with a different general formulae for transmission. Finally, both treatments were used with a minimal model for a biphenyl system in an asymmetric sample-biphenyl-tip configuration and analyzed in terms of a SJ point of view bringing another perspective to the main features of this system.

Magnetic field effect on the electronic states and the intraband optical absorption spectrum of a laser dressed double quantum dot molecule

The simultaneous effects of intense terahertz (THz) laser, a homogeneous magnetic fields, and the modification of the structural parameters on the electronic states, and the intraband optical absorption spectrum in a two-dimensional double quantum dot molecule are theoretically investigated. The crossing and anticrossing are observed in the energy dependence on the magnetic field induction between the third and the fourth energy levels. Additionally, it is shown that an intense THz laser field always shifts the energy spectrum to higher values. The variation of the structural parameters leads to the change of the positions of the energy levels and the anticrossing point. Finally, we have found that the intraband optical absorption spectrum, particularly the absorption intensity and the peak position, can be effectively regulated by an intense THz laser and a magnetic fields, as well as by the variation of the structural parameters of the double quantum dot molecule.

Control of electronic and optical properties of a laser dressed double quantum dot molecule by lateral electric field

We study the confined states of a molecule formed by two laterally coupled quantum dots. The static electric field and incident plane wave laser field are considered with different polarization. The respective Shrödinger equation is obtained in the framework of the high-frequency limit of Floquet theory. It is shown that the electric field has a qualitative impact on the spatial symmetry of electronic states. An in-depth study of electric and laser field combined effects show a strong correlation between them. Additionally, we show that the linear, nonlinear and total optical absorption coefficients and relative refractive index of the considered structure can be effectively tuned by changing electric field strength and intense laser field parameter. Our results indicate the possibility of controlling the optical response of double quantum dots molecule for future applications in nanostructure-based quantum information science.

Light drag via electron tunneling and incoherent pumping in semiconductor three-level InAs/GaAs double quantum dot molecules

We theoretically investigate the lateral and rotary light-drag in semiconductor three-level InAs/GaAs double quantum dot molecules. No coherent laser fields are used and the coherence is created by interdot electron tunneling. The effect of interdot electron tunneling on lateral and rotary light-drag is discussed. Moreover, it is shown that applying an incoherent pumping field to the probe transition changes the subluminal to superluminal condition. It is observed that the light polarization state drags opposite and along with the medium motion in superluminal and subluminal propagating regions, respectively.

Light emission properties in a double quantum dot molecule immersed in a cavity: Phonon-assisted tunneling

Two main mechanisms dictate the tunneling process in a double quantum dot: overlap of excited wave functions, effectively described as a tunneling rate, and phonon-assisted tunneling. In this paper, we study different regimes of tunneling that arise from the competition between these two mechanisms in a double quantum dot molecule immersed in a unimodal optical cavity. We show how such regimes affect the mean number of excitations in each quantum dot and in the cavity, the spectroscopic resolution and emission peaks of the photoluminescence spectrum, and the second-order coherence function which is an indicator of the quantumness of emitted light from the cavity.

Voltage-controllable multifunctional spin polarizer based on side-coupled quantum dots

Abstract Quantum dot system is considered as the prime candidate for nanospintronics devices, within which to generate spins one by one is the long term goal for quantum information and quantum computation. In this paper, with the aid of the Wilson’s numerical renormalization group method, we find a side-coupled double quantum dot molecule could be used to implement a multifunctional spin polarizer in the strongly correlated regime without switching the direction of the magnetic field. The spin polarizer reveals almost 100 % efficiencies, and the polarizing scenarios could be controlled easily by the initial charge state through external gate voltage, leading to various kinds of spin polarizers, e.g., unidirectional, bidirectional, and dual spin polarizers. We demonstrate the Zeeman effect and the inter-dot transport processes are responsible for the spin-polarized transport. Our scheme is robust for any inter-dot coupling and may also be useful for quantum dots connected with magnetic leads.

Phonon-assisted tunnelling in a double quantum dot molecule immersed in a cavity

The effects caused by phonon-assisted tunnelling (PhAT) in a double quantum dot (QD) molecule immersed in a cavity were studied under the quantum Markovian master equation formalism in order to account for dissipation phenomena. We explain how for higher PhAT rates, a stronger interaction between a QD and the cavity at off-resonance takes place through the resonant interaction of another QD and the cavity, where the QDs interact through tunnelling. A closer look at the system's allowed optical transitions as a function of PhAT rates showed coalescence of the transition energies, leading to a loss of spectroscopic resolution in the photoluminescence (PL) spectrum. This coalescence occurs at several exceptional points, suggesting a quantum dynamical phase transition.

Controllable optical bistability in double quantum dot molecule

Controllable optical bistability in double quantum dot molecule