Asymmetric Quantum Dot Molecule Research Articles

Spatially varying optical characteristics in quantum-dot molecules through interdot tunneling

In this paper, we investigate the spatially dependent absorption characteristics in structurally asymmetric quantum-dot molecules (QDMs), characterized by four energy levels interacting with position-dependent beams and varying system parameters. We explore the effects of detuning, standing wave intensity, and the relative phase of applied fields on the absorption patterns within the medium. A diverse array of patterns emerges, including cross-like structures, ring-like formations, and localized absorption maxima, illustrating the intricate interplay between these parameters and the spatial distribution of absorption features. The introduction of a vortex-shaped control field adds a new dimension to the study, revealing azimuthal dependence and providing a novel perspective for manipulating absorption and gain properties based on the orbital angular momentum of the control field. This work contributes to a comprehensive understanding of the intricate dynamics governing spatially dependent absorption in QDMs, offering valuable insights for controlled manipulation and practical applications in quantum systems.

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.

Electronic States and Optical Transitions in an Asymmetric Quantum Dot Molecule

In the framework of adiabatic approximation the electronic states and direct interband absorption of light in the asymmetric double quantum dot molecule (QDM) having a shape of Cassini lemniscate revolution are discussed. Analytical expressions for the wave functions and energy spectrum of the electron in the QDM are derived. Nonmonotonic split and step-like behavior of the energy spectrum is revealed due to the possibility of the electron tunneling between quantum dots (QDs) in the molecule. The corresponding selection rules of quantum transitions for the direct interband absorption of light are obtained. The absorption edge characteristics depending on the QDM geometrical sizes are also revealed.

Coupling effects on photoluminescence of exciton states in asymmetric quantum dot molecules.

We present a theoretical study of photoluminescence from exciton states in InAs/GaAs asymmetric dot pairs, where interdot coupling is reached via magnetic field in the Faraday configuration. Electronic structure is obtained by finite element calculations, and Coulomb effects are included using a perturbative approach. According to our simulated spectra, bright excited states may become optically accessible at low temperatures in hybridization regimes where intermixing with the ground state is achieved. Our results show effective magnetic control on the energy, polarization and intensity of emitted light, and suggest these coupled nanostructures as relevant candidates for implementation of quantum optoelectronic devices.

Influences of strain on binding energies of excitons in InAs/GaAs quantum dot molecules

We investigate theoretically strain effects on bonding properties of excitons in both symmetric and asymmetric InAs/GaAs quantum dot molecules (QDMs). The dependences of exciton binding energies on interdot spacings (d) were systematically explored. A critical dot separation dc can be found where the binding energy reaches a minimum value. The decoupling of electron (hole) and the subsequent degeneracy of hole (electron) respectively lead to the minimum and maximum of binding energies in unstrained (strained) QDMs. The strain can significantly tune binding energies of excitons by changing the height of QD in symmetric QDMs. Furthermore, as compared to unstrained systems, strain results in the transition from direct to indirect excitons in asymmetric QDMs. The variation of binding energies in large asymmetric structures is 2 times higher than that in symmetric ones, showing the obvious modulation character by strain in non-symmetric QDMs.

Control of indirect exciton population in an asymmetric quantum dot molecule

Abstract We analyze the problem of coherent population transfer to the indirect exciton state in an asymmetric double semiconductor quantum dot molecule that interacts with an external electromagnetic field. Using the controlled rotation method, we obtain analytical solutions of the time-dependent Schrodinger equation and determine closed-form conditions for the parameters of the applied field and the quantum system that lead to complete population transfer to the indirect exciton state, in the absence of decay effects. Then, by numerical solution of the relevant density matrix equations we study the influence of decay mechanisms to the efficiency of population transfer.

Tuning of electron g factors by electric fields in asymmetric InAs quantum dot molecules

Most self-assembled quantum dot molecules (QDMs) are composed of dots of different sizes stemming from imperfect control of crystal growth. In this paper we use the intrinsic structural asymmetry to tune spin-polarized states by reversing electric fields. We calculate electron g factors of InAs/GaAs QDMs with inequivalent dots under external electric fields. The g factor in the asymmetric system exhibits remarkable tunability under the forward applied field. In contrast, it is not very sensitive to the backward electric field within the resonant region. Hence, different spin-polarized states can be switched by reversing an applied field. Furthermore, g factors for both bonding and anti-bonding states can be tuned to zero under the same positive field (6.6 mV nm−1) in the asymmetric QDM.

Singlet-Triplet Dephasing in Asymmetric Quantum Dot Molecules

Dephasing of singlet-triplet superpositions in twoelectron quantum dot molecules is important for a possible implementation of quantum information processing in semiconductor systems [1]. In a recent work [2], we showed that elastic phonon scattering via virtual transitions to doubly occupied states, which is only possible in a singlet configuration, induces distinguishability of spin configurations and, therefore, leads to pure dephasing of spin superpositions. In Ref. [2], we studied a system of two identical dots. However, as quantum dots are artificial systems, one has to take into account unavoidable inhomogeneity of dot parameters when modeling the properties of the system. Therefore, in the present contribution, we generalize our previous result and study the phonon-induced dephasing process in an asymmetric QDM. We show that in the asymmetric QDM, an additional dephasing channel appears, as compared to the symmetric one. Nonetheless, the dephasing rate is very weakly affected by the asymmetry unless the latter becomes very strong. The system under consideration is composed of two electrons in an asymmetric quantum dot molecule (QDM) built from two different gate-defined quantum dots [3, 4]. The electrons are coupled to phonons by the usual charge-phonon interactions (deformation potential and piezoelectric couplings). We do not take any spin-environment interactions into account (neither direct, with nuclear spins, nor indirect, via spin-orbit coupling). The Hamiltonian of the system is then

Dynamics of excitons and Coulomb beats in a quantum dot molecule

The authors present results for the energy spectrum and the dynamical behavior of excitons in an asymmetric quantum dot molecule. Two main resonances are found by using parameters from GaAs-based systems. Excitonic and binding energies are calculated as functions of an external electric field. They fully explore the dynamics of the exciton, showing the time evolution of both the electron and hole components. The effect of the Coulomb interaction is found to be quite dramatic and responsible for considerably large resonance fields as well as for some underlying beatings found in the main oscillation pattern.

Optical detection of asymmetric quantum-dot molecules in double-layer InAs/GaAs structures

Self-assembled quantum dots (QDs) in double-layer InAs/GaAs structures are studied by resonant photoluminescence and photoluminescence excitation spectroscopy. A weakly correlated (50%) double-layer system with an array of vertically coupled QDs (asymmetric quantum-dot molecules) was formed in a structure consisting of the 1.8-monolayer-thick first and the 2.4-monolayer-thick second InAs layers separated by 50 monolayers of GaAs. The nature of discrete quantum states in this system was studied and resonances corresponding to vertically coupled QDs were clearly observed for the first time.