A study of the excitonic states in artificial molecules set up of two vertically coupled double quantum dots is presented. The electron and hole eigenstates are calculated for the quantum dot molecule. In particular, the coupling effect of the barrier and the consequent tunneling is analyzed following the evolution of the absorption spectra as function of the distance between the two dots. On the other hand, the role of the Coulomb correlation between the confined particles is also studied. We present results of relevant interactions in these systems and discuss how the optical properties of double quantum dots are affected by the interdot coupling, by the geometry of the dots and by an applied electric field.
The cover picture of this issue of physica status solidi (c) has been taken from the article [1].
We study the excitonic polaron formation in two InAs/GaAs coupled quantum dots. We calculate the coupling between the exciton and the LO-phonon states by using the Fröhlich Hamiltonian, from which we determine the excitonic polaron states varying the quantum dot separation. We study the dependence on the excitonic polaron formation with the coupling between the dots. Polaron formation strongly modifies the energy spectra due to the appearance of several anticrossings in the excited states.1 Introduction Various experimental and theoretical results have shown that in InAs/GaAs self assembled quantum dots (QDs) the electron are strongly coupled to longitudinal optical (LO) phonons [1,2]. The so-called electron polarons are formed due to this interaction. Many optical experiments performed on QDs involve intraband optical transitions and probe directly the polaron levels instead of purely electronic states [2][3][4]. Analogously, Far-infrared (FIR) intraband magneto-optical experiments evidence the formation of hole polarons [5]. Optical interband transitions between valence and conduction QD states include Coulomb-correlated electron-hole pairs (in short, excitons) which are electrically neutral. The coupling between LO phonons and excitons could be small since this coupling is basically electrical (Fröhlich interaction). However, recent experimental and theoretical works have demonstrated that excitons in QDs strongly couple to LO phonons in spite of their electrical neutrality [6,7]. The eigenstates of the interacting exciton and phonon systems are called excitonic polarons. In this Letter, we focus on the excitonic polaron formation in a quantum dot molecule (QDM) i. e., two coupled quantum dots. We calculate the coupling between the exciton and the LO-phonon states by using the Fröhlich Hamiltonian, from which we determine the excitonic polaron states. Polaron formation strongly modifies the optical spectra due to the appearance of several anticrossings in the excited states. We study the dependence on the excitonic polaron formation with the coupling between the dots.
The THz nonlinear absorption of asymmetric double quantum wells is calculated. The dependence of the absorption on different parameters of the quantum wells is studied. In particular, different values of the barrier and wells width are considered. Also, a DC field in the direction of growth of the wells is introduced in the calculations and the nonlinear absorption is obtained for different DC and AC field intensities. The nonlinear absorption is obtained by using a perturbation calculation of the nonlinear susceptibility up to the third order within a density matrix approach. The absorption spectra are studied near the anticrossing of the first two excited sub-bands. Special attention is given to the nonlinear peaks due to the intensities of both fields. In particular some interesting negative nonlinear peaks are found and their dependence on the intensities of the fields is discussed. IntroductionThe terahertz band is of great importance due to the rich physical and chemical processes that occur in that region and the lack of sources for applications. Quantum well semiconductors are model systems for the study of the light-matter interaction. West and Eglash have observed large oscillator strengths associated with intersubband transitions [1]. Later other authors predicted that asymmetric quantum well would display giant second-order susceptibilities [2] and it was supported by measurements [3]. In addition large third order susceptibilities in asymmetric wells were observed [4]. More recently Adriano A. Batista et. al. presented a complete study of non linear dynamics in a triple well obtaining a region of optical bistability [5]. In this work we report a systematic study of the role of the geometric interaction (barrier width) between two 2D diluted electron gases confined in an asymmetric double quantum well separated by a barrier (see Fig. 1), which plays the role of coupling the two wells. We calculate the transition energies, linear and nonlinear absorption coefficients for these devices. Then we add a second interaction to the system by applying a external static electric field, which can be varied many orders of magnitude because an exact calculation method is used [6]. Furthermore, we study the dependence of the absorptions spectra on the intensity of the electromagnetic radiation in order to obtain and discuss the nonlinear steady state absorption.
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