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Journal of Contemporary Physics (Armenian Academy of Sciences)

Spontaneous Emission of a Semiconductor Quantum Dot without the Dipole Approximation

G. A. Muradyan, L. R. Arzumanyan, A. Zh. Muradyan

Published Oct 1, 2020• 0 citations
Publisher

Abstract

Abstract The spontaneous emission of a spherical semiconductor quantum dot is considered without using the dipole approximation. It is shown that the dipole-forbidden radiation of a shallow potential well with one electron level becomes more intense than the dipole-permitted radiation of a deeper potential well and that the potential deepening makes the dipole approximation closer. It is also shown that the cylindrically symmetric anisotropic radiation can be obtained from a spherical quantum dot if the direction of the quantization axis remains unchanged for the acts of photon emission.

DOI
10.3103/s106833722004012x

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Abstract The spontaneous emission of a spherical semiconductor quantum dot is considered without using the dipole approximation. It is shown that the dipole-forbidden radiation of a shallow potential well with one electron level becomes more intense than the dipole-permitted radiation of a deeper potential well and that the potential deepening makes the dipole approximation closer. It is also shown that the cylindrically symmetric anisotropic radiation can be obtained from a spherical quantum dot if the direction of the quantization axis remains unchanged for the acts of photon emission.
We investigate spontaneous emission from excitons in quantum dots beyond the dipole approximation and show how the symmetry of the exciton wavefunction plays a crucial role. We show explicitly that for spherically symmetric excitons, the Purcell effect is independent of the exciton size and is governed by the local density of optical states at the center of the exciton only, which is identical to the result derived with the dipole approximation. This surprising result is a spontaneous emission counterpart to the shell theorem of classical mechanics and electrostatics and provides new insights to the physics of mesoscopic emitters as well as great simplifications in practical calculations.
We investigate the collective spontaneous emission in a system of two identical quantum dots (QDs) strongly coupled through the dipole-dipole ($d\ensuremath{-}d$) interaction. The QDs are modeled as two-level quantum objects, while the $d\ensuremath{-}d$ interaction is described as the exchange of a virtual photon through the photonic reservoir. The master equation approach is used in the analysis. The main attention is focused on antenna characteristics of the two-QD system---the radiation intensity dependence on the meridian and azimuthal angles of observation. We show that the radiation pattern of such a system is nonstationary and its temporal behavior depends on the initial quantum state. In particular, for entangled initial states the radiative pattern exhibits oscillations on the frequency which corresponds to the $d\ensuremath{-}d$ interaction energy. We also analyze spectral properties of the directional diagram. The comparison of radiation patterns is carried out for two QDs and two classical dipoles. The concept of quantum nanoantenna is proposed based on collective spontaneous emission in QD ensembles.
We investigate spontaneous emission from excitons beyond the point source dipole approximation and show how the symmetry of the exciton wave function plays a crucial role. We find that for spherically symmetric wave functions, the Purcell effect is independent of the wave function size and therefore is given exactly by the dipole approximation theory. This surprising result is a spontaneous emission counterpart to the shell theorems of classical mechanics and electrostatics and provides insights into the physics of mesoscopic emitters as well as great simplifications in practical calculations.
Spontaneous emission from two two-level atoms within an anisotropic photonic crystal is investigated when direct dipole-dipole interaction is taken into account. The radiation properties of the present two atoms system are similar to that of a V-type three-level atom embedded in an anisotropic photonic crystal. The direct dipole-dipole interaction can enhance or depress the spontaneous emission depending on the relative position of the upper levels from the band edge of the photonic crystals. Owing to quantum interference and the dipole-dipole interaction, the spontaneous emission spectra are complicated non-Lorentzian shapes and depend strongly on the relative position of the upper levels from the bandgap and the initial state of the system.
We analyze the properties of spontaneous emission of a two-level atom in anisotropic media where the geometry of physical dispersion relations is characterized by an ellipsoid or a hyperboloid. Within the framework of quantum optics the rate of spontaneous emission in the above media is explicitly given with the orientation of the dipole transition matrix element taken into account. It indicates that for the ellipsoid case the intensity of the photons coupled into different modes can be tuned by changing the direction of the matrix element relative to the optical axis. For the hyperboloid case it is found that spontaneous emission in the hyperbolic medium (HM) can be dramatically enhanced compared with the case in the background medium. Moreover, in the HM the spontaneous emission exhibits strong directivity and gets the maximum in the asymptote direction.
Semiconductor quantum dots coherently driven by pulsed laser are fundamental physical systems which allow studying the dynamical properties of confined quantum states. These systems are attractive candidates for a solid-state qubit, which open the possibility for several investigations in quantum-information processing. In this work we study the effects of a specific decoherence process, the spontaneous emission of excitonic states, in a quantum dot molecule. We model our system considering a three-level Hamiltonian and solve the corresponding master equation in the Lindblad form. Our results show that the spontaneous emission associated with the direct exciton helps to build up a robust indirect exciton state. This robustness against decoherence allows potential applications in quantum memories and quantum gate architectures. We further investigate several regimes of physical parameters, showing that this process is easily controlled by tuning of external fields.