Strong Electron-Phonon Coupling Regime in Quantum Dots: Evidence for Everlasting Resonant Polarons

Abstract

Using far-infrared magnetospectroscopy in self-assembled InAs quantum dots, we have investigated the electronic transitions from the ground s levels to the excited p levels. The experiments consist of monitoring, by means of Zeeman tuning of the excited level, a resonant interaction between the discrete ( p, 0 LO phonon) state and the continuum of either (s, 1 LO phonon) or (s, 2 LO phonons). We show that the electrons and the LO phonons are always in a strong coupling regime and form an everlasting mixed electron-phonon mode. PACS numbers: 73.40.Kp, 73.20.Dx, 78.20.Ls Electrons in excited atomic states can relax towards lower lying levels by spontaneous emission of photons. Artificial atoms like semiconductor quantum dots display discrete levels. For electrons (or holes) placed in excited levels the spontaneous emission of photons is inefficient for the relaxation due to the characteristic energy splitting of the dot states (ϳ50 meV in a ϳ20 nm dot). The carriers bound to these artificial atoms are however in interaction with phonons which display a continuum of finite width, unlike photons. It has been shown that the intradot relaxation through acoustical phonons is totally inefficient, the energy mismatch between electron states being much too large [1,2]. In semiconductors, the most powerful energy relaxation channel is (by far) the irreversible emission of longitudinal optical (LO) phonons through the Fröhlich coupling. Despite its effectiveness, this electron-phonon coupling is weak, to the extent that the initial discrete level (e, 0 phonon) irreversibly decays into the continuum (g, 1 phonon) where e and g, respectively, denote an excited state and the ground electronic state. Such a weak coupling is very well described by the Fermi golden rule in bulk, quantum well (2D) or quantum wire (1D) structures. Because the optical phonons show very little dispersion, it has been argued that the LO phonon assisted relaxation in semiconductor quantum dots could be efficient only if the energy separation between the electronic states differs by one (or several) LO phonons. Here we present experimental evidence supported by theoretical modeling that the very idea of an electron emitting LO phonons and relaxing irreversibly to a less excited state (as in bulk, 2D, and 1D heterolayers) is wrong in a quantum dot. What happens in reality is that the electrons and the LO phonons are in a strong coupling regime and form everlasting mixed electron-phonon modes, as recently suggested by Inoshita and Sakaki in the case of one phonon [3]. Using far-infrared (FIR) magnetospectroscopy, we have investigated the g ! e transition in self-assembled doped InAs quantum dots. The experiments consist of monitoring, by means of Zeeman tuning of the dot excited level e, a resonant interaction between the discrete (e, 0 LO phonon) state and the continuum of either (g, 1 LO phonon) or (g, 2 LO phonons). We show that the (e, 0 LO phonon) state does not dissolve when entering into the continuum but forms a hybrid mode with (g, 1, or 2 LO p...

One- and two-phonon capture processes in quantum dots

“…In this article, we concentrate on QD structures ͑GaAs/AlGaAs, 22 for instance, and QD laser structures 1-3 ͒, where the material characteristics that define the spatial distribution of phonon modes do not differ significantly in the QD and the barrier material. For such structures, the bulk model for phonons has provided a good description of dephasing rates, 22,23 polaron effects, 24 and capture processes. 7 Note, also, that LO-phonon carrier relaxation rates in quantum wells do not differ significantly, when calculated by either using bulk or confined LO phonons.…”

Optophononics with coupled quantum dots

“…In the following, we verify this phonon-induced Fano effect and demonstrate the molecular polaron and its characteristics as an optophononic transparency. Similar in nature, resonant polaron states were described for the regime of cyclotron resonance 29,31 and in multiquantum wells 30 .…”

“…The concept of a polaron was first introduced by Landau as a quasi-particle formed by a charge and the deformation of the lattice it causes 28 . Particularly favourable are resonant polaron states as they can exhibit very long coherence times [29][30][31] . Here, a novel resonant polaron, a molecular polaron in a QDM, is being introduced.…”

The role of phonons for exciton and biexciton generation in an optically driven quantum dot

“…The carriers in the QD also interact with optical phonons, e.g., via the Fröhlich interaction [57,58,59,60], which under certain conditions can also contribute to the generation of biexcitons [61]. In contrast to acoustic phonons, optical phonons, in particular those which are efficiently coupled to the electrons and holes in a QD, can typically be taken to be dispersionless.…”