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...

The discretization of the electronic spectrum in semiconductor quantum dots implies a strong coupling behavior between the optical phonons and the electron-hole pairs, despite the fact that a pair is electrically neutral. The excitonic polarons strongly modify the optical spectra. In particular, the ground excitonic polaron contains one or two phonon components, which leads to the existence of phonon replicas in the luminescence. The population and coherence decay times of the optical transition associated with the ground excitonic polaron are calculated.

We show that the anharmonicity driven instability of optical phonons leads in semiconductor quantum dots to a decay of polaron states which otherwise would be everlasting. By such a mechanism a single electron in an excited dot state can relax down to the ground state even if the electron energy difference markedly differs from the optical phonon energy.

No abstract

No abstract