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

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 ( $p,0\mathrm{}$ LO phonon) state and the continuum of either ( $s,1\mathrm{}$ LO phonon) or ( $s,2\mathrm{}$ 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.