Second-harmonic generation resonant with s-p transition in InAs/GaAs self-assembled quantum dots

We have investigated the second-harmonic generation with the conduction states of InAs/GaAs self-assembled quantum dots. The harmonic generation is resonant with intersublevel transitions, and in particular with the $s\ensuremath{-}p$ transition. This transition, polarized along the [110] and the [-110] directions, exhibits a large dipole matrix element (3 nm). The frequency doubling is achieved around 20 \ensuremath{\mu}m wavelength using the $s\ensuremath{-}p$ and the $p\ensuremath{-}d$ transitions. The double resonance leads to a resonant enhancement of the susceptibility with a linewidth lower than 1 meV. We show that the polarization dependence of the susceptibility follows the polarization selection rule of the intersublevel transitions. The susceptibility amplitude is deduced by comparison with phase-matched second-harmonic generation in bulk GaAs. A susceptibility as large as $2.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\mathrm{m}/\mathrm{V}$ for one quantum dot plane is measured. This susceptibility is four orders of magnitude larger than the susceptibility of bulk GaAs.