Unveiling the spin-singlet states of two electron-hole pair complexes using two-photon excitation in a GaAs/AlAs quantum dot

We use two-photon excitation to create biexcitonic complexes with two electron-hole pairs in different orbital levels of a GaAs/AlAs quantum dot. In addition to p-shell emission of the biexcitonic triplet states generated by two-photon excitation, we observe additional higher-energy resonances which are a signature of the radiative cascade of two-photon excited singlet states. The detection of these signals obtained in a high excitation regime is made possible by the use of a waveguiding structure in which the quantum dots are inserted, allowing an orthogonal excitation and detection geometry with an excellent laser rejection.