II-VI semiconductor quantum dots with high confinement potentials and large Coulomb correlation energies provide promising systems for quantum optical experiments. Recently we were able to observe ultrafast Coulomb renormalization and single photon gain in negatively charged CdSe/ZnSe dots [1]. Here we report on two-color femtosecond pump-probe measurements on a single self-assembled CdSe/ZnSe quantum dot. The fundamental trion (X-) emits at 2.168 eV. Two additional emission lines at 2.160 eV and 2.156 eV (XX1- & XX2-) are identified as charged biexcitons via polarization resolved PL spectroscopy. PLE measurements and theoretical calculations [2] enable us to identify the two lowest energetic resonances as the 1D_3/2 to 1S_1/2 and the 1P_3/2 to 1P_1/2 transitions. The system was resonantly pumped on the 1P_3/2 to 1P_1/2 transition with a 660 fs laser pulse of 5 meV spectral width. The charge carrier dynamics of the system was spectro-temporally investigated across both the trion line and the biexcitonic emission lines with a probe pulse duration of 200 fs and a spectral width of 17.8 meV. Differential transmission spectra for time delays between -100 ps and +200 ps were aquired. For negative delays an absorption feature at the position of the trion line is clearly identified. This consists of the perturbed free induction decay component and possibly fine-structure splitting due to a long-lived nuclear spin polarization, which is currently under investigation. After hot exciton creation by the pump, for positive time delays the trion resonance exhibits an initial bleaching followed by stimulated emission, which occurs after relaxation to the ground state [1]. Additionally, negative pump-probe features emerge on the low-energy side of the X- line. We attribute those to an induced absorption into the biexciton states, switched on by the Coulomb renormalization of the X- ground state due to the excitation by the pump. This picture is corroborated by matching transient absorption and photoluminescence emission lines. Exciting the system on the 1D_3/2 to 1S_1/2 transition instead leads to a suppression of the absorption lines due to the full occupation of the S-level.    

References:

[1] F. Sotier, T. Thomay, T. Hanke, J. Korger, S. Mahapatra, A. Frey, K. Brunner, R. Bratschitsch, and A. Leitenstorfer, “Femtosecond fewfermion dynamics and deterministic single-photon gain in a quantum dot”, Nature Phys. 5, 352-356 (2009).

[2] J. Huneke, I. D'Amico, P. Machnikowski, T. Thomay, R. Bratschitsch, A. Leitenstorfer, and T. Kuhn, “Role of Coulomb correlations for femtosecond pump-probe signals obtained from a single quantum dot”, Phys. Rev. B 84,115320(2011).