Resonant fluorescence in a single quantum dot: coherent versus incoherent effects.

Communication oraleSingle photon emission is one of the most striking optical properties of semiconductor quantum dots (QDs), and the corresponding photon anti-bunching effect was observed by photoluminescence experiments in single QDs under non-resonant excitation [1], and more recently under strictly resonant excitation of the fundamental interband transition [2,3]. This configuration minimizes the residual excitation of the QD environment that acts as a fluctuating reservoir for spectral diffusion [4], and allows the generation of indistinguishable photons [3]. In this paper, we report on the resonant fluorescence of the neutral exciton in single InAs/GaAs QDs embedded in a planar microcavity [2]. We show in figure 1(a) the resonant Rayleigh scattering (RRS) spectrum as a function of the laser detuning with the neutral exciton line. We observe two Lorentzian lines of width 4μeV, separated by 9μeV and corresponding to two linear orthogonal polarizations, as expected for the fine structure splitting. We show in figure 1(b) a clear photon anti-bunching under strictly resonant excitation that proves that the RRS spectrum is dominated by resonant fluorescence of the neutral exciton. This situation is only rarely observed. The most frequent case corresponds to no resonance in the RRS spectrum and to a doublet in the non-resonant photoluminescence (PL) spectrum as shown in figure 2(a). The two lines are identified as the neutral and charged excitons by auto- and cross-correlation measurements of the g(2)(τ) function, as illustrated in figure 2(b). Strikingly, we observe the charged exciton line in the PL excitation spectrum of the neutral exciton line (figure 2(c)). This effect reveals the existence of residual charge carriers in the QD environment even under strictly resonant excitation. This favors incoherent population transfer between the charged and neutral excitons that competes with the coherent RRS effect and most frequently suppresses the resonant fluorescence of the neutral exciton. We conclude that the optical response of the QD remains affected by its environment even in case of coherent resonant pumping. [1] P. Michler, A. Kiraz, C. Becher, W. V. Schoenfeld, P. M. Petroff, L. Zhang, E. Hu, and A. Imamoglu, Science 290, 2282 (2000) [2] A. Muller, E. B. Flagg, P. Bianucci, X. Y. Wang, D. G. Deppe, W. Ma, J. Zhang, G. J. Salamo, M. Xiao, and C. K. Shih, Phys. Rev. Lett. 99, 187402 (2007) [3] S. Ates, S. M. Ulrich, S. Reitzenstein, A. Löffler, A. Forchel, and P. Michler, Phys. Rev. Lett. 103, 167402 (2009) [4] A. Berthelot, I. Favero, G. Cassabois, C. Voisin, C. Delalande, Ph. Roussignol, R. Ferreira, and J. M. Gérard, Nature Physics 2, 759 (2006