Résumé
Motional narrowing refers to the striking phenomenon where the resonance line of a system coupled to a reservoir becomes narrower when increasing the reservoir fluctuation. I will present our recent data on the experimental evidence of motional narrowing in the optical spectrum of a semiconductor quantum dot broadened by the spectral diffusion phenomenon [1]. Semiconductor quantum dots are often referred to as artificial atoms in solid state and the decoherence dynamics is a sensitive probe of the influence of the environment on the quantum dot optical and electronic properties. In fact, impurities, defects or localized charges in the vicinity of a quantum dot induce micro-electric fields that shift the quantum dot emission line through the quantum confined Stark effect. The fluctuation of the quantum dot environment thus randomize the emission frequency and result in the so-called spectral diffusion effect. I will present the experimental evidence of a motional narrowing regime for the spectral diffusion of a single InAs/GaAs quantum dot. High-resolution Fourier-transform spectroscopy on the photoluminescence intensity of a single InAs/GaAs quantum dot brings the evidence of a crossover from Gaussian to Lorentzian line profiles on decreasing incident power or temperature. This observation shows that motional narrowing is surprisingly achieved on decreasing incident power or temperature, in contrast with the standard phenomenology observed for nuclear magnetic resonance. I will discuss some consequence of this non-standard type of motional narrowing on the dephasing dynamics in semiconductor quantum dots [2]. [1] A. Berthelot, I. Favero, G. Cassabois, C. Voisin, C. Delalande, Ph. Roussignol, R. Ferreira, and J. M. G´erard, Nat. Phys. 2, 759 (2006). [2] I. Favero, A. Berthelot, G. Cassabois, C. Voisin, C. Delalande, Ph. Roussignol, R. Ferreira, and J. M. G´erard, Phys. Rev. B 75, 073308 (2007).