Phonon-Assisted Population Inversion of a Single Quantum Dot

Abstract

We demonstrate an approach to realize the population inversion of a single InGaAs/GaAs quantum dot, which is driven by a laser pulse tuned within the neutral exciton phonon sideband. The inversion is achieved by rapid thermalization of the optically dressed states via phonon-assisted relaxation. A maximum exciton population of 0.67 ± 0.06 is measured for a laser tuned 0.83 meV to higher energy and the phonon sideband is mapped using a two-color pump-probe technique. Our experiments reveal that, in accordance with theory, the phonon-bath provides additional functionality for an optically driven quantum dot qubit.It is a basic tenet of laser physics that a population inversion cannot be achieved through incoherent excitation of a two-level atom. At best, a laser pulse with duration longer than the coherence time T 2 can only drive the system to the transparency point where the populations of the upper and lower levels are equal [1]. However, if the two-level atom is coupled to a vibrational continuum, it has been predicted that inversion can be possible even in the incoherent regime through the interaction of the dressed states with the Boson bath [2]. Excitons in semiconductor quantum dots (QDs) form a near ideal system for investigating these effects, since their behavior approximates well to that of a two-level atom [3], while their coupling to the acoustic phonons in the crystal provides a mechanism to thermalize the dressed states.The possibility of creating population inversion in QDs through phonon coupling was first investigated for microwave-driven electrostatic quantum dots [4]. Recently, it has been demonstrated that the conditions for population inversion can be met via a microwave Raman effect [5,6]. Theoretical work has indicated that similar effects should be possible for optically-driven excitons [7][8][9][10]. The underlying mechanism is the coupling of the excitons to longitudinal acoustic (LA) phonons through the deformation potential [11], which generates sidebands in the excitonic spectra [12] that can also be observed in four-wave mixing [13] and resonance fluorescence experiments [14], as well as through off-resonant coupling of excitons to nano-cavities [15,16]. In a strong driving field regime evidence for phonon induced relaxation between optically dressed states is observed in the intensity damping of Rabi rotations [17][18][19], and more recently in adiabatic rapid passage experiments [20,21].In this letter we report a population inversion of the excitonic two-level system of a single InGaAs/GaAs QD under strong laser pumping at positive detuning with the driving laser to higher energy than the QD zero phonon transition. We compare the results to the theoretical predictions of Ref. [9]. The population inversion is achieved in an incoherent regime where the dephasing time is shorter than the laser pulse duration. Pump-probe measurements are presented, where the phonon-assisted population inversion is observed clearly as a gain-like dip in the photocurrent absorption spectrum. Furthermore...