Exciton Rabi Oscillation in Single Quantum Dot

Hidehiko Kamada and Hideki Gotoh
Physical Science Laboratory

    Optical Rabi oscillation is the most fundamental examples of coherent nonlinear light-matter interactions. It is the essential physics that coherent optical control of the quantum states relies on. Excitons in semiconductor quantum dots (QD's) are above all promising elements for such coherent manipulation because of long-lived coherence favored by discrete energy level structure. We observed the exciton Rabi oscillation in QDs.
    In a single isolated InGaAs dot, an excited state resonance was chosen as upper level of relevance. Upon resonant excitation, the exciton emission was found to split into doublet for increasing excitation density [1]. This splitting increased proportional to the electric field as expected for Rabi splitting (Fig. 1 left). To observe the oscillation in time-domain, a single-dot exciton dipole interferometry was then conducted. Under low excitation, the coherent dipole oscillation induced by the first pulse persists as long as 40 ps: Correspondingly the dipole interference fringe decay with this time constant was observed (Figs. 1(a) and 1(b)). This demonstrates a long-lived exciton coherence. As the excitation increased more than one order of magnitude, the radiation induced not only the dipole oscillation but also the population oscillation, leading to a mixing of the Rabi frequency into the dipole interference fringe (Fig. 1(d)) as well as another oscillatory behavior with a period of 10-20 ps in the fringe envelope (Fig. 1(c)). The frequency of this slowly varying oscillation was found to increase proportional to pulse area, manifesting the relevance of the Rabi oscillation [2]. Observation of the coherent population flopping promises the coherent control of the quantum states in semiconductor QDs.

[1] H. Kamada et al., ICPS25, Osaka, (2000) H169.
[2] H. Kamada et al., CLEO Pacific Rim 2001, Chiba, (2001) ThG4-3.

Fig. 1. Energy splitting of exciton PL for increasing excitation density (left). Exciton interference as functions of pulse-pair interval (right): (a) fringe envelopes for a power density P1 (0.067 μJ/cm2 /pulse) and (b) decomposed fring near 30 ps, (c) envelopes for 12 P1 (0.8 μJ/cm2 /pulse) and (d) fringe near 0 ps.

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