Explanation of Photon Correlations in the Far-Off-Resonance Optical Emission from a Quantum-Dot–Cavity System

Abstract

In a coupled quantum-dot nano-cavity system, the photoluminescence from an off-resonance cavity mode exhibits strong quantum correlations with the quantum dot transitions, even though its autocorrelation function is classical. Using new pump-power dependent photon-correlation measurements, we demonstrate that this seemingly contradictory observation that has so far defied an explanation stems from cascaded cavity photon emission in transitions between excited multiexciton states. The mesoscopic nature of quantum dot confinement ensures the presence of a quasicontinuum of excitonic transitions part of which overlaps with the cavity resonance.A quantum dot (QD) coupled to a photonic crystal cavity provides a promising system for studying cavity quantum-electrodynamics (QED) in the solid state [1,2]. In contrast to their atom-based counterparts, these systems exhibit features that arise from their complex environment. A common effect that surfaced in previous experiments is strong off-resonant emission of a cavity mode (CM) containing one or multiple QDs. Photon correlation measurements revealed that the cavity-mode emission is anti-correlated with the QD excitons at the level of single quanta, proving that cavity feeding is mediated solely by a single QD [3,4]. Surprisingly, however, the photon stream emitted by the far off-resonant CM did not show any significant quantum correlations. Previous experimental [5,6,7] and theoretical [8,9] investigations have focused on explaining cavity feeding in terms of dephasing of the QD excitons mediated either by coupling to acoustic phonons or to free carriers. However, all of the attempts to describe cavity feeding using Markovian dephasing of the fundamental exciton line fail to explain the above mentioned photon correlation signatures that appear to be true for all studied QD cavity-QED systems.In this Letter, we unequivocally demonstrate that the far off-resonant excitation of the CM is solely due to the mesoscopic nature of quantum dot confinement, which in turn leads to an energetically broad cascaded emission of the QD. In this setting, cavity feeding and its photon correlation signatures can be regarded as an intrinsic feature of QD-cavity systems that arises from the complicated QD multi-exciton level structure. We carry out pump-power dependent photoluminescence (PL) as well as photon auto-and cross-correlation measurements on a nano-structure incorporating a single QD embedded in a photonic crystal (PC) defect cavity [3]. To explain our experimental observations, we develop a new theoretical model for the QD-cavity system, perform numerical calculations of its semi-classical dynamics and compare its predictions with the new experimental findings. While a quantitative comparison between numerical and experimental results is intrinsically difficult, the qualitative agreement we achieve is excellent. In particular, the unusual correlation features found experimentally are naturally reproduced by the model and the simulations.Before proceeding, we remark that acoustic...

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