The semiconductor quantum dot - microcavity pillar system represents an attractive platform for studying fundamental light-matter interaction as well as for demonstrating novel quantum devices, ultra-low threshold lasers and sub-ps optical switching. In this work we present a novel tapered GaAs/AlAs micropillar design where Bloch-wave engineering is employed to significally enhance the cavity mode confinement in the submicron diameter regime. We demonstrate a record-high vacuum Rabi splitting of 85 µeV of the strong coupling for pillars incorporating quantum dots with modest oscillator strength f ≈ 10. It is well-known that light-matter interaction depends on the photonic environment, and thus proper engineering of the optical mode in microcavity systems is central to obtaining the desired functionality. In the strong coupling regime, the visibility of the Rabi splitting is described by the light-matter coupling constant g proportional to Q/√V, where Q is the quality factor and V is the mode volume. A high Q and a low V are thus desirable.The mode volume V can be minimized by reducing the pillar diameter. However, for the standard micropillar design, the poor mode matching between the cavity mode and the DBR Bloch mode limits the Q to about 2000. [1] In our optimized design we have replaced the standard λ-spacer with a 3 segment tapered region. The layer thicknesses of these GaAs/AlAs segments are gradually reduced towards the center, effectively detuning the bandgap relative to that of the DBRs and allowing for a single localized mode inside the cavity. The fundamental Bloch mode experiences an adiabatic transition, leading to an improved mode matching and a reduced coupling to propagating Bloch modes in the DBRs. The central GaAs layer incorporating quantum dots is only 60 nm thick corresponding to ≈ λ/5, and regular cavity concepts are thus insufficient to explain the localization of the cavity mode, demonstrating the necessity of Bloch-wave formalism in the analysis of the design.We compare our adiabatic design to a reference incorporating a λ-spacer. A theoretical improvement of Q of two orders of magnitude and an experimentally measured improvement of ≈ 5, limited by fabrication imperfections, are obtained. Thus our novel approach allows us to demonstrate remarkably high quality factors exceeding 10,000 for MP cavities with diameters below 1 µm. [2] Whereas previous studies of strong coupling in micropillars relied on quantum dots with high oscillator strengths f 50, our advanced design allows for the observation of strong coupling for submicron diameter quantum dot-pillars with standard f ≈ 10 oscillator strength. A quality factor of 13600 and a vacuum Rabi splitting of 85 µeV are observed for a small mode volume micropillar with a diameter of 850 nm.
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