Characterization of Strong Light-Matter Coupling in Semiconductor Quantum-Dot Microcavities via Photon-Statistics Spectroscopy

Abstract

It is shown that spectrally resolved photon-statistics measurements of the resonance fluorescence from realistic semiconductor quantum-dot systems allow for high contrast identification of the twophoton strong-coupling states. Using a microscopic theory, the second-rung resonance is analyzed and optimum excitation conditions are determined. The computed photon-statistics spectrum displays gigantic, experimentally robust resonances at the energetic positions of the second-rung emission.PACS numbers: 42.50. Pq, 42.50.Ar, 78.67.Hc, 78.90.+t Even though light-quantization effects, such as squeezing [1], antibunching [2], and entanglement [3], have been observed under a wide range of conditions, one can rarely detect the discrete energy levels of the different photonnumber states directly as resonances. Under strongcoupling conditions between a two-level resonance at the energy ω and the quantized resonant light field, the resulting dressed-state resonances appear at ωn±g √ n + 1 where n is the occupation of the photon-number state |n and g is the light-matter coupling constant [4]. The resulting discrete energy structure resembles a ladder with n-dependent rungs. The first rung shows the so-called vacuum Rabi splitting 2g, the second rung shows the splitting 2g √ 2, and so on. For sufficiently small damping and dephasing, the different rungs are clearly visible as discrete resonances, e.g., in transmission spectra.The first observations of strong coupling [5,6] and the second rung [7] were reported for atoms in high-quality cavities. Besides the demonstration of the discrete nature of light, this research has lead to major advances, e.g. in utilizing entanglement effects as a basis for quantum-logic applications [8,9]. The clear identification of the secondrung resonance can be considered as the critical step if one wants to use other materials, such as solid-state systems for strong-coupling applications. A promising candidate are semiconductor quantum dots (QD) in microcavities. Recent experiments [10,11,12,13] have already demonstrated that one can see clear vacuum Rabi splitting effects in the photoluminescence (PL) of such systems. However, despite continuing attempts, the second rung resonance has not yet been observed. The most likely reason for this failure is that dephasing and the other broadening effects in the real systems smear out the expected discrete features.In this Letter, we propose a novel scheme to observe the second-rung strong coupling effects even under the realistic broadening conditions of the currently available samples. In our approach, we use a fully microscopic theory to determine the exact conditions to obtain unambiguous evidence for the presence of the second-rung resonance in QD microcavities. Our results confirm the difficulty of the second-rung observations in standard semiconductor experiments that monitor PL after carrier capture of electrons, initially created to the wetting layer. However, we demonstrate that it should be completely feasible to observe the second rung in re...

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