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Wavelength-tunable open double-microcavity to enhance two closely spaced optical transitions

Microcavities have long been recognized as indispensable elements in quantum photonic research due to their usefulness for enhanced light extraction and light-matter interaction. A conventional high-Q cavity structure typically allows only a single optical transition to be tuned into resonance with a specific mode. The transition to a more advanced double-cavity structure, however, introduces new and interesting possibilities such as enhancing two spectrally close optical transitions at the same time with two distinct cavity modes. Here, we investigate a cavity structure composed of a monolithic planar cavity enclosed between two semiconductor distributed Bragg reflectors (DBR) and a top dielectric mirror deposited on a fiber tip. While the bottom cavity is formed by the two DBRs, the mirror on the fiber tip and the top DBR of the semiconductor chip create a second tunable cavity. These coupled cavities exhibit mode hybridization when tuned into resonance and their splitting can be adjusted to match with the spectral separation of closely spaced optical transitions by a suitable sample design. Furthermore, we report on the simultaneous resonance tuning of the exciton and biexciton transition of a semiconductor quantum dot, each to a separate mode of the open fiber-based double cavity. Decay time measurements at simultaneous resonance showed a Purcell-factor of $F_P^X$=1.9$\pm$0.4 for the exciton transition.