Strong interactions between excitons and photons, e.g. in a semiconductor microcavity, lead to the formation of hybrid light-matter quasiparticles called exciton-polaritons. In recent years, polaritons have attracted special attention as their bosonic character features new emergent phenomena like non-equilibrium condensation or superfluidity. Most of these seminal experiments were performed by using inorganic semiconductor microcavities (based on e.g. the GaAs material system). This requires the use of low-temperature facilities owing to the instability of Wannier-Mott excitons at elevated temperatures. In contrast to this, Frenkel excitons, characteristic of organic semiconductors, possess much larger binding energies and are stable at room temperature, making polariton experiments at ambient air conditions feasible. Organic materials further exhibit very large oscillator strengths and thus strongly interact with a cavity field. However, the implementation of organic semiconductors in optical microcavities is challenging because organic materials are very sensitive to the depositing of semiconductor layers on top of them. Circumventing these issues, we use an open cavity system, which makes non-invasive investigation of the active material possible. Open cavities are tunable systems and comprise a bottom semiconductor distributed Bragg reflector (DBR) with the active material (the organic semiconductor) on top and a concave top DBR separated by a micrometer sized air gap. This configuration allows a 3D photonic confinement and brings unprecedently high quality factors into reach. The concave top DBRs were prepared by focused ion beam (FIB) milling on Si02 substrates and subsequent deposition of a DBR structure. The organic material was spin-coated onto the plane bottom DBR that consists of the same DBR layout as the top mirror. Both mirrors are attached to nanopositioners allowing the spectral tuning of cavity modes by changing the mirror distance. We demonstrate the versatility of open cavities by performing reflectivity and photoluminescence measurements in Fourier imaging configuration and investigate the strong exciton-photon coupling between different organic systems (J-aggregates, proteins and cyanines) and the dielectric cavity. We emphasize that the open cavity approach can easily be extended to more complex active regions including two-dimensional monolayer materials or hybrid organic-inorganic bilayers.
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