Vibrational Strong Light-Matter Coupling in an Open Microcavity Based on Reflective Germanium Coatings.

Open microcavities (OMCs) enable tuning of the optical resonances of a system and insertion of different materials between the mirrors. They are of large scientific interest due to their many potential applications. Using OMCs, we can observe strong light-matter coupling while tuning the cavity wavelength. Typically, dielectric Bragg reflectors (DBRs) and Au mirrors are used to form microcavities and observe vibrational strong coupling (VSC) in the middle-infrared (MIR) spectral region. Here, we make the mirrors of the OMC using thin film coatings of the semiconducting material germanium (Ge) and demonstrate VSC in the MIR region. We deposited a uniform coating of poly(methyl methacrylate) (PMMA) on one of the OMC mirrors' inner surfaces, and then we tuned the cavity to the carbonyl stretch mode resonance at 1731 cm -1 . Comparing VSC using Ge mirrors to DBRs or Au mirrors, we achieve enhanced optical transmission through the polaritonic resonances and large Rabi splitting, with Rabi-splitting values of 8.8 meV for the Ge mirror-based OMC compared to 7.0 and 7.4 meV for the DBR- and Au-based microcavities, respectively. The use of Ge mirror components can simplify the microcavity structure and offer a new and simple alternative for MIR semiconductor mirrors, which may be particularly useful for polariton chemistry applications.