Microcavity devices exhibiting strong light-matter coupling in the mid-infrared spectral range offer the potential to explore exciting open physical questions pertaining to energy transfer between heat and light and can lead to a new generation of efficient wavelength tunable mid-infrared sources of coherent light based on polariton Bose-Einstein Condensation. Vibrational transitions of organic molecules, which often have strong absorption peaks in the infrared and considerably narrower linewidths than organic excitonic resonances, can generate polaritonic states in the mid-infrared spectral range using microcavity devices.

Here, narrow linewidth polaritonic resonances are exhibited in the mid-infrared by coupling the carbonyl stretch vibrational transition of a polymethyl methacrylate (PMMA) film to the photonic resonance of a low optical-loss mid-infrared microcavity, which consisted of two Ge/ZnS dielectric Bragg reflectors. Rabi-splitting of 14.3 meV is observed, with a 4.4 meV polariton linewidth at anti-crossing. The large Rabi-splitting relative to linewidth indicates efficient impedance-matching between the bare vibrational and photonic states.

Furthermore, polariton states from coupling a mixture of two vibration modes to the microcavity resonance are revealed using organic film composed from solid PMMA and liquid dimethylformamide (DMF). The solid and liquid components, both possess spectrally narrow carbonyl stretch peaks, resulting in three branches in the polariton dispersion relation. The upper branch (UB) is composed largely of the PMMA phonon and photon. The middle branch (MB) contains all three components, and the lower branch (LB) is mostly the DMF phonon mixed with the photon. Rabi splitting of 9.6 meV and 5.2 meV is found between the UB and the MB, and between the MB and LB, respectively. The sum of the Rabi splitting values, 14.8 meV, is similar to that of the PMMA cavity splitting.

Molecular-vibration polaritons incorporated in dielectric microcavities can be an enabling step towards realizing polariton optical switching, polariton condensation, and new hybrid coupled states of light and matter in both solid and liquid form, in the mid-infrared spectral range.