Ann. Phys. (Berlin) 528, No. 3–4, 313–320 (2016) / DOI 10.1002/andp.201500282 Strong light-matter coupling between a molecular vibrational mode in a PMMA flm and a low-loss mid-IR microcavity Merav Muallem, Alex Palatnik, Gilbert D. Nessim, and Yaakov R. Tischler ∗ Received 8 September 2015, revised 24 September 2015, accepted 6 October 2015 Published online 19 November 2015 Microcavity devices exhibiting strong light-matter cou- pling in the mid-infrared spectral range ofer the poten- tial to explore exciting open physical questions pertain- ing to energy transfer between heat and light and can lead to a new generation of efcient wavelength tunable mid- infrared sources of coherent light based on polariton Bose- Einstein Condensation. Vibrational transitions of organic molecules, which ofen 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 flm to the photonic resonance of a low optical-loss mid-infrared microcavity, which consisted of two Ge/ZnS dielectric Bragg refectors. Rabi-splitting of 14.3 meV is observed, with a 4.4 meV po- lariton linewidth at anti-crossing. The large Rabi-splitting relative to linewidth indicates efcient impedance-matching between the bare vibrational and photonic states, and sug- gests molecular-vibration polaritons incorporated in dielec- tric microcavities can be an enabling step towards realizing polariton optical switching and polariton condensation in the mid-infrared spectral range. 1 Introduction For over 15 years, the excitonic resonances of organic materials have been used to study strong light-matter coupling in the visible spectral range [1, 2]. By coupling thin films of organic dye molecules, polymers and/or J-aggregates to planar optical cavities [3–10], researchers have demonstrated exciton-polariton states with gi- ant Rabi-splitting at room temperature [2], polariton electroluminescence [7], the ultra-strong coupling regime [11], polariton lasing [12] and Bose–Einstein condensate (BEC) [13], novel long-range energy transfer phenomena [14], 3-way exciton-polarition hybridiza- tion between molecular and Mott excitons [15], and fast exciton-polariton optical switching [16]. A ma- jor lingering drawback of such systems is the broad linewidth of the excitonic transitions which prevents efficient coupling to higher quality factor (Q) cavities and leads to strongly-coupled states with short lifetimes. To achieve polariton lasing or optical switching, higher peak power excitation lasers are needed to overcome the fast lifetimes of organic polaritonic states. The situation is apparent in the equation for the Rabi-splitting, ℏ R , which measures the strength of the light-matter coupling actually observed in the system, given by:   R = 2  N TLS (μ · E vac ) 2 − 1 4 (κ − γ ) 2 (1) The Rabi-splitting equation describes strong light- matter coupling between a number of two level systems (TLS), N TLS , with transition dipole μ, situated in a cav- ity of vacuum field strength, E vac , where the linewidth of the cavity and ensemble of TLS are κ and γ , respectively, and the cavity resonance is equal to the optical transition of the TLS [17]. The equation shows that to obtain the largest observable splitting, one must match linewidths, i.e. κ = γ . For systems of organic excitons, which have rel- atively large linewidth γ , κ is necessarily also large, as are the resulting polariton linewidths Ŵ, which on resonance are given by the formula Ŵ = 1 2 (κ + γ ). ∗ Corresponding author E-mail: yrt@biu.ac.il, Tel: 972-3-738-4514, Fax: 972-3-738-4053 The Department of Chemistry and the Bar-Ilan Institute for Nanotechnology and Advanced Materials, Bar-Ilan University, Ramat Gan, 52900, Israel C  2015 by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 313 Original Paper