Multiple micro-cavity vibro-polaritons formation with different vibrational bands of the methylene group

We present an experimental technique to accurately predict the formation of vibro-polaritons from a molecular polymeric film embedded in a resonant mid-infrared cavity. Using simple Fourier-transform reflectance measurement, we extract the complex dielectric function of a polyethylene film using Kramers-Kronig relations. The fitted dielectric function can be plugged into a numerical code to predict the strength and dispersion of the strong light-matter coupling regime between the quantized electromagnetic modes of a microcavity and the vibrational bands of the molecules. As a demonstration, we experimentally resolve the simultaneous formation of multiple vibro-polariton modes issued from the strong coupling of some vibrational bands of the methylene group (CH₂) in a 2.5-μm-thick polyethylene film embedded in a microcavity. We measure a Rabi splitting of 6.3 THz for the stretching doublet around 87.5 THz and a Rabi splitting of 1.1 THz for the scissoring doublet around 43.7 THz, in excellent agreement with numerical predictions.