Vibrational Ultra Strong Coupling of Water and Ice

Water is of vital importance for life and human activities on Earth—it exhibits unique properties due to its interlinked and multipoint hydrogen bonding network. Here, we experimentally show that water can undergo vibrational ultra strong coupling (V-USC) in both the liquid and solid forms when the OH stretching mode of water or ice is resonantly coupled with an optical mode of an infrared Fabry−Pérot cavity. The light-coupled H₂O under V-USC reveals the largest Rabi splitting ever reported, reaching 22% and 26% of the vibrational energy for water and ice, respectively. We confirm that the extraordinarily large Rabi splitting stems from the densely packed minuscule molecular structures, large vibrational energies, and broad and intense absorptions due to intermolecular hydrogen bonding. These new findings offer a brand-new platform in polaritonic chemistry for controlling the properties of water with an ultra strong light-matter interaction.