Suppressed terahertz dynamics of water confined in nanometer gaps

Nanoconfined waters have been extensively studied within various systems, demonstrating low permittivity under static conditions; however, their dynamics have been largely unexplored due to the lack of a robust platform, particularly in the terahertz (THz) regime where hydrogen bond dynamics occur. We report the THz complex refractive index of nanoconfined water within metal gaps ranging in width from 2 to 20 nanometers, spanning mostly interfacial waters all the way to quasi-bulk waters. These loop nanogaps, encasing water molecules, sharply enhance light-matter interactions, enabling precise measurements of refractive index, both real and imaginary parts, of nanometer-thick layers of water. Under extreme confinement, the suppressed dynamics of the long-range correlation of hydrogen bond networks corresponding to the THz frequency regime result in a significant reduction in the terahertz permittivity of even 'non-interfacial' water. This platform provides valuable insights into the long-range collective dynamics of water molecules which is crucial to understanding water-mediated processes such as protein folding, lipid rafts, and molecular recognition.