  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.
