Water Monolayers: An Overlooked Loss Mechanism in Mid-Infrared Photonics

Mid-infrared (MIR) photonics promises transformative advances in environmental monitoring, chemical sensing and molecular spectroscopy, yet its practical deployment remains hindered by the persistently high propagation losses. Here we identify and quantify a previously underappreciated but dominant loss mechanism in air-clad MIR photonic devices: light absorption in water monolayers formed on the device surface from ambient humidity. Although only a few molecular layers thick, such layers become an appreciable source of loss as adsorbed water exhibits absorption coefficients in the MIR that are orders of magnitude larger than in the visible and near-infrared, where most photonic platforms were originally developed. We show that the formation of merely one to several monolayers is sufficient to induce propagation losses of tens of decibels per centimeter in sensing-optimized waveguides, in some cases completely suppressing transmission. Using silicon-on-insulator slot waveguides and suspended silicon nitride waveguides operating near 3.27 µm and 4.35 µm, we correlate relative humidity, surface chemistry, and waveguide geometry with measured propagation losses. The effect is strongly wavelength- and geometry-dependent and is amplified in sensing-optimized structures with enhanced modal overlap at air interfaces. We further demonstrate that the observed behaviour can be accurately captured by incorporating nanometre-scale water/ice layers into standard electromagnetic band-structure models, providing a predictive design tool.