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

Water Monolayers: An Overlooked Loss Mechanism in Mid-Infrared Photonics | Research Square Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Help Center Sign In Submit a Preprint Cite Share Download PDF Article Water Monolayers: An Overlooked Loss Mechanism in Mid-Infrared Photonics Jana Jágerská, Antonia Torres-Cubillo, Martin Feiler, Roman Zakoldaev, and 4 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-9284522/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 4 You are reading this latest preprint version Abstract 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. Physical sciences/Optics and photonics/Applied optics/Mid-infrared photonics Physical sciences/Optics and photonics/Optical materials and structures Full Text Additional Declarations There is NO Competing Interest. Supplementary Files Watermonolayersupplementsubmission.pdf Water Monolayers: An Overlooked Loss Mechanism in Mid-Infrared Photonics - Supplement Cite Share Download PDF Status: Under Review Version 1 posted Unknown event 05 Jun, 2026 Editorial decision: Reject before peer review 13 May, 2026 Editor assigned by journal 09 Apr, 2026 First submitted to journal 31 Mar, 2026 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information

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