Structure and mid-infrared optical properties of spin-coated polyethylene films developed for integrated photonics applications

Polyethylene is a promising polymer for mid-infrared integrated optics due to its broad transparency and optimal refractive index. However, simple fabrication protocols that preserve its optical characteristics are needed to foster a wide range of applications and unlock its full potential. This work presents investigations of the optical and structural properties of spin-coated linear low-density polyethylene films fabricated under humidity-controlled conditions. The film thickness on polymer concentration dependence shows a non-linear behavior, in agreement with previously reported theoretical models and allowing predictive concentration-dependent thickness deposition with high repeatability. The surface roughness is on the nanometer-scale for all investigated concentrations between 1% and 10%. The crystallinity of the films was studied with the Raman spectroscopy technique. Mid-infrared ellipsometry measurements show a broad transparency range as expected for bulk material. Layer exposure to solvents revealed good stability of the films, indicating that the fabricated layers can outlast further fabrication steps. These investigations confirm the excellent properties of spin-coated thin films fabricated with our novel method, creating new opportunities for the use in photonic integrated circuits Published by Optica Publishing Group under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.