Thermo-optic coefficient of Ormocomp and comparison of polymer materials in athermal replicated subwavelength resonant waveguide gratings

Abstract

We study the thermo-optic coefficient (dn/dT) of UV-curable Ormocomp® thin films by three methods, based on calculations using analytical models and data measured by ellipsometry, fitted with the Cauchy model in the wavelength range 380<λ<1000nm. The obtained values of dn/dT by the three methods are −1.02×10−4°C−1, −0.81×10−4°C−1, and −1.27×10−4°C−1. These values are employed to achieve an athermal operation of resonant waveguide gratings (RWGs). We describe the design, fabrication and characterization of three RWGs with different polymer materials (polycarbonate, cyclic olefin copolymer and Ormocomp®) and investigate athermal filtering device operation over a wide range of temperatures. The thermal spectral shifts of RWGs are described in terms of thermal expansion and thermo-optic coefficients of the selected materials. Furthermore, the spectral shifts are explained on the basis of shrinkage strains, frozen-in stresses and the molecular chain orientation in polymeric materials.