A comparative study of the refractive index of silk protein thin films towards biomaterial based optical devices

Over the last two decades, silk fibroin has been exploited as a versatile optical material in biological applications due to a combination of unique properties. Recently, protocols have been developed to produce a silk fibroin negative tone resist that is UV crosslinkable, thereby allowing micro and nanoscale patterning of the protein using traditional photolithographic tools. The same protocol has been applied to the silk protein sericin to develop a sericin resist. Despite the immense potential of these biomaterials to develop micro optical patterns on silicon and glass surfaces, as well as self-standing components, their refractive indexes are not well characterized. In this work, optimizing a method to obtain extremely smooth, thin films, the refractive index (RI) of fibroin and sericin proteins and resists were characterized using ellipsometry. The parameters of the Sellmeier and Cauchy dispersion laws have been determined to obtain the RI over a large wavelength range. A complete morphological study of the films has been conducted. In addition, the effect of solvent on the optical properties of silk fibroin and sericin thin films are reported, with differences in values explained by examining the change in the protein secondary structure.