Photo-elasticity of silk fibroin harnessing whispering gallery modes

Silk fibroin is an important biomaterial for photonic devices in wearable systems. The functionality of such devices is inherently influenced by the stimulation from elastic deformations, which are mutually coupled through photo-elasticity. Here, we investigate the photo-elasticity of silk fibroin employing optical whispering gallery mode resonation of light at the wavelength of 1550 nm. The fabricated amorphous (Silk I) and thermally-annealed semi-crystalline structure (Silk II) silk fibroin thin film cavities display typical Q-factors of about 1.6 × 10⁴. Photo-elastic experiments are performed tracing the TE and TM shifts of the whispering gallery mode resonances upon application of an axial strain. The strain optical coefficient K' for Silk I fibroin is found to be 0.059 ± 0.004, with the corresponding value for Silk II being 0.129 ± 0.004. Remarkably, the elastic Young's modulus, measured by Brillouin light spectroscopy, is only about 4% higher in the Silk II phase. However, differences between the two structures are pronounced regarding the photo-elastic properties due to the onset of β-sheets that dominates the Silk II structure.