Biological glass fibers: Correlation between optical and structural properties

Biological systems have, through the course of time, evolved unique solutions for complex optical problems. This is often achieved through a sophisticated control of fine structural features. Here we present a detailed study of the optical properties of basalia spicules, from the glass-sponge Euplectella aspergillum and reconcile them with structural characteristics. We show these biosilica fibers to have a distinctive layered design with specific compositional variations in the glass/organic composite, and a corresponding non-uniform refractive index profile with a high index core and a low index cladding. The spicules can function as single-mode, few-mode or multi-mode fibers, with spines serving as illumination points along the spicule shaft. The presence of the lens at the end of the fiber increases the light-collecting efficiency. While free-space coupling experiments emphasize the similarity of these spicules to commercial optical fibers, the absence of any birefringence, the presence of technologically inaccessible dopants in the fibers and their improved mechanical properties highlight the advantages of the low-temperature synthesis employed by biology to construct these remarkable structures.