Hexactinellid sponges — also known as glass sponges because of their silica-based skeletal elements called spicules — offer an example of natural systems with notable optical properties.
Previous studies have showed that the spicules of such sponges can guide light thanks to total internal reflection. Now Hermann Ehrlich and collaborators have observed supercontinuum generation (SG) in the anchoring needle-like spicules of Sericolophus hawaiicus (pictured; left), which lives in the sea at depths of around 400 m in Hawaii (Adv. Opt. Mater. http://doi.org/10.1002/adom.201600454; 2016).
First, the team investigated the structure of the spicules. Scanning electron microscopy indicated that the spicules consist of three distinct regions: a central 1-μm-wide hollow channel, a surrounding solid region of highly pure silica with a thickness between 25 and 100 μm, and an outer region of concentric silica–chitin layers (right panels).
Second, when Ehrlich and co-workers coupled pulsed beams with optical peak intensities between 1 and 100 TW cm−2 and wavelengths ranging from 650 to 900 nm into 30- to 50-mm-long fragments of spicules, they observed the emission of a supercontinuum with wavelength-dependent spectral width at the output. The widest spectrum was obtained for an input wavelength around 800 nm; higher excitation intensities and longer spicules both led to increased spectral broadening, providing further evidence that white-light emission was not due to fluorescence and that the whole spicule served as the active medium for SG.
According to the authors, the origin of SG might be due to the composite structure of the spicules, where the high isotopic purity of the silica could favour light transmission and the chitinous matrix is thought to enhance the flexibility of the anchoring stalk, which is normally composed of hundreds of spicules. As Ehrlich told Nature Photonics, a promising feature of the spicules of S. hawaiicus is their minimal use of materials, that is, their “high strength-to-weight ratio.”
Whereas the fabrication of commercial glass fibres requires very high temperatures, hexactinellid sponges such as S. hawaiicus produce the spicules responsible for their anchorage at temperatures close to 4 °C. The authors speculate that a better understanding of the dispersive and nonlinear properties of these natural optical fibres could facilitate the synthesis of new light-guiding materials.
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Donati, G. Bright sponges. Nature Photon 10, 625 (2016). https://doi.org/10.1038/nphoton.2016.199