Bio-Pigment Xanthommatin and Cephalopod Skin Photonics

Biomaterials and biopolymer materials such as biopigments (melanin, xanthommatin, etc.) have fascinating optical properties, such as bright and structural color and advanced camouflage. These material properties are explored in basic research to learn about how the skin of cephalopods (e.g., squid, octopus) functions optically, based on pigment, texture, and photon-scattering granules (~50-500 nm in size). Natural pigment molecules and bio-polymers are attractive for photonics applications: displays, adaptive lenses, underwater optical communications, etc.<br/>Small-molecule biopigments readily dissolve in polymer hosts, resist ultraviolet light, are toxicologically benign, and can be fabricated using large-scale chemical methods. Over 500 mg was synthesized and dried in our laboratory for toxicological analysis. Because of their small-molecule nature, their ability to combine with polymers such as polyvinyl alcohol (PVA), their manufacturability (biological and chemical synthesis), their high refractive index (RI), and their sustainability (they're eaten by animals), the bio-pigments and associated bio-polymers are useful for photonically functional thin films, anti-reflective coatings, photonic scattering for color, etc. Large-scale environmentally friendly synthesis may enable new designs of lightweight optical systems that can adapt and change shape, have a high numerical aperture, and aid in underwater detection or imaging.<br/>The chromatophores in squid skin change from an expanded, pancake-like shape to a punctate, spherical shape and back again to influence photonic scattering and color; they are colored yellow, red, and brown, and are responsible for the bulk color changes. Underneath the chromatophore lie less-obvious iridocyte cells - multilayered Bragg reflectors that reflect color iridescently. The chromatophore cell, in its expanded state, contains multiple layer of sub-monolayers of photon-scattering granules.<br/>Our group discovered that the cephalopod’s Xa pigment has a high RI (<i>n </i>&gt; 1.55), at 589 nm and at 532 nm. For the latter experiment, <i>n</i> may have been artificially low. Recently, we have carried out more comprehensive broadband (300 nm – 3200 nm) ellipsometric spectroscopy to determine the optical indices (<i>n</i> and absorption coefficient <i>k</i>) of Xa, extracting natural Xa pigment from the squid’s chromatophores and forming smooth, high-quality films with concentrations of Xa as much as 90% in polyvinyl alcohol (PVA). Many samples containing natural Xa, and some with artificial Xa, show the same trends, providing a more complete description of Xa’s optical properties (<i>n</i>,<i> k</i>) across the entire wavelength range of interest. We have observed an unreported infrared absorption peak and associated reduction in <i>n</i>, and explain possible origins.<br/>Armed with ellipsometric data, we model optical properties (e.g., reflectivity) using a four-flux model (forward and backwards-propagating specular and diffuse light), to gain a more complete and quantitative understanding of squid skin optics, improving on an analytical model of light scattering and absorption developed in 2008 that made important breakthroughs, but lacked separate experimental data on fundamentals for the Xa, and did not comprehensively treat the full optical problem. Here, we update this model analytically, taking better account of the optical coupling between chromatophore and underlying iridocyte. Our 4-flux optical model of the chromatophore will not ignore the background reflectivity and scattering.<br/>The to-be-reported physics modeling and theory, with experimental characterization, will enable new bio-pigment and bio-polymer materials and applications. By understanding how the color of the chromatophores changes, another important step can be taken to harnessing artificial and novel material platforms for more efficient control of light at the nanoscale. We discuss possible new functional nanophotonic systems, based on the bio-pigments, and possible applications.