Meeting Abstract
While most research in biophotonics has focused on structural colors, and – to a lesser extent – structural whites, special structures are also required to make exceptionally black surfaces. Deep blacks are important for camouflage against bioluminescent searchlights in the deep-sea, and also for enhancing the contrast of color signals, as has been documented in the Superb Bird-of-Paradise (Lophorina superba) wings of the papilionid butterfly Trogonoptera brookiana are characterized by iridescent green triangles on an unusually black surround. We used specialized spectroscopic techniques to measure the reflectance of the black surface and found that, even at perpendicular incidence, which maximizes the specular contribution to the reflected light, the reflectance was only 0.05%, as low as the blackest known natural and artificial surfaces. Investigation by scanning electron microscopy (SEM) showed that the scales of the wings had an unusual honeycomb structure with a periodicity on the order of a wavelength of visible light. The structure contained melanin, but the importance of the structure itself was made clear by the fact that the gold-coated SEM samples remained black. The honeycomb structure strongly resembled the honeycomb radar-absorbing materials used in stealth aircraft technology. Indeed, if one scales the periodicity of the radar materials by the difference in wavelength between radar and visible light, one arrives at the periodicity of the honeycomb in the scales of T. brookiana. Similar structures have been found in other less-black papilionid butterflies ( Papilio paris,Troides helena, Pachliopta aristolochiae) and have been investigated with an aim to increasing the efficiency of solar cells. We discuss the biological significance of this potentially convergent trait and its implications for signaling.
