Highly specular carbon nanotube absorbers

Specular black materials have important applications, such as in absolute cryogenic radiometers, space-borne spectroradiometers, and some energy conversion devices. While vertically aligned carbon nanotubes (VACNT) can have close-to-unity absorptance, so far the reported reflection has been essentially diffuse. This letter describes a highly specular black absorber made of VACNT. Both the bidirectional reflectance distribution function and specular reflectance were measured at the wavelength lambda = 635 nm using a laser scatterometer. The ordinary and extraordinary optical constants were obtained by fitting the specular reflectance, calculated from modified reflectance formulae for light incident from air to a uniaxial medium, considering surface roughness. Furthermore, the absorptance at lambda = 635 nm was shown to be 0.994 +/- 0.002, based on the measured directional-hemispherical reflectance. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3502597]