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 λ=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 λ=635 nm was shown to be 0.994±0.002, based on the measured directional-hemispherical reflectance.
Bibliographical noteFunding Information:
X.J.W., L.P.W., and Z.M.Z. thank the support of NSF (Grant No. CBET-0828701). O.S.A. and B.A.C. appreciate support from the DARPA NTI program, Georgia Tech startup funds, and Georgia Tech’s Nanotechnology Research Center. Thanks to Mr. Sheng Xu for help taking SEM images.