The group says the technology works because the nanotubes' "index of refraction [is] very close to that of air".
This means they slow down light to a similar degree.
As a result there is very little scattering of light as it passes from the air into the layer of nanotubes.
Applied Physics Letters - Low density carbon nanotube forest as an index-matched and near perfect absorption coating
By absorbing instead of scattering light, carbon nanotube coatings could cloak an object against a black background, such as that of deep space, the researchers note. In such cases the carbon nanotube forest "acts as a perfect magic black cloth that can completely conceal the 3-D structure of the object," the researchers write.
We demonstrate broadband, near perfect absorption with a conformal coating of a multi-walled carbon nanotube (CNT) forest on an arbitrarily shaped surface. The complex refractive index of such a CNT forest is retrieved from the measured transmission and reflection spectra using Kramers-Kronig constrained variational analysis, which gives a typical value of neff = 1.04 + 0.01i at visible wavelengths. Therefore, when used as a conformal coating on an object, a thick layer of the CNT forest can provide an excellent impedance match to air and near perfect absorption, preventing any detectable light reflection and scattering from the object.
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