  Circular dichroism is widely used for characterizing organic and biological
materials, but measurements at a single molecule level are challenging because
differences in absorption for opposite helicities are small. Here we show that
extrinsic chirality can induce giant circular dichroism in individual carbon
nanotubes, with degree of polarization reaching 65%. The signal has a large
dependence on the incidence angle, consistent with the interpretation that
mirror symmetry breaking by the optical wave vector is responsible for the
effect. We propose that field-induced charge distribution results in an
efficient polarization conversion, giving rise to the giant dichroism. Our
results highlight the possibility of polarization manipulation at the nanoscale
for applications in integrated photonics and novel metamaterial designs.
