  High intrinsic mobility and small, biologically-compatible size make
single-walled carbon nanotubes (SWNTs) in demand for the next generation of
electronic devices. Further, the wide range of available bandgaps due to
changes in diameter and symmetry give SWNTs greater versatility than
traditional semiconductors. Single-stranded DNA has been employed to make these
desirable properties accessible for large scale fabrication of devices. Because
single-stranded DNA can helically wrap a SWNT, forming a stable hybrid
structure, DNA/polymer wrapping has been used to disperse bundles of
intrinsically hydrophobic SWNTs into individual tubes in aqueous solution. The
ability to isolate individual tubes, make them soluble, and separate them
according to symmetry would enable fabrication of SWNT optoelectronic devices
that benefit from the unique electronic properties of specific nanotube
structures. Envisioning optoelectronic applications of nanotubes, we
investigate whether the optical properties of DNA-wrapped SWNT materials are
different than those of pristine SWNTs. Our previous work found that
bandstructures of DNA-SWNTs were indeed affected by the charged wrap. That is,
the direct optical bandgap, $E_{11}$, decreases, but changes are fairly small.
This is consistent with the available experimental data in standard
experimental geometry in which incident light is polarized along the SWNT axis.
Here we consider optical absorption of light with perpendicular (or circular)
polarization with respect to the tube axis, which has been measured
experimentally for SWNTs dispersed using a surfactant. In this geometry we find
qualitative changes in the absorption spectra of SWNTs upon hybridization with
DNA, including strong optical circular dichroism in non-chiral SWNTs.
