Environmental Effect on the UV Optical Absorption of Single-Walled Carbon Nanotubes

We studied optical absorption of single-walled carbon nanotubes by varying the dielectric environment. For the two different components of the broad UV absorption feature conventionally referred to as the pi-plasmon, we find that the component at 5.0 - 5.3 eV exhibits remarkable spectral changes, based on which we attribute this to a dipolar radial surface plasmon. However, the component at ~4.5 eV remains unchanged, raising a fundamental question as to its conventional attribution. We discuss its relation with the absorption feature at ~4.5 eV in graphite arising from an interband transition.