(Invited) Non-Resonant Absorption in Carbon Nanotubes

Absorption spectroscopy of large ensembles of carbon nanotubes show marked resonances, attributed to the different exitonic levels of each chiral species, as well as a relatively smooth background. This signal consists of a large number of different intrinsic and extrinsic contributions, such as phonon sidebands, amorphous carbon absorption, light scattering... It is therefore challenging to extract any quantitative information from these direct absorbance measurements.

In contrast, photoluminescence excitation measurements in semi-conducting nanotubes give access to the intrinsic non-resonant absorption contribution between the excitonic resonances. Using a global analysis method, we were able to delineate the contribution of each chiral species including its tiny non-resonant component that shows up as 0.5 eV wide plateaus between the excitonic resonances. By comparison with the recently reported excitonic absorption cross-section on the S₂₂ resonance, we found a universal non-resonant absorption which turns out to be of the order of one half of that of an equivalent graphene sheet. This value as well as the absorption line-shape in the non-resonant window is in excellent agreement with microscopic calculations. This non-resonant absorption cross-section is the same (within our resolution) for all the chiral species we measured in this study.