Universal non-resonant absorption in carbon nanotubes

TL;DR

This study reveals a universal, frequency-independent non-resonant absorption in semiconducting carbon nanotubes, comparable to graphene, consistent across chiral species and supported by microscopic calculations.

Contribution

We identified a universal non-resonant absorption in carbon nanotubes, characterized its properties, and validated it with microscopic density matrix formalism, enhancing understanding of nanotube optical behavior.

Findings

Non-resonant absorption is approximately half of graphene's.

Absorption is frequency independent over 0.5 eV windows.

Same non-resonant cross-section across different chiral species.

Abstract

Photoluminescence excitation measurements in semi-conducting carbon nanotubes show a systematic non-resonant contribution between the well known excitonic resonances. Using a global analysis method, we were able to delineate the contribution of each chiral species including its tiny non-resonant component. By comparison with the recently reported excitonic absorption cross-section on the $S_{22}$ resonance, we found a universal non-resonant absorbance 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 based on the density matrix formalism. This non-resonant absorption of semi-conducting nanotubes is essentially frequency independent over 0.5~eV wide windows and reaches approximately the same value betweeen the $S_{11}$ and $S_{22}$ resonances or between the $S_{22}$ and $S_{33}$ resonances. In addition, the non-resonant absorption cross- section turns out to be the same for all the chiral species we measured in this study. From a practical point of view, this study puts firm basis on the sample content analysis based on photoluminescence studies by targeting specific excitation wavelengths that lead to almost uniform excitation of all the chiral species of a sample within a given diameter range.