Broadband complex refractive index spectra of structure-sorted single-walled carbon nanotube assembly at high temperatures

Structure-sorted single-walled carbon nanotube (SWCNT) assembled membranes, with pronounced excitonic resonances, hold promise for high-temperature near-infrared photonic applications. However, the temperature dependence of their broadband optical spectra remains unclear due to spectral complexity from residual impurities. We present the temperature-dependent broadband complex refractive index spectra (300–800 K) of a highly purified (6,5) SWCNT membrane, measured via transmission spectroscopy. Both first and second subband exciton peaks showed red shifts and spectral broadening, whereas their integral oscillator strengths were conserved. The first subband exciton energy shifts followed the Varshni formula, matching parameters for isolated SWCNTs. This finding facilitates reliable peak energy prediction, enabling accurate spectral modeling at a given temperature and local temperature probing based on its spectra. Our results provide fundamental insights into the excitonic thermal response, reinforcing their potential for high-temperature thermal photonic devices.