Enriched carbon nanotubes materials, whether purified in electronic type, diameter, filling or even specific chirality, have deepened researchers' understanding of the fundamental optical properties of single-wall carbon nanotubes (SWCNTS). In particular, density gradient ultracentrifugation and aqueous two-phase extraction techniques have enabled the production of sizable aqueous suspensions and aligned films, allowing the interrogation of chiral-specific absorption, emission and scattering properties. In this talk, I will discuss two particular aspects: the chiral-specific nature of the Raman M-mode in small-diameter semiconducting SWNCTs and the large orbital magnetic susceptibility anisotropy of large-diameter metallic SWCNTs.

In the first part of the talk, I examine the unusual frequency dependence and excitation profile shape of the M-mode (~1620-1800 cm^-1) in small-diameter semiconducting species such as the (5,4), (6,4), (8,3), (9,1), (6,5), (7,6), (11,0), (9,4) & (8,6). In particular, semiconducting species exhibit a mod- and diameter-dependent "fanning-out" behavior for E₂₂ excitation, reminescent of that observed for optical transition energies. Even more interesting is the strongly asymmetrical excitation profile of the M-mode for a specific (n,m) species, deviating from the standard symmetrical profile observed for the radial breathing mode and the non-Condon-violating profile observed for the G-band. At certain energies in the excitation profile, the M-mode exceeded the G-band in intensity and unlike the G-band, the most intense peaks in the M-mode profile do not correspond to either the incoming or outgoing resonances. The M-mode seems to exhibit a new type of excitation profile shape.

In the second part of the talk, I discuss the magneto-optical properties of metallic SWCNTs. Using the strongly anisotropic optical and magnetic properties arising from the one-dimensional nature of carbon nanotubes, we aligned SWCNTs in aqueous suspension in magnetic fields up to 22T. Specifically, the orbital magnetic susceptibility anisotropy (Δχ) was measured, via magnetic linear dichroism spectroscopy enabled by the unique Florida Split-Helix magnet system, of multiple large-diameter metallic and semiconducting SWCNT species. In particular for the metallic species, Δχ’s on the order of 10^-4 emu/mol, an order of magnitude larger than any Δχ measured previously for SWCNTs, were observed. Additionally, broadening and the beginnings of peak splitting of metallic optical absorption features, due to the Aharanov-Bohm effect, were also seen. The emergence of such magneto-optical behavior at such relatively “low” magnetic fields, combined with new developments in carbon nanotube sample preparation science, opens the door to observing new many-body effects through magneto-optics.

Lastly, I will share a few memories of my time with my friend and mentor, Robert Hauge.