Intersubband Plasmon Resonances in Single Chirality Single-Wall Carbon Nanotubes

Intersubband plasmons (ISBPs) in carbon nanotubes (CNTs) arise from collective charge oscillations between quantized subbands and occur only when the incident light is polarized perpendicular to the nanotube axis, making the alignment of CNTs essential for their observation. Since their first experimental demonstration in aligned CNT films under ionic liquid gating [1], ISBPs have been intensively studied for their strong oscillator strength, gate tunability, and potential for novel quantum photonic phenomena. However, all reported observations so far have relied on ensembles with mixed chiralities and/or not well-aligned samples, leading to significant inhomogeneous broadening that obscures the intrinsic plasmonic response. Here we present the first observation of ISBP in a single-chirality, highly aligned CNT ensemble, revealing a sharp and well-defined spectral feature. This unlocks the possibility of coupling an ISBP with optical cavities in the strong and ultrastrong coupling regimes, opening a new path toward exploring quantum vacuum effects in carbon nanotubes.





[1] Yanagi et al., Nat. Com. 9, 1121 (2018)