Intersubband plasmons in gated and aligned single-wall carbon nanotubes*

Intersubband transitions (ISBTs) occur in doped semiconductor quantum wells (QWs) as resonant optical transitions between subbands within the conduction or valence band. They are the elementary optical process in quantum-engineered infrared (IR) optoelectronic devices, such as the QW IR photodetector and the quantum cascade laser. From a fundamental point of view, ISBTs represent the collective, plasmonic response of quantum-confined carriers to an oscillating electric field applied along the confinement direction, whose resonance frequency is strongly influenced by a variety of many-body effects. Here, we describe the first clear evidence for ISBTs in single-wall carbon nanotubes. We observed the appearance of a large optical absorption peak in the near-IR for excitation light polarized perpendicular to the nanotube axis only when carriers were created in the lowest-energy subband, either in the conduction or valence band, through ionic-gel gating.