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.