Selection-rule breakdown in plasmon-induced electronic excitation of an isolated single-walled carbon nanotube

Optical transitions between electronic levels are restricted according to the selection rules governed by the parities of electronic wavefunctions(1). If this restriction can be removed, then drastic improvements in the efficiency and diversity of photoreactions may be realized. Because the selection rules are based on the long wavelength approximation, wherein the field amplitude is assumed to be constant within a molecule, a localized plasmon that has an electromagnetic field with a significantly high gradient is expected to violate the conventional limitations of the optical response(2). Here, we demonstrate the possibility of controlling the optical electronic transitions of a single-walled carbon nanotube (SWNT) using metal nanodimers with a controlled nanogap distance. Highly systematic identification of the excitations of individual SWNTs through well-resolved Raman signals implies a breakdown of the selection rules due to the strong field gradient in the radial direction of the SWNTs.