Researchers’ observation of gate-controlled quantum plasmons in aligned carbon nanotubes could pave the way for the development of carbon-based NIR optoelectronic devices and enable researchers to study the collective dynamic response of interacting electrons in one dimension. Rice University researchers achieved tight alignment of carbon nanotubes in wafer-sized films. These films caught the attention of a team at Tokyo Metropolitan University (TMU) that had developed a gating technique for controlling the density of electrons in nanotube film. “The gating technique is very interesting, but the nanotubes were randomly oriented in the films I had used. . . . I could not get precise knowledge of the one-dimensional characteristics of nanotubes in such films, which is most important,” said TMU professor Kazuhiro Yanagi. A wafer of highly aligned carbon nanotubes, seen in gray on a piece of glass, facilitated a novel quantum effect in experiments at Rice University. Courtesy of Jeff Fitlow/Rice University. Combining these technologies, the researchers pumped electrons into nanotubes that were slightly more than one nanometer wide. The narrow width of the nanotubes trapped the electrons in quantum wells, confining the electrons to subbands. When the electrons were excited, they began to oscillate and act as plasmons. “Plasmons are collective charge oscillations in a confined structure. If you have a plate, a film, a ribbon, a particle or a sphere and you perturb the system (usually with a light beam), these free carriers move collectively with a characteristic frequency,” said Rice professor Junichiro Kono. Plasmonic effect is determined by the number of electrons and size and shape of the object. Kono said that because the nanotubes were so thin, the energy between the quantized subbands was comparable to plasmonic energy. “This is the quantum regime for plasmons, where the intersubband transition is called the intersubband plasmon. People have studied this in artificial semiconductor quantum wells in the very far-infrared wavelength range, but this is the first time it has been observed in a naturally occurring low-dimensional material and at such a short wavelength,” said Kono. Rice University researchers Junichiro Kono (left) and Fumiya Katsutani prepare a nanotube film for testing. The lab observed a novel quantum effect in their carbon nanotube film that could lead to the development of near-infrared lasers and other optoelectronic devices. Courtesy of Jeff Fitlow/Rice University. The teams were surprised to detect a complicated gate voltage dependence in the plasmonic response and to detect it in both metallic and semiconducting single-walled nanotubes. “By examining the basic theory of light-nanotube interactions, we were able to derive a formula for the resonance energy,” Kono said. “To our surprise, the formula was very simple. Only the diameter of the nanotube matters.” The researchers believe the phenomenon could lead to advanced devices for communications, spectroscopy and imaging, as well as highly tunable NIR quantum cascade lasers. In contrast to traditional semiconductor lasers, quantum cascade lasers do not depend on the width of the lasing material’s bandgap. “Our laser would be in this category. Just by changing the diameter of the nanotube, we should be able to tune the plasma resonance energy without worrying about the bandgap,” said researcher Weilu Gao. Kono believes the gated and aligned nanotube films could also give physicists the opportunity to study Luttinger liquids — theoretical collections of interacting electrons in one-dimensional conductors. “One-dimensional metals are predicted to be very different from 2D and 3D. Carbon nanotubes are some of the best candidates for observing Luttinger liquid behaviors. It’s difficult to study a single tube, but we have a macroscopic one-dimensional system. By doping or gating, we can tune the Fermi energy. We can even convert a 1D semiconductor into a 1D metal. So this is an ideal system to study this kind of physics,” said Kono. The research was published in Nature Communications (doi:10.1038/s41467-018-03381-y).