Researchers from Rice University and Tokyo Metropolitan University have stated that an innovative quantum effect detected in a carbon nanotube film could result in the development of distinctive lasers and other optoelectronic devices.
The Rice-Tokyo group has described an enhancement to the potential to regulate light at the quantum scale by using single-walled carbon nanotubes as plasmonic quantum confinement fields.
The phenomenon discovered in the Rice lab of physicist Junichiro Kono could play a significant role in developing optoelectronic devices such as near-infrared, nanoscale lasers emitting continuous beams at wavelengths very short to be generated by adopting prevalent techniques.
The new study has been described in the Nature Communications journal.
The study was the result of the discovery of a method by Kono’s team to accomplish very tight arrangement of carbon nanotubes in wafer-sized films. These films enabled the researchers to perform experiments which were highly difficult to be conducted on tangled or single aggregates of nanotubes and attracted the interest of Kazuhiro Yanagi, Tokyo Metropolitan physicist who analyzes condensed matter physics in nanomaterials.
He brought the gating technique (which controls the density of electrons in the nanotube film), and we provided the alignment technique. For the first time, we were able to make a large-area film of aligned nanotubes with a gate that allows us to inject and take out a large density of free electrons.
The gating technique is very interesting, but the nanotubes were randomly oriented in the films I had used. That situation was very frustrating because I could not get precise knowledge of the one-dimensional characteristics of nanotubes in such films, which is most important. The films that can only be provided by the Kono group are amazing because they allowed us to tackle this subject.
Their combined methodologies enabled them to pump electrons into nanotubes with a width of just over 1 nm and then stimulate them by using polarized light. The width of the nanotubes enabled the electrons to be confined into quantum wells, where the energy of subatomic particles and atoms is “confined” to specific states, or subbands.
Then, light-induced them to oscillate rapidly between the walls. Kono stated that if there are adequate numbers of electrons, they start functioning like plasmons.
“Plasmons are collective charge oscillations in a confined structure,” stated Kono. “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.”
The object’s shape and size and also the number of electrons govern this effect.
According to Kono, the nanotubes used in the Rice experiments were so thin that the energy between the quantized subbands was analogous to the plasmon 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.”
It was astonishing to observe a highly complex dependence on gate voltage in the plasmonic response, as well as its appearance in both semiconducting and metallic single-walled nanotubes. “By examining the basic theory of light-nanotube interactions, we were able to derive a formula for the resonance energy,” stated Kono. “To our surprise, the formula was very simple. Only the diameter of the nanotube matters.”
The scientists are confident the phenomenon could result in the advancement of spectroscopic, communications, and imaging devices, and also highly adjustable near-infrared quantum cascade lasers.
According to Weilu Gao, one of the co-authors of the study who is a postdoctoral researcher in Kono’s team that has been pioneering device development by using aligned nanotubes, in contrast to conventional semiconductor lasers that are dependent on the bandgap width of the lasing material, quantum cascade lasers are not dependent on that. “The wavelength is independent of the gap,” he stated. “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.”
Kono also anticipates that the aligned and gated nanotube films will enable the physicists to investigate 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.
The lead author of the paper is Yanagi, a professor of condensed matter physics at Tokyo Metropolitan University. Graduate student Ryotaro Okada, graduate student Yota Ichinose and assistant professor of condensed matter physics Yohei Yomogida, all from Tokyo Metropolitan; and graduate student Fumiya Katsutani from Rice are the co-authors of the study. Kono is a professor of electrical and computer engineering, of physics and astronomy, and of materials science and nanoengineering.
Japan Society for the Promotion of Science Grant-in-Aid for Scientific Research (KAKENHI) grants, a Japan Science and Technology Core Research of Evolutional Science and Technology grant, the Yamada Science Foundation and the Basic Energy Sciences program of the U.S. Department of Energy, the National Science Foundation, and the Robert A. Welch Foundation supported the study.