Scientists have developed a number of sophisticated methods using lasers or magnetic traps to confine atoms and molecules. An alternative approach, which relies on some clever chemistry, is to encapsulate molecules — like carbon buckyballs (C60) — within the narrow interior of a carbon nanotube and study how this confinement affects the molecule's motion and properties.

Optical spectroscopy is helpful in the study of such encapsulated molecules, but the carbon atoms in the nanotube can modify the intensity of light in ways that are not completely understood. Through simulations using Japan's Earth Simulator supercomputer, Hong Zhang of Sichuan University in China and Yoshiyuki Miyamoto at NEC in Japan1 have explored how light’s electric field is enhanced in the interior of several types of carbon nanotubes.

The team applied a method called time-dependent density functional theory to study how electrons in carbon nanotubes with different chiralities (how the tube is rolled) respond to electric fields oscillating at high frequencies, as they do in light. An important new aspect of their work is that they calculated the response of the electrons when the electric field of the light was polarized perpendicular to the long axis of the carbon nanotube — a geometry called ‘cross-polarization’.

Fig. 1: Calculations show that when light is polarized with its electric field perpendicular to the length of a carbon nanotube, the electric field in the interior of the nanotube can be enhanced at certain frequencies.

They found that for certain resonant frequencies of light, specifically those that excite some of the valence electrons of the nanotube into the conduction band, the electric field in the center of the carbon nantoube was twice that of the light (Fig. 1). “For metallic nanotubes, the electrons should completely shield the electric field inside the nanotube,” says Miyamoto. “But in these semiconducting nanotubes, the field is not zero and at the resonant frequency, some electrons oscillate with large amplitude. This creates an ‘anti-field’ in the interior of the nanotube, which is stronger than the light’s field and in the opposite direction.”

Zhang and Miyamoto’s results suggest a way to enhance the optical electric field for a molecule encapsulated in a carbon nanotube and may be interesting for applications in which the nanotubes are used as miniature antennas. Moreover, their computational technique could be applied to other problems where a carbon nanotube is exposed to rapidly changing electric fields.