Nanotubes Act Like Antennae for Light

In a series of experiments with random arrays of multiwalled carbon nanotubes, scientists in Zhifeng Ren's group at Boston College, in collaboration with the US Army's Natick Soldier Center and Mega Wave Corp. in Boylston, all in Massachusetts, and Florida International University in Miami, have demonstrated that the structures interact with optical radiation in a fashion analogous to antennae receiving and transmitting radio waves. They suggest that such arrays may find application in the development of detectors for use at terahertz frequencies. A report of their findings appears in the Sept. 27 issue of Applied Physics Letters.

The scientists produced arrays of the nanotubes by plasma-assisted chemical vapor deposition, altering the growth times to yield samples of 50-nm tubes with varying lengths of up to 1000 nm. In one experiment, they monitored the intensity of white unpolarized light reflected from a sample through a rotating polarizer at different angles relative to the direction of growth of the tubes. In another, they measured the wavelength of light reflected from a sample with gradually varying tube lengths, correlating the reflected light and the length of the tubes.

The group confirmed that the nanotube arrays display two phenomena consistent with radio antenna theory: the polarization effect and the length-matching antenna effect. The former describes the suppression of a dipole antenna's response when its axis is perpendicular to the electric field of the incoming radiation. The latter describes the wavelength-dependent response of an antenna, so that its response is maximized when its length is a multiple of the half-wavelength of the incoming radiation.