An Antenna for Visible Light

An antenna for visible light, analogous to antennas for radio waves, can be made with carbon nanotubes. In a radio antenna, whose size is equal to the wavelength of the incoming wave or a fair fraction of it, the wave excites electrons into meaningful currents . Such a response, amplified and tuned, is the backbone of radio and TV broadcasting.

At optical wavelengths, where the wavelength is hundreds of nm, this is harder to do. Nevertheless, a rudimentary antenna effect for visible light has now been observed by scientists at Boston College using an array of carbon nanotubes, in which infalling light excites miniature electrical currents.

According to Yang Wang (wangyq@bc.edu,617-552-3436) one would like to measure these electrical excitations directly, but this requires nano-diodes capable of processing electrical pulses oscillating at optical frequencies (10¹⁵ Hz), and these are not yet available. The next best thing is to observe the secondary radiation emitted by the faint excitations. The nanotubes used in the experiment are in effect little metallic antennas about 50 nm wide and hundreds of nm long (see figure).

Not only can the nanotubes respond in the manner of dipole radio antennas to incoming light, but they also exhibit a polarization effect; when the incoming light is polarized at right angles to the orientation of the nanotubes, the response disappears.

Possible applications for visible-light antennas? Optical television: a TV signal, superimposed on a laser beam sent down an optical fiber, is demodulated at the customer end by an array of nanotubes (each functionalized by a fast diode). Or efficient solar energy conversion: incoming light is turned into charge which is stored in a capacitor. (Wang et al., Applied Physics Letters, 27 September 2004; contact Zhifeng Ren, Boston College, 617-552-2832, renzh@bc.edu)