Transmitting Light Through a Coaxial Cable

Taking a cue from radio and microwave transmission lines, researchers at Boston College in Chestnut Hill and at NanoLab Inc. in Newton, both in Massachusetts, as well as at the Center of Advanced European Studies and Research in Bonn, Germany, have demonstrated a coaxial cable for visible light. The technique, which is described in the Jan. 8 issue of Applied Physics Letters, could be used to route and manipulate light at the nanometer scale.

Using the principles of conventional coaxial cables (far left), researchers have developed a nanoscale coax that transmits visible light. Scanning electron microscope images show the structure of a nanocoax (center) and an array of coax ends in a glass substrate (right). Reused with permission of Applied Physics Letters.

In a conventional coaxial cable, there is a central conductor, an outer metallic sheath and an interelectrode dielectric between the two. The cable transmits in the transverse electromagnetic mode, with attenuation resulting largely from resistance in the metal. The cable’s length is much greater than the wavelength of the transmitted radio and microwaves, while its width is much less.

In building a nanoscale analog of a coax, the researchers exploited the fact that visible light can propagate in transverse electromagnetic modes if certain conditions involving the frequency of the light, the spacing between electrodes and the inner electrode diameter are met. They satisfied these requirements by constructing a demonstration array using deposited carbon nanotube cores with a radius of 50 nm. Onto this, they sputtered an aluminum oxide dielectric 100 nm thick for the interelectrode dielectric and followed that with a layer of chromium for the outer electrode.

They encased the array in spun-on glass and polished the glass back, exposing the ends of the structures. Because of the nontransparent chromium, light could pass through only via the interior of the nanocoaxes.

Tests with lasers operating at 680 and 532 nm showed no drop-off in transmitted light intensity through nanocoaxes several microns long. The scientists estimated a maximum transmission length of 50 μm. A comparison indicated that the nanocoaxial array behaved similarly to a glass substrate for light at 680 nm.

In addition to transmission, the researchers said that the coaxes could manipulate light through the materials in the interelectrode dielectric and conductors. The right choices would enable the mixing, switching and phase conjugation of visible light on the nanoscale.