Carbon nanotube arrays imitate insect antennae

Research documenting the remarkable mechanical and electrical properties of carbon nanotube (CNT) arrays has led to the realization of new composite materials, electronic components, and nanofabricated devices. For several years, our team at the Natick Soldier Research Development and Engineering Center, along with researchers from Boston College and MegaWave Corporation, have been studying the passive optical response of CNT arrays. These studies have revealed the manner in which the optical properties are related to nanotube and array geometries.^1–4

Periodic CNT arrays exhibit Bragg diffraction,⁵ photonic bandgap properties,⁶ and plasmonic resonance. Nonperiodic CNT arrays interact with light waves similarly to the way in which radio antennae interact with radio waves.⁷ This allows us to use standard antenna theory to describe and predict nonperiodic CNT/light wave interaction. Furthermore, the growth of periodic nanotube arrays can be precisely controlled to maximize response to specific wavelengths and nanotube length and width can be controlled for maximum coupling efficiency. Hence, it is possible to create highly tuned CNT nanostructures that interact strongly with predetermined wavelengths of light.

Recently, the research team has demonstrated a CNT antenna-lobe pattern effect. Figure 1 shows the optical response of a nonperiodic array of CNT light antennae illuminated by a 5mW, 543.5nm, continuous wave laser. In general, a half-wavelength dipole antenna responds with a single intensity lobe. With each half-wavelength increase in antenna length, a new side lobe appears. Additionally, the main lobe becomes directional and begins to radiate directly back to the source as each half-integer segment is excited sequentially out of phase and fires back in precisely the same order. The lobes are concentric about the length of the CNT, projecting a conic intensity pattern. The projection screen at θ = 50° effectively slices the cone, creating the parabolic pattern shown in the top photo of Figure 1. The lower image depicts the theoretical response of a dipole antenna with the same effective length.

The experimental photo is particularly noteworthy because it represents the first time, to the authors' knowledge, that such an antenna intensity pattern has been visible to the human eye. Not so for insects, which have been using light antennae as the basis of their visual sensors for thousands of years. Some intriguing work was performed in this regard in the 1970s by Robert L. Bailey, then at Goddard Space Flight Center, and Philip Callahan, a University of Florida entomologist.They detailed the workings of the Saturnidae moth antennae⁸ published a paper⁹ in 1975, the centerpiece of which is shown in Figure 2. They explain that the antenna spine (A) is a transparent dielectric. The tapered geometry decreases side lobes and enhances main lobe directionality for navigational purposes. The spine focuses light to the probe at K, itself an antenna, which is attached to a coax (E) through which light energy exits the antenna. Interfaces between regions at J, C, and I are tapered to create slowly varying indices of refraction to minimize reflective losses. A small portion of light becomes trapped via total internal reflection inside the dielectric waveguide wall. The light exits at the base and is directed toward the probe at K. This is clearly a highly efficient design that has been optimized over thousands of years of evolution.

As research into light antennae continues, it is helpful to consider nature's designs. Indeed, numerous antenna configurations have been created, both in nature and artificially by humans, where the complex optical response of light antenna arrays has been optimized for maximum efficiency and functionality.