Polarization Defines Nanowires

Hank Hogan

Researchers at Martin Luther Universität Halle-Wittenberg and Fraunhofer Institute for Mechanics of Materials, both in Halle, Germany, have demonstrated a technique that may lead to novel photonic devices. Heinrich Graener, a professor of physics at the university, and his colleagues have used a femtosecond laser to create wires only a few hundred nanometers wide that display markedly different electrical and optical properties depending on their direction relative to the polarization of the laser.

Nanowires generated by a pulsed laser display different properties depending on their relative direction to the polarization of the laser. Here a scanning electron micrograph of gold nanowires illustrates the result of exposing nanoparticles to the ultrafast pulses. The arrow indicates the polarization of the laser. Courtesy of Heinrich Graener.

The researchers produced nanowire arrays by firing <150-fs pulses from a Spectra-Physics Ti:sapphire laser at a target of 20- to 50-nm-diameter silver or gold particles embedded in a 60-nm-thick organic film. Using 800- and 400-nm light for the gold and the silver, respectively, they created nanowires with a spacing equal to the width -- in gold, 300 to 450 nm; in silver, 260 to 320 nm. In both cases, the resulting nanowires were aligned to the polarization of the pulses.

Parallel to the polarization of the pulses, the electrical conductivity was nearly unchanged before and after laser treatment. Perpendicular to the polarization, however, conductivity dropped by 10 orders of magnitude following the procedure. Optically, both the parallel and the perpendicular displayed different extinction spectra after laser treatment.

That directional performance difference could be useful in photonic applications. "It should be possible to generate optical elements with special features," Graener said. In particular, the group is considering applications in fiber optics.

Photonic aspect crucial

Although the mechanism of nanowire formation is unknown, it is clear that the photonic aspect of the phenomenon is crucial. Thermally treated nanoparticles, for example, do not exhibit altered optical characteristics. Because of the short duration of the pulses, the researchers believe that heat transfer cannot occur, and that no meaningful material transport is possible.

Nevertheless, Graener said that understanding the mechanics of nanowire formation is only part of the ongoing research. The team also is investigating local optical properties, with the goal of putting the nanowires to use.