Researchers at IISER Pune demonstrate that it is possible to propagate as well as localize light on a v-shaped silver nanowire, a phenomenon that could find application in designing plasmonic circuit elements and nano-optical detectors.
Plasmonics is a branch of science dealing with interaction of light with metal. As photons of light hit a metal, they encounter a sea of electrons near the metal surface. The collective oscillations of these electrons are in the nanometer scale and are called surface plasmons. The light-surface plasmon combination (called polariton) can now travel much faster along the metal surface. Now, how efficiently this technology can be applied for transfer of information across circuits depends on how far plasmons can carry light.
“Not far enough,” says Danveer Singh, a fifth year integrated Masters student working with principal investigator Pavan Kumar and one of the primary authors on their recently published work. “The energy is eventually spent or lost before the light can propagate a reasonable distance. Therefore, a key challenge in this field is to develop technologies that help in propagation and localization of light–and that is what our work is about.”
Along with another undergraduate student Rohit Chikkaraddy, the team has chosen to study v-shaped silver nanowire pairs. “We were expecting to see linear nanowires under the microscope when we found the method we used yielded end-to-end coupled nanowire pairs as well. These pairs caught our attention because of the presence of a junction. Few reports in literature have studied this kind of nanowire configuration where the wires are chemically continuous,” says Chikkaraddy.
As the microscopy images revealed the nanowire pairs to be joined at one of their ends, the team posed following question: if you were to illuminate an end of one of the nanowires, would light jump across the junction and hop onto the accompanying nanowire. They found that indeed light does propagate across the junction and up to the distal end of the accompanying nanowire, thus traveling nearly 10 to 15 microns. The significance of this result is brought out by the fact that none of the earlier reports in literature have achieved more than 5-10 microns of propagation.
Another important phenomenon they checked was the ability of light to localize at the nanowire pair junction. They monitored this using Raman imaging which reports on the vibrational and rotational energy status of a molecule.
“Raman images suggested an intense signal at the junction that is much greater in intensity than from other parts of the silver nanowire pair. This suggests localization of electrons at the junction,” describes Singh. Adds Chikkaraddy, “Such a selective enhancement in intensity at the junction can potentially be used for single molecule sensing.” The junction thus acts as an electromagnetic hot-spot facilitating enhanced electric field. This can be harnessed as a light source to detect a diffusing single molecule.
The study titled “Plasmon assisted light propagation and Raman scattering hot-spot in end-to-end coupled silver nanowire pairs” is authored by Rohit Chikkaraddy, Danveer Singh and G. V. Pavan Kumar and has been published in a recent issue of Applied Physics Letters (100:43108).
Pavan Kumar is a Ramanujan Fellow at IISER Pune and his group studies interaction of light with materials and molecules at micro- and nano-scales. This work was partially supported by DST Nanoscience Unit Grant.
-Reported by Shanti Kalipatnapu