Oct 28, 2009
Nanogap enhances scattering intensity of optical antennas
The manipulation of electromagnetic waves via antennas has been fascinating researchers for more than 100 years. Not only antennas in their most simple form – long metallic bars with a small feed gap in the middle, termed dipole antennas, which are nowadays found in many applications – but all types of more complicated structures.
Antennas facilitate easy communication and information transport between distant places, for example using radio frequencies. In the last decade, the investigation of antennas that operate in the visible wavelength regime has become more and more prominent. So-called optical antennas are designed to actively couple far-field and near-field information, and specifically use the strongly localized near-field response for applications such as nanomanipulation.
Gold nanorod design
Gap dipole antennas – two tightly coupled gold nanorods (antenna arms) with dimensions on the order of a few tens of nanometres – have now experimentally been shown to yield a significantly higher scattering intensity response (a factor of about eight) than single rods of the same size as one antenna arm would generate. To achieve this result, we applied dark-field microscopy to both single arm and two arm antennas, collecting their respective scattering spectra and the scattered intensity. By varying the gap size, we also observed that the enhancement for gap antennas is strongly dependent on the distance between the two rods – a gap of about 30 nm or less is required to allow for the enhancement to take place.
Our study, which also varies the antenna arm length and width, gives further insight into the performance of optical antennas and specifically the antenna gap, which is not only crucial for enhanced localized near-fields, but also has significant influence on the scattering spectra and intensity observable in the far-field. It also paves the way for additional fundamental studies on antenna behaviour, and applications of such structures including optical sensors and antenna-enhanced solar cells.
The researchers presented their work in the journal Nanotechnology.
About the author
Matthias Wissert is a PhD student in the DFG Heisenberg Group "Nanoscale Science" headed by Dr Hans-J Eisler, DFG Heisenberg Fellow. The research team is hosted by the Light Technology Institute, Universität Karlsruhe (TH), Germany, and sponsored by Deutsche Forschungsgemeinschaft (DFG) under the project number DFG EI 442/2-1 and DFG EI 442/3-1. The group currently explores the nanofabrication and characterization of functional nanostructures, such as optical antennas.