An electromagnetic field may promote a concerted oscillation of electrons on a metal surface, allowing light to be concentrated in spaces that are smaller than its wavelength. To visualize these waves (or plasmonic modes), researchers use cathodoluminescence, where a highly focused electron beam excites the surface electrons, which then emit light on recombination.

Credit: © 2012 ACS

In Nano Letters (http://doi.org/jrw; 2012), Naomi Halas at Rice University (USA) and co-workers report the plasmonic modes of an aluminium nanorod with spatial resolution of about 20 nm. The nanorod functions as an optical antenna concentrating light in specific regions. Transitions from circular emission (top row in the figure), where the modes from the longitudinal and transverse directions are degenerate, to dipolar and even quadrupolar emission, which arise from the longitudinal confinement, are clearly visible as the rod length increases. The modes remain intense throughout the investigated regions and can be tuned from the ultraviolet to the visible range by changing the nanorod length.

It is only recently that aluminium has been regarded as a serious contender for practical applications of plasmonics. Gold and silver have been the main players since the inception of the field because they allow long-distance propagation of plasmons with minimal loss of energy. However, for some applications propagation distance can be compromised, and the focus therefore shifted to the ability of nanorods to confine plasmons in tighter spaces. Here, aluminium outperforms both silver and gold. A particularly relevant application of this type involves the coupling between the semiconductor technology used in computers and the plasmonic modes of a nanorod. Aluminium is already compatible with fabrication technology for complementary metal-oxide semiconductors, and optimization of its optical properties at the nanoscale could lead to the integration of plasmonics and semiconductor electronics. Therefore, the spatially resolved characterization of the plasmonic properties of the aluminium nanoantenna reported by Halas and co-workers is an essential step in this direction. Add to the mix the fact that aluminium is the third most abundant element in the Earth's crust, and the potential for an aluminium rush in plasmonic science and technology is easily envisaged.