The ability of plasmonic nanowires to confine and guide light at a subwavelength scale gives them wide appeal for use as optical sensors and nanoscale building blocks in photonic integrated circuits. Although combining many individual nanowires could enable plasmonic routing for the development of logic networks, damping unfortunately limits the propagation of surface plasmons.

Credit: © 2012 ACS

To gain a better understanding of the loss mechanism involved, Primoz Kusar and colleagues from Karl Franzens University in Austria used scattered light spectroscopy to investigate the relative damping contributions from metal crystallinity and substrate absorption (Nano Lett. 12, 661–665; 2012). The researchers studied chemically synthesized single-crystalline silver nanowires with diameters of around 90 nm and lengths of up to 40 μm, as well as lithographically fabricated polycrystalline silver and gold nanowires spin-coated on either quartz or indium tin oxide-covered glass substrates with cross sections of 100 nm × 75 nm.

By analysing the nanowires' spectral signatures — interference resulting from multiple surface-plasmon reflection in the Fabry–Pérot longitudinal cavity modes — the researchers were able to deduce the spectral-dependent damping of the surface-plasmon modes and thus reconstruct the surface-plasmon dispersion. They found the strongest modulation contrast occurred for single-crystalline nanowires on quartz, indicating not only that non-absorbing substrates exhibit low damping, but also that damping increases with nanowire length. They also found that absorption is the dominant factor in determining the short propagation length of surface plasmons in nanowires on indium tin oxide-covered glass substrates.

The researchers then fabricated surface-plasmon splitters by dividing the ends of silver and gold polycrystalline nanowires in two. They concluded that polycrystalline nanowires, which were previously expected to exhibit strong damping due to scattering at grain boundaries, can be used for nanowire networks. This finding certainly broadens the potential of nanoscale plasmonic routing.