Launching low-energy surface plasmons in purple gold (AuAl2)

We confirm that the unusual purple color of the intermetallic compound AuAl 2 is of a plasmonic origin by launching surface plasmons (SPs) in thin AuAl 2 films. We measure the SP dispersion relation and also use the films to measure the index of refraction of sucrose solutions using standard SP resonance sensing. We find that the SP energy in planar AuAl 2 is approximately 2.1 eV, about 0.4 eV lower than in gold, and the material is highly resistant to oxidation. This is close to what is expected from previously reported measurements of the dielectric function of AuAl 2 . On this basis, we predict that AuAl 2 nanoparticles will a have very strong, spectrally nearly uniform light absorbance about an order of magnitude greater than standard carbon black. Such particles may therefore find applications as obscurants or as an alternative to more complex light-absorbing gold structures in areas such as photothermal therapy or solar steam generation, or in plasmonic catalysis.