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

Abstract

We confirm that the unusual purple color of the intermetallic compound AuAl2 is of a plasmonic origin by launching surface plasmons (SPs) in thin AuAl2 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 AuAl2 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 AuAl2. On this basis we predict that AuAl2 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.

Recent Developments in Plasmonic Alloy Nanoparticles: Synthesis, Modelling, Properties and Applications

“…Several reports addressed the intense purple colour of AuAl 2 surfaces suggesting the origin was due to interband absorption [118] . Samaimongkol and Robinson recently verified the existence of a low energy surface plasmon after that some OCs’ calculations alluded to it [168] . They launched surface plasmon in AuAl 2 thin film with Kretschmann configuration and the surface plasmon resulted quite lossy and lying at lower energy (2.1 eV).…”

“… [118] Samaimongkol and Robinson recently verified the existence of a low energy surface plasmon after that some OCs’ calculations alluded to it. [168] They launched surface plasmon in AuAl 2 thin film with Kretschmann configuration and the surface plasmon resulted quite lossy and lying at lower energy (2.1 eV). This is unusual for gold but may be interesting in spherical NPs due to the high absorption cross‐section in the red spectral range.…”