Dielectric function and its predicted effect on localized plasmon resonances of equiatomic Au-Cu

Equiatomic (Au, Cu) solid solution orders below 658 K to form a tetragonal AuCu (I) phase with significant changes in physical properties and the crystal structure. The effect of ordering on the dielectric function of the material is controversial however, with inconsistent results reported in the literature. Since the nature of any localized surface plasmon resonance (LSPR) in the nanostructures is very sensitive to the dielectric function, this uncertainty hinders the use of AuCu in plasmonic devices or structures. Therefore, we re-examine the question using a combination of measurements and computations. We find that no significant change in the dielectric function occurs when this material becomes ordered, at least over the range of photon energies relevant to LSPRs. The likely properties of LSPRs in plasmonic devices made of AuCu are analyzed. Use of the alloy offers some advantages over pure Cu, however pure Au would still be the superior option in most situations.