Numerical study of propagating surface plasmons in complex alloy thin films

In this work, we numerically study the plasmonic response of heterogeneous two phase Ag0.45-Cu0.55 alloy thin films. This alloy undergoes spinodal decomposition and forms Ag-rich and Cu-rich phases in the microstructure. We combine phase-field modelling with FEM simulations to understand the optical response of the Ag0.45-Cu0.55 alloy thin films. We show that the far field response is insensitive to the microstructure unlike the nearfield which exhibits significant dependence on the morphology of phases in the microstructure. The inhomogeneous near fields on alloy thin films can have potential in enhancing the spin-orbit coupling of light.

I. Introduction

The conventional metals Ag, Au, and Cu are the metals of choice for visible range plasmonics [1]. Although these are high quality plasmonic materials, their tunability is limited due to the ohmic losses and their fixed optical response for a given wavelength. In this respect, alloying has been one of the traditional routes to tailor the properties of pure metals and can potentially result in novel functionalities [2]. However, this approach for increasing the materials available for visible plasmonics has only gained attention in the recent times. The plasmonic response of alloy thin films of Ag-Au, Ag-Cu, and Au-Cu was studied by Gong and Leite as a function of alloy composition [3]. They showed that the plasmon resonances supported by the alloy films change as a function of composition and lie within the resonance range of pure metal constituents. Furthermore, amongst these plasmonic alloys, Ag-Cu is a eutectic system that is cost-effective and forms compositionally inhomogeneous two-phase microstructures under appropriate fabrication conditions [4]. Depending on the composition of the alloy, the phase separation can happen either by spinodal decomposition or by nucleation and growth. This kind of two-phase microstructure is a unique and interesting system to investigate the behavior of surface plasmons. However, the effect of such complex microstructures on the plasmonic behavior has not yet been investigated.