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. © 2022 IEEE.