Anisotropic Optical Response of Gold–Silver Alloys

Gold and silver alloys present innovative possibilities for engineering metals with unique optical properties. However, earlier theoretical methods have obtained the permittivity of alloys by linearly interpolating the response of their pure counterparts or averaging out the spatially dependent anisotropic results of small cells owing to their tetragonal geometry. Here, the permittivity of gold–silver alloys (Ag x Au 1– x ) employing first‐principles density functional theory for gold concentrations of 25, 50, and 75% is calculated. The structures consisting of 4 atoms, along with supercells of 32 and 108 atoms obtained via the special quasirandom structure approach to capture critical short‐range correlations in atomic spatial distribution, are examined. Herein, the larger structures exhibit anisotropy in the permittivity, especially in the infrared part of the electromagnetic spectrum. The novel approach of projecting the permittivity over atomic orbitals is used to elucidate the findings on the nonlinear optical characteristics of Ag x Au 1– x alloys, demonstrating that the orbital contributions of the atoms differ along each direction. The subsequent investigations indicate that nonlinear optical properties, including second‐harmonic generation and third‐order susceptibility, could uncover other anisotropic behaviors, deepening the comprehension of electronic transitions in bimetallic alloys.