Effect of change in number of electrons to optical properties and surface plasmon resonance of noble metals

We have performed first-principles calculations to investigate the effect of change in the number of electrons on optical properties of Cu, Ag, and Au metals in visible and near-infrared energy ranges for surface plasmon resonance (SPR) applications in Kretschmann configuration. We find that an increase in the deviation of the number of electrons leads to a decrease in the real part of the optical conductivity, ${\sigma }_1$, and an increase in the real part of the dielectric constant, ${\epsilon }_1$, for Ag and Au, but the decrease occurs in Cu. The changes in optical properties correspond to changes in the characteristics of the SPR curves; for Ag and Au, the SPR angle decreases, and the minimum reflectance increases, and in contrast, for Cu, the SPR angle increases, and the minimum reflectance decreases. Band-by-band decomposition analysis identifies that the prominent peak of optical conductivity arises from the interband transitions between the unoccupied uppermost d state and the conduction sp-like state, where an increase in the number of electrons causes a decrease in the prominent peak of optical conductivity in the metal, and vice versa. SPR simulation based on the calculated optical properties delineates the observed trend in SPR measurements. The results provide a scenario to improve the SPR biosensor’s performance by applying an electric field through the change in the number of electrons.