In recent decades, plasmonic devices are widely interested because of the capability of subwavelength confinement. The plasmon phenomena is generated by oscillation of free charges in optical frequency. Hence, the metallic component is general used in plasmonic device. Metal provides large amount of free charges and the negative real part of permittivity, which is the essential property of plasmonic material. However, the loss of metal is critical issue of the devices, which occur from the interband transition in visible and ultra-violet range. Thence, the engineering of permittivity is the important topic for plasmonic devices.
The interband transition of noble metal is known as the electrons transition from occupied d bands to unoccupied states. The concepts of searching materials for low loss alloy is based on the structure of density of state.