Replacing noble metals with alternative metals in MID-IR frequency: A theoretical approach

An ideal plasmonic material should respect different parameters as tunable plasma frequency, low energy loss, high chemical, mechanical and thermal stability, low cost and high integrability with existing technology. Obviously, not all these properties in a single material are reachable; this will depend on the specific application. Noble metals (especially Au and Ag) are considered the plasmonic materials per excellence, and are largely used for several applications as realizing micro-antennas for energy harvesting. However, the imaginary part of the permittivity and high-energy losses at THz frequency prevent their use imposing to find alternative materials. The permittivity response plays an important role in the design process and efficiency of a micro-antenna. In this paper the permittivity response of some metals such as Gold (Au), Silver (Ag), Copper (Cu) and Aluminum (Al) are investigated at MID-IR frequencies. Comparison among simulated dispersion curves allows us to investigate the origin of the dissipative behavior of the materials, which are an unavoidable prerequisite for any realistic application. Relationships among several parameters are under investigation. Moreover. the performance of each material is evaluated relying on quality factors Q or figure of merit (FOM) defined for each metal. Finally, the suitability and limitation of each one of these plasmonic materials will be discussed, starting with the more traditional noble metals to end up with alternative plasmonic metals at IR wavelengths.