Comparative Analysis of Metals and Alternative Infrared Plasmonic Materials

In the past decade or two, the field of nanophotonics has seen rapid development, empowered by introducing the concepts of plasmonics and metamaterials. The enabling feature behind this progress has been the use of noble metals that exhibit negative dielectric permittivities over a wide spectral range of visible and infrared wavelengths that allowed for manipulating the light on the subwavelength scale. Consequently, numerous interesting phenomena that otherwise do not exist in nature have been demonstrated in laboratories, but their transitions to practical applications have been painfully slow due to the large ohmic losses that are inherent in all metals. Doped semiconductors with lower losses have been proposed as new plasmonic materials to replace noble metals. Because the electron densities that are introduced with the artificial doping are always a few order of magnitude lower than what naturally available in metals, their plasma frequencies are shifted considerably toward longer wavelengths to the infrared (IR). This work compares these two categories of plasmonic media in structures that support either localized or propagating surface plasmon polaritons (SPPs) in mid-IR. We have found that in both cases new plasmonic materials underperform noble metals in terms of enhancing optical field in localized SPPs and reducing SPP propagation loss in plasmonic waveguides. The cause of this subpar performance is the inherently low electron density that yields a significantly reduced plasma frequency compared to noble metals. This fundamental property associated with all new plasmonic materials dictates that, while new materials do hold a number of advantages, including tunability and ability to withstand high temperatures, noble metals, even with their ohmic losses, are not likely to be replaced in the foreseeable future.