Replacing noble metals with alternative materials in plasmonics and metamaterials: how good an idea?

Noble metals that currently dominate the fields of plasmonics and metamaterials suffer from large ohmic losses. Some of the new plasmonic materials, such as doped oxides and nitrides, have smaller material loss, and using them in place of metals carries the promise of reduced-loss plasmonic and metamaterial structures, with sharper resonances and higher field concentrations. This promise is put to a rigorous analytical test in this work, which reveals that having low material loss is not sufficient to have reduced modal loss in plasmonic structures. To reduce the modal loss, it is absolutely necessary for the plasma frequency to be significantly higher than the operational frequency. Using examples of nanoparticle plasmons and gap plasmons one comes to the conclusion that, even in the mid-infrared spectrum, metals continue to hold an advantage over alternative media when it comes to propagation distances and field enhancements. Of course, the new materials still have an application niche where high absorption loss is beneficial, e.g. in medicine and thermal photovoltaics.This article is part of the themed issue 'New horizons for nanophotonics'.