Replacing metals with alternative materials for mid-IR to THz plasmonics and metamaterials – does it make sense?

Metals, which dominate the fields of plasmonics and metamaterials suffer from large ohmic losses which tends to dampen the lofty promise of nanoplasmonics and metamaterials. Therefore it would be greatly beneficial to identify alternative materials with smaller loss that have negative real part of dielectric constant and can potentially replace the metals.
New plasmonic materials, such as highly doped semiconductors including oxides and nitrides, have smaller material loss combined with the negative epsilon in the IR range, and using them in place of metals carries a promise of reduced-loss plasmonic and metamaterial structures, with sharper resonances and higher field concentration.  This promise is put to a rigorous analytical test in this work and it is revealed that having low material loss is not sufficient to have a reduced modal loss in plasmonic structures, unless the plasma frequency is 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 advantage over the alternative media.
Aside from alternative plasmonic materials in the mid and far infrared ranges of the spectrum there exists a viable alternative to metals – polar dielectrics and semiconductors in which dielectric permittivity (the real part) turns negative in the Reststrahlen region. This feature engenders the so-called surface phonon polaritons (SPhPs) capable of confining the field in a way akin to their plasmonic analogues, the SPPs. Since the damping rate of polar phonons is substantially less than that of free electrons, it is not unreasonable to expect that “phononic” devices may outperform their plasmonic counterparts. Yet a more rigorous analysis of the comparative merits of phononics and plasmonics reveals a more nuanced answer, namely that while phononic schemes do exhibit narrower resonances and can achieve a very high degree of energy concentration, most of the energy is contained in the form of lattice vibrations so that enhancement of the electric field, and hence the Purcell factor, is rather small compared to what can be achieved with metal nanoantennas. Still, the sheer narrowness of phononic resonances is expected to make phononics viable in applications where frequency selectivity is important.
Overall, for most of the application, such as waveguide propagation and antenna the metals outperform the alternative materials but these new materials may still find application niche where the high absorption loss is beneficial, e.g. in medicine and thermal photovoltaics.
 
About the research of the speaker: https://engineering.jhu.edu/ece/faculty/khurgin-jacob-b/