Highly confined, low-loss plasmonics based on two-dimensional solid-state defect lattices

Plasmons, collective excitations of electrons in solids, are associated with strongly confined electromagnetic fields, with wavelengths far below the wavelength of photons in free space. Such strong confinement nominally holds the potential to enable optoelectronic technologies that bridge the size difference between photonic and electronic devices. However, despite decades of research in plasmonics, many applications remain limited by plasmonic losses, thus motivating a search for new engineered plasmonic materials with lower losses. Among the promising candidates for low-loss plasmonic materials are solid-state lattices with flat and energetically isolated metallic bands-with commensurately small phase spaces for phonon-assisted optical losses, a major contributor to short plasmonic lifetimes. Such electronic band structures may be created by judiciously introducing an ordered lattice of defects in an insulating host material. Here, we explore this approach, presenting several low-loss, highly confined, and tunable plasmonic materials based on arrays of carbon substitutions in hexagonal boron nitride monolayers. From our first-principles calculations based on density functional theory, we find plasmonic structures with midinfrared plasmons featuring very high confinements (lambda(vacuum)/lambda(plasmon) exceeding 1400). In addition, we find that one of our materials exhibits a confinement of 700 while avoiding second-order-phonon-assisted-losses entirely (infinite quality factor at this order of perturbation theory). We provide a systematic explanation of how crystal structure, electronic bandwidth, local-field, and many-body effects inform the plasmonic dispersions and losses of these materials. The results are thus of relevance to low-loss plasmon engineering in other flat band systems.