  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. This strong confinement promises the
realization of optoelectronic devices that could 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. A
promising pathway for low-loss plasmonic materials is the engineering of
materials with flat and energetically isolated metallic bands, which can
strongly limit 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 (hBN) monolayers. From our
first-principles calculations based on density functional theory (DFT), we find
plasmonic structures with mid-infrared plasmons featuring very high
confinements ($\lambda_{\text{vacuum}}/\lambda_{\text{plasmon}}$ exceeding
2000) and quality factors in excess of 1000. We provide a systematic
explanation of how crystal structure, electronic bandwidth, and many-body
effects affect the plasmonic dispersions and losses of these materials. The
results are thus of relevance to low-loss plasmon engineering in other flat
band systems.
