  Phosphorene has emerged as an atomically-thin platform for optoelectronics
and nanophotonics due to its excellent nonlinear optical properties and the
possibility of actively tuning light-matter interactions through electrical
doping. While phosphorene is a two-dimensional semiconductor, plasmon
resonances characterized by pronounced anisotropy and strong optical
confinement are anticipated to emerge in highly-doped samples. Here we show
that the localized plasmons supported by phosphorene nanoribbons (PNRs) exhibit
high tunability in relation to both edge termination and doping charge
polarity, and can trigger an intense nonlinear optical response at moderate
doping levels. Our explorations are based on a second-principles theoretical
framework, employing maximally localized Wannier functions constructed from
ab-inito electronic structure calculations, which we introduce here to describe
the linear and nonlinear optical response of PNRs on mesoscopic length scales.
Atomistic simulations reveal the high tunability of plasmons in doped PNRs at
near-infrared frequencies, which can facilitate synergy between electronic band
structure and plasmonic field confinement in doped PNRs to drive efficient
high-harmonic generation. Our findings establish phosphorene nanoribbons as a
versatile atomically-thin material candidate for nonlinear plasmonics.
