  Within the past few years, atomically thin black phosphorus (BP) has been
demonstrated as a fascinating new 2D material that is promising for novel
nanoelectronics and nanophotonics applications, due to its many unique
properties such as direct and widely tunable bandgap, high carrier mobility and
remarkable intrinsic in-plane anisotropy. However, its important extreme
nonlinear behavior and ultrafast dynamics of carriers under strong-field
excitation have yet to be revealed to date. Herein, we report nonperturbative
high harmonic generation (HHG) in monolayer BP by first-principles simulations.
We show that BP exhibits extraordinary HHG properties, with clear advantages
over three major types of 2D materials under intensive study, i.e.,
semimetallic graphene, semiconducting MoS$_2$, and insulating hexagonal boron
nitride, in terms of HHG cutoff energy and spectral intensity. This study
advances the scope of current research activities of BP into a new regime,
suggesting its promising future in applications of extreme-ultraviolet and
attosecond nanophotonics, and also opening doors to investigate the
strong-field and ultrafast carrier dynamics of this emerging material.
