  Phosphorene, a well-studied 2D allotrope of phosphorus, features unique
properties such as widely tunable bandgap, high carrier mobility, and
remarkable intrinsic in-plane anisotropy. Utilizing these structural and
electronic properties, we investigate ultrafast electron dynamics and high
harmonic generation (HHG) from phosphorene subject to band structure
engineering through external strain, based on ab initio time-dependent
density-functional theory approach. We show that strong field processes in such
systems can be optimized and controlled by biaxial tensile and compressive
strain engineering, that results in electronic structure modification. While
-10% strain resulted in closing of band gap, 2% strain increased the gap by 22%
with respect to 0.9 eV in pristine phosphorene, consequently affecting the high
harmonic yield. With reduction of gap, by applying strain from 2% to -10%, the
valence band near $\Gamma-$point becomes more flat and discreet, resulting in
large electronic density of states and enhanced electronic excitation, which
reflects in their ultrafast sub-cycle dynamics under laser excitation.
Moreover, due to its intrinsic in-plane anisotropy, harmonic yield with laser
polarization along the armchair (AC) direction is found to be higher than that
of the zigzag (ZZ) direction for all the strain cases. Nearly, an order of
magnitude enhancement of harmonic intensity is achieved for -10% strain along
AC direction. The current study expands the research possibilities of
phosphorene into a previously unexplored domain, indicating its potential for
future utilization in extreme-ultraviolet and attosecond nanophotonics, and
also for efficient table-top HHG sources.
