  Second harmonic generation (SHG) in van der Waals (vdWs) materials has
garnered significant attention due to its potential for integrated nonlinear
optical and optoelectronic applications. Stacking faults in vdWs materials, a
typical kind of planar defect, can introduce a new degree of freedom to
modulate the crystal symmetry and resultant SHG response, however, the physical
origin and tunability of stacking-fault-governed SHG in vdWs materials remain
unclear. Here, taking the intrinsically centrosymmetric vdWs RhI3 as an
example, we theoretically reveal the origin of stacking-fault-governed SHG
response, where the SHG response comes from the energetically favorable AC-
Cstacking fault of which the electrical transitions along the high symmetry
paths Gamma-M and Gamma-K in the Brillion zone play the dominant role at 810
nm. Such stacking-fault-governed SHG response is further confirmed via
structural characterizations and SHG measurements. Furthermore, by applying
hydrostatic pressure on RhI3, the correlation between structural evolution and
SHG response is revealed with SHG enhancement up to 6.9 times, where the
decreased electronic transition energies and huger momentum matrix elements due
to the stronger interlayer interactions upon compression magnify the SHG
susceptibility. This study develops a promising foundation based on
strategically designed stacking faults for pioneering new avenues in nonlinear
nano-optics.
