Nonlinear optical
properties of materials such as second and higher
order harmonic generation and electro-optic effect play pivotal roles
in lasers, frequency conversion, electro-optic modulators, switches,
and so forth. The strength of nonlinear optical responses highly depends
on intrinsic crystal symmetry, transition dipole moments, specific
optical excitation, and local environment. Using first-principles
electronic structure theory, here we predict giant second harmonic
generation (SHG) in recently discovered two-dimensional (2D) ferroelectric–ferroelastic
multiferroics–group IV monochalcogenides (i.e., GeSe, GeS,
SnSe, and SnS). Remarkably, the strength of SHG susceptibility in
GeSe and SnSe monolayers is more than 1 order of magnitude higher
than that in monolayer MoS<sub>2</sub>, and 2 orders of magnitude
higher than that in monolayer hexagonal BN. Their extraordinary SHG
is dominated by the large residual of two opposite intraband contributions
in the SHG susceptibility. More importantly, the SHG polarization
anisotropy is strongly correlated with the intrinsic ferroelastic
and ferroelectric orders in group IV monochalcogenide monolayers.
Our present findings provide a microscopic understanding of the large
SHG susceptibility in 2D group IV monochalcogenide multiferroics from
first-principles theory and open up a variety of new avenues for 2D
ferroelectrics, multiferroics, and nonlinear optoelectronics, for
example, realizing active electrical/optical/mechanical switching
of ferroic orders in 2D multiferroics and in situ ultrafast optical
characterization of local atomistic and electronic structures using
noncontact noninvasive optical SHG techniques