  Photostriction is predicted for SnS and SnSe monolayers, two-dimensional
ferroelectrics with rectangular unit cells (the lattice vector $\mathbf{a}_1$
is larger than $\mathbf{a}_2$) and an intrinsic dipole moment parallel to
$\mathbf{a}_1$. Photostriction in these two-dimensional materials is found to
be induced by a screened electric polarization in the photoexcited electronic
state (i.e., a converse piezoelectric effect) that leads to a compression of
$a_1$ and a comparatively smaller increase of $a_2$ for a reduced unit cell
area. The structural change documented here is ten times larger than that
observed in BiFeO$_3$, making monochalcogenide monolayers an ultimate platform
for this effect. This structural modification should be observable under
experimentally feasible densities of photexcited carriers on samples that have
been grown already, having a potential usefulness for light-induced, remote
mechano-opto-electronic applications.
