  In a solid, the electronic subsystem can exhibit incipient order with lower
point group symmetry than the crystal lattice. External fields that couple to
electronic order parameters have rarely been investigated, however, despite
their potential importance to inducing exotic effects. Here, we show that when
inversion symmetry is broken by the antiferromagnetic (AFM) order in Cr2O3,
transmitting a linearly polarized light pulse through the crystal gives rise to
an in-plane rotational symmetry breaking (from C3 to C1) via optical
rectification. Using interferometric time-resolved second harmonic generation,
we show that the ultrafast timescale of the symmetry reduction is indicative of
a purely electronic response; the underlying spin and crystal structures remain
unaffected. The symmetry-broken state exhibits a dipole moment, and its polar
axis can be controlled with the incident light. Our results establish a
coherent nonlinear optical protocol by which to break electronic symmetries and
produce unconventional electronic effects in solids.
