Light-induced electronic polarization in antiferromagnetic Cr2O3

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