Midinfrared-light-induced structural symmetry breaking via nonlinear phononics

There is much interest in accessing the broken-symmetry phases of crystals using light. However, an arbitrary light pulse, whose oscillating electric field integrates to zero by definition, imparts a zero total force to the electric dipole present in the material.  Therefore, light pulses cannot in general be used to break the symmetries present in crystals.  I present theoretical results that show that this limitation can be overcome if a strong quartic coupling of the type $Q_1^2Q_{\textrm{IR}}^2$ is present between a high-frequency infrared mode $Q_{\textrm{IR}}$ and a low-frequency mode $Q_1$ with a negative coupling constant $g$.  Realistic calculations show that such a coupling is present in oxide paraelectrics and can be used to induce ferroelectric phases using midinfrared pulses.