Speaker
Description
Coherent driving of phonons with intense laser pulses usually does not lower the symmetry of a crystal. It generally causes large oscillations about the equilibrium structure or displaces the lattice along a fully symmetric coordinate. This makes it difficult to use light to reach lower-symmetry structures. I will discuss how nonlinear phononics can overcome this limitation through biquadratic couplings between two different phonon coordinates. In strained KTaO3, such a coupling between zone-center modes can induce ferroelectricity. I will then show that the same idea can be extended to zone-boundary phonons, where it provides a way to break translational symmetry. In KTaO3 this effect requires unrealistically high laser intensities, but it illustrates a mechanism for light-induced symmetry lowering beyond the zone center.