THz Pump Driven Dynamics of Polar Nanoregions in Strontium Titanate*

We use ultrafast x-ray diffraction to elucidate the dynamics the lattice of SrTiO₃ after resonantly driving the ferroelectric soft mode using strong, single-cycle THz pulses. We observe a strong lattice response with a characteristic length scale in the range 10-60 nm indicative of spatially inhomogeneous absorption of THz pulse. The inhomogeneity occurs mainly along the surface normal. We observe the lattice response at this length-scale to be odd with the field and inverts upon inversion of the THz field. We interpret the inhomogeneous absorption as arising from polar nano regions (PNR)s within the quantum paraelectric phase of SrTiO₃. We further observe that the THz pump excites the transverse acoustic (TA) modes coupled to the soft transverse optical (TO) modes at the 10-60 nm length scale. Ultrafast X-ray diffraction allows us to resolve the PNR response and, more broadly, observe ultrafast dynamics at short length scales away from zone center.