The ultrafast response of polar vortices under optical excitation*

Polar vortices were recently discovered in PbTiO$_{\mathrm{3}}$-SrTiO$_{\mathrm{3}}$ superlattices [1], stabilized at the nanoscale in the presence of depolarizing fields and mechanical stresses. We have found that these exotic structures, non-existent in the bulk ferroelectrics, possess rich ultrafast responses to fs laser excitation, which is attractive for high-density information storage and nanoscale actuation. Using optical pulse excitation above the bandgap of PbTiO$_{\mathrm{3}}$ layers and structural and second harmonic generation probes of ferroelectric order dynamics, a series of reversible and irreversible nano-ordering transitions were observed. X-ray diffraction and diffuse scattering on nanodomains aided the phase identification during time-resolved measurements. In this talk, we will discuss the dynamic interplay between collinear and toroidal ferroelectric domains aided by understanding from phase field modelling. [1] A.K. Yadav, R. Ramesh et el., "Observation of Polar Vortices in Oxide Superlattices", accepted, Nature (2015).