Professor Venkatraman Gopalan (PSU) will give a colloquium entitled
"Ultrafast Light Activated Emergent Phenomena in Polar Oxides"
The fast-paced improvements in ultrafast radiation from X-ray and ultraviolet to terahertz (THz) frequencies is enabling simultaneous probe of electron, phonon, and spin dynamics on the ps-to- ns time scales, as well as sub-micrometer length scales. There is emerging recognition now that ultrafast dynamics and the appearance of new phenomena depend not only on the intrinsic properties of the compound, but also on the mesoscale structure such as surfaces, domains, walls, and interfaces that govern the coupling between various order parameters. Thus the study of emergent phenomena on the mesoscale requires new spatio-temporal imaging techniques, as well as the engineering of the mesostructure to create competing highly degenerate states that can be perturbed by light into new metastable states. In this talk, I will present two examples of ultrafast pump-probe studies in ferroelectrics. In the first example, femtosecond light pulses transform carefully counterpoised polar states inan oxide heterostructure into a metastable supercrystal phase with tens of nanometers periodicity. In the second example, ultrafast pumping of ferroelectric crystal reveals a large light-induced electric field near the surface that excites GHz waves with differing dynamics near the surface versus subsurface regions. A dynamical phase-field modeling (DPFM) method is developed to understand these rich spatio-temporal domain dynamics.