Ultrafast modulation of covalency in GeTe driven by a ferroelectric soft mode

The general idea of using ultrashort light pulses to control ferroic order parameters has recently attracted attention as a means to achieve control over material properties on unprecedented time scales. Much of the challenge in such work is in understanding the mechanisms by which this control can be achieved, and in particular how observables can be connected to structural and electronic properties. Here we report on a combination of experimental and computational methods to study the electronic structure of the semiconducting ferroelectric GeTe when driven out of equilibrium by absorption of a femtosecond pulse of light. We observe coherent modulations of second harmonic generations on the order of 50%, which we attribute to a combination of atomic and electronic structure changes due to a coherently excited soft mode. Comparison of the observed experimental data with model calculations indicates that this effect is predominantly due to an ultrafast modulation of the covalency of the bonding between Ge and Te ions. This stands in contrast to previously held assumptions in other systems, indicating that care should be exercised in using indirect measurements of electronic structure to make strong conclusions about the magnitude of nuclear motions.