  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.
