  We have systematically investigated the spatial and temporal dynamics of
crystallization that occurs in the phase-change material Ge2Sb2Te5 upon
irradiation with an intense THz pulse. THz pump and optical probe spectroscopy
revealed that Zener tunneling induces a nonlinear increase in the conductivity
of the crystalline phase. This result indicates that the large electric field
associated with the THz pulses causes a nonlinear increase only at the edge of
the crystallized area. The electric field concentrating in this area causes a
temperature increase via Joule heating, which in turn leads to nanometer scale
crystal growth parallel to the field and the formation of filamentary
conductive domains across the sample.
