  Time-resolved multi-terahertz (THz) spectroscopy is used to observe an
ultrafast, non-thermal electronic phase change in SnSe driven by interband
photoexcitation with 1.55 eV pump photons. The transient THz photoconductivity
spectrum is found to be Lorentzian-like, indicating charge localization and
phase segregation. The rise of photoconductivity is bimodal in nature, with
both a fast and slow component due to excitation into multiple bands and
subsequent intervalley scattering. The THz conductivity magnitude, dynamics,
and spectra show a drastic change in character at a critical excitation fluence
of approximately 6 mJ/cm^2 due to a photo-induced phase segregation and a
macroscopic collapse of the band gap.
