Ultrafast Photoinduced Phase Change in SnSe

Time-resolved multiterahertz (THz) spectroscopy is used to observe an ultrafast, nonthermal 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 $6\text{ }\text{ }\mathrm{mJ}/{\mathrm{cm}}^{2}$ due to a photoinduced phase segregation and a macroscopic collapse of the band gap.