Ultrafast Photo-induced Phase Change in SnSe

SnSe is a quasi-two dimensional semiconductor exhibiting record setting thermoelectric properties driven by anharmonic phonons and intrinsic lattice instabilities. In this work, we use time-resolved multi-THz spectroscopy to observe a non-thermal, ultrafast electronic phase change in SnSe driven by above band gap photoexcitation. The THz photoconductivity is polaronic in nature and exhibits bimodal sub-picosecond dynamics with both fast (sub-30 fs) and slow (100-200 fs) components attributed to the injection of both free and polaronic charge carriers, respectively. Both the sub-picosecond dynamics and the conductivity spectra show a drastic change at a critical excitation fluence of approx. 6 mJ/cm2. Above this fluence, the THz mobility is reduced and polaron formation is suppressed indicative of a photo-induced phase change and an accompanying increase in charge scattering. We discuss the origin of this transition in light of several recent works reporting photo-induced structural phase changes sensed by light and electron scattering techniques. This work provides important insights into the electronic response of functional materials driven far beyond thermal perturbation.