Transient photoinduced cubic-like phase transition in orthorhombic SnSe

SnSe, a low-symmetry layered semiconductor, features weak-interatomic bonding, pronounced anharmonicity, and other characteristics that make it highly attractive for thermoelectric applications. Recent studies have demonstrated that SnSe undergoes a non-thermal phase transition from the Pnma phase to a non-equilibrium Immm phase when exposed to ultrafast laser irradiation at room temperature [Sundaram, et al. Nat. Mater. 1, 217 (2002)]. Here, we employ polarization-resolved ultrafast two-color pump–probe reflectivity spectroscopy to investigate the anisotropic hot carrier dynamics in the photoinduced Immm phase SnSe. We demonstrate that a coherent acoustic phonon mode at ∼46 GHz provides additional confirmation of the photoinduced phase transition. Concurrently, the incoherent dynamics reveal two polarization-dependent exponential decay components. Based on the two-temperature model, we extract the electron–phonon coupling strength to be 7.3 × 1014 W m−3 K−1 along the armchair axis and 8.7 × 1014 W m−3 K−1 along the zigzag axis—weak anisotropy of the in-plane carrier–phonon interactions in the photoinduced Immm phase SnSe. These findings unveil a non-equilibrium lattice structure with direction-dependent electron–phonon coupling, offering valuable insights into the excited-state behavior and ultrafast phase transition mechanisms in the low-symmetry materials.