Probing Photoinduced Structural Phase Transitions via Nonlinear Spectroscopy

The symmetry of a material is crucial to defining its electronic and structural properties. By manipulating this symmetry through photoinduced phase transitions, one can explore innovative methods for controlling material characteristics on ultrafast time scales. It is essential to employ techniques that can probe symmetrical changes in the temporal domain to capture these transitions. Here, by utilizing time-resolved third-order nonlinear spectroscopy, we demonstrate that a time-domain analysis of the coherent phonon dynamics can effectively reveal alterations in the symmetry of the lattice potential. This nonlinear approach serves as a fully optical method for investigating structural transitions. Focusing on the photoinduced structural phase transition in the α-CaF₂ dielectric crystal, we observe that as photoexcited carriers increase, the coherent phonon mode initially exhibits a softening effect. Subsequently, the transition from α-CaF₂ to γ-CaF₂ occurs at higher carrier density, corresponding to a switch from the high-symmetry Fm3̅m to the low-symmetry Pnma space group. The immediate emergence of equilibrium-phase phonon modes beyond the transition threshold indicates a nonthermal mechanism for the photoinduced symmetry changes, where significant perturbation of the lattice potential alters its symmetry before any ionic rearrangement takes place. Our findings open new avenues for investigating structural transitions on the femtosecond time scale.