Ultrafast photocarrier dynamics of black phosphorus under pressure

In situ ultrafast photocarrier dynamics of multilayer black phosphorus (BP) are investigated under pressure using optical pump-probe spectroscopy. Below 10 GPa, the transient reflectivity exhibits sub-picosecond saturable absorption (SA) followed by oscillations arising from longitudinal coherent acoustic phonons (CAPs). With increasing pressure, pronounced anomalies in carrier relaxation and CAP behavior are observed, including a strong enhancement of CAP amplitude around 2.0 GPa, associated with a pressure-induced Lifshitz transition. Above 10 GPa the ultrafast response switches from SA to absorption enhancement (AE), accompanied by the complete disappearance of CAPs, indicating the transition to a cubic metallic phase. The pressure-dependent behavior of the CAPs reflects an enhanced interlayer coupling along the cross-plane direction, while the transition from SA to AE dynamics signifies a fundamental shift in in-plane carrier transport. Meanwhile, first-principles calculations reveal a pressure-induced reconstruction of the electronic structure and an increase in the longitudinal acoustic phonon group velocity across Lifshitz transition, supporting the microscopic understanding for anomalous CAP dynamics based on enhanced deformation-potential coupling and electron temperature of Dirac carriers. The study provides critical insights into the pressure-tuned topological transitions and the role of Dirac fermions in the nonequilibrium dynamics of compressed BP.