Probing and controlling coherent and incoherent dynamics of phase transitions via multipulse excitation

Ultrafast laser pulses can selectively induce either wide-sized coherent or localized incoherent dynamics in solids, yet harnessing these complex dynamics to achieve controlled phase transitions remains challenging. Here, we use real-time time-dependent density functional theory (rt-TDDFT) to investigate a double-pulse laser scheme in VO₂, elucidating the roles of these two distinct types of dynamics and proposing two more energy-efficient phase transition routes. In the first scenario, a weak initial pulse induces coherent oscillations of V-V dimers. When the second pulse is applied at the dimer stretching time, transient bandgap narrowing enhances carrier excitation, thereby reducing the total laser fluence required for the phase transition. In the second scenario, a stronger but subthreshold first pulse activates localized structural distortions resembling photoinduced polarons. A subsequent long-wavelength pulse promotes accumulation and spatial propagation of the polarons, ultimately triggering a global phase transition. Our findings establish multipulse excitation as an energy-efficient and general approach for controlling structural phase transitions and disentangling distinct ultrafast dynamics in solids.