  A major challenge in condensed matter physics is active control of quantum
phases. Dynamic control with pulsed electromagnetic fields can overcome
energetic barriers enabling access to transient or metastable states that are
not thermally accessible. Here we demonstrate strain-engineered tuning of
La2/3Ca1/3MnO3 into an emergent charge-ordered insulating phase with extreme
photo-susceptibility where even a single optical pulse can initiate a
transition to a long-lived metastable hidden metallic phase. Comprehensive
single-shot pulsed excitation measurements demonstrate that the transition is
cooperative and ultrafast, requiring a critical absorbed photon density to
activate local charge excitations that mediate magnetic-lattice coupling that,
in turn, stabilize the metallic phase. These results reveal that strain
engineering can tune emergent functionality towards proximal macroscopic states
to enable dynamic ultrafast optical phase switching and control.
