  Achieving fundamental understanding of insulator-to-metal transitions (IMTs)
in strongly correlated systems and their persistent and reversible control via
nonequilibrium drive are prime targets of current condensed matter research.
Photoinduced switching between competing orders in correlated insulators
requires a free-energy landscape with nearly degenerate ground states, which is
commonly reached through doping, strain, or static electric field. The
associated spatial inhomogeneity leads to a photoinduced phase transition that
remains confined near the illuminated region. Here we report optical
spectroscopy experiments at the first-order IMT in the 4d-electron compound
Ca$_3$(Ru$_{0.99}$Ti$_{0.01}$)$_2$O$_7$ and show that specific Ru d-d interband
transitions excited by light with a threshold fluence corresponding to the
planar density of Ru atoms can trigger reversible, avalanche-like coherent
propagation of phase interfaces across the full extent of a macroscopic sample,
in the absence of assisting external stimuli. Based on detailed comparison of
spectroscopic data to density functional calculations, we attribute the
extraordinary photo-sensitivity of the IMT to an exceptionally shallow
free-energy landscape generated by the confluence of electron-electron and
electron-lattice interactions. Our findings suggest
Ca$_3$(Ru$_{0.99}$Ti$_{0.01}$)$_2$O$_7$ as an ideal model system for building
and testing a theory of Mott transition dynamics in the presence of strong
electron-lattice coupling and may pave the way towards nanoscale devices with
quantum-level photosensitivity.
