Light-induced dimension crossover dictated by excitonic correlations*

Strong electronic correlation coupled with reduced dimensionality is a key driver for novel states in condensed matter, giving rise to the fractional quantum Hall effect, unconventional superconductivity, and Wigner crystallization. When these low-dimensional systems are subjected to an ultrafast laser pulse, the carriers excited abruptly modify the many-body Coulombic interaction, yielding a variety of metastable states that greatly expand the equilibrium phase diagram of quantum materials. The central challenge in understanding such phenomena is to determine how dimensionality and many-body correlations govern the pathway of a non-adiabatic transition. To this end, we examine a layered compound, 1T-TiSe₂, whose three-dimensional charge-density-wave (3D CDW) ground state also features exciton condensation due to strong electron-hole interactions. Using ultrafast electron diffraction, we find that light excitation suppresses the equilibrium 3D CDW while creating a nonequilibrium 2D density wave. Remarkably, the dimensional reduction in 1T-TiSe₂ does not occur unless bound pairs of electrons and holes are first broken. This relation suggests that excitonic correlations maintain the out-of-plane CDW coherence, settling a long-standing debate over their role in the CDW transition. Our findings demonstrate how optical manipulation of electronic interaction enables one to control the dimensionality of a broken-symmetry order, paving the way for realizing other emergent states in strongly correlated systems.