Laser tuning the electronic and structural order and electron-phonon couplings in 1<i>T</i>-TaSe₂

Ultrafast laser excitation can produce states of matter that are thermally inaccessible, enabling the exploration of new properties as well as providing insight into the microscopic interactions. In this work, we use time- and angle-resolved photoemission spectroscopy to study the electronic structure and electron-phonon couplings throughout a newly-uncovered laser-enriched phase diagram in the charge density wave (CDW) material 1T-TaSe₂. First, we drive the material into new metastable intermediate CDW states by finely tuning the laser fluence, to achieve mode-selective electron-phonon coupling. As a result, the transient heat capacity is substantially reduced, which supports an energy-efficient phase transformation route. Moreover, the data suggest a switching of the dominant coupling mechanism between the coherent amplitude mode and electrons. Second, we detect a transient inverted CDW state by strongly exciting the material, to coherently over-drive the periodic lattice distortion. The dynamic electronic band structures show signatures of novel high metallicity, and an inversion of the momentum dependence of the coupling between the amplitude mode and Ta 5d band. These results demonstrate an ultrafast and coherent route to steer and control quantum materials using light.