In the presence of electron-phonon coupling, an excitonic insulator harbors two degenerate ground states described by an Ising-type order parameter. Starting from a microscopic Hamiltonian, we derive the equations of motion for the Ising order parameter in the phonon coupled excitonic insulator ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$ and show that it can be controllably reversed on ultrashort timescales using appropriate laser pulse sequences. Using a combination of theory and time-resolved optical reflectivity measurements, we report evidence of such order parameter reversal in ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$ based on the anomalous behavior of its coherently excited order-parameter-coupled phonons. Our Letter expands the field of ultrafast order parameter control beyond spin and charge ordered materials.

• Received 15 June 2020
• Revised 25 August 2020
• Accepted 24 November 2020

DOI:https://doi.org/10.1103/PhysRevLett.125.267602