Multi-dimensional high harmonic spectroscopy of ultrafast phonon dynamics in hBN

High harmonic spectroscopy has recently been pioneered as a promising method for probing ultrafast electron dynamics in solids. However, its possible applications for tracking structural changes and phonon dynamics are not yet clear. We explore here pump-probe high harmonic generation (HHG) from monolayer hexagonal-Boron-Nitride, where a terahertz pump excites optical phonons that are subsequently probed with intense laser pulses that drive HHG. Through state-of-the-art ab-initio calculations, we show that the structure of the HHG spectrum is attenuated by the phonons and comprises a continuous emission in the plateau region. The HHG yield strongly oscillates with the pump-probe delay, corresponding to femtosecond-scale changes in the lattice such as bond compression or stretching. Remarkably, we show that the HHG process can become sensitive to the carrier-envelope-phase (CEP) of the driving laser, even when the pulse duration is long. We find that the degree of CEP sensitivity vs. pump-probe delay is a highly selective measure for instantaneous structural changes in the lattice, providing a new approach for ultrafast multi-dimensional HHG-spectroscopy. Our work shows new routes for probing ultrafast structural changes and provides a mechanism for controlling solid HHG emission.