Probing phonon dynamics with multi-dimensional high harmonic carrier envelope phase spectroscopy

TL;DR

This paper demonstrates a novel ultrafast spectroscopy technique using high harmonic generation sensitive to phonon dynamics and structural changes in monolayer materials, with femtosecond temporal resolution.

Contribution

It introduces a multi-dimensional HHG spectroscopy method that probes phonon-induced structural dynamics via CEP sensitivity, achieving sub-cycle temporal resolution.

Findings

HHG spectrum structure is affected by coherent phonons, becoming continuous in the plateau region.

HHG yield oscillates with pump-probe delay, reflecting ultrafast lattice changes.

CEP sensitivity in HHG reveals instantaneous structural dynamics, enabling femtosecond resolution.

Abstract

We explore pump-probe high harmonic generation (HHG) from monolayer hexagonal-Boron-Nitride, where a terahertz pump excites coherent optical phonons that are subsequently probed by an intense infrared pulse that drives HHG. We find, through state-of-the-art ab-initio calculations, that the structure of the emission spectrum is attenuated by the presence of coherent phonons, and is no longer comprised of discrete harmonic orders, but rather of a continuous emission in the plateau region. The HHG yield strongly oscillates as a function of the pump-probe delay, corresponding to ultrafast changes in the lattice such as bond compression or stretching. We further show that in the regime where the excited phonon period and the pulse duration are of the same order of magnitude, the HHG process becomes sensitive to the carrier-envelope-phase (CEP) of the driving field, even though the pulse duration is so long that no such sensitivity is observed in the absence of coherent phonons. The degree of CEP sensitivity vs. pump-probe delay is shown to be a highly selective measure for instantaneous structural changes in the lattice, providing a new approach for ultrafast multi-dimensional HHG-spectroscopy. Remarkably, the obtained temporal resolution for phonon dynamics is ~1 femtosecond, which is much shorter than the probe pulse duration because of the inherent sub-cycle contrast mechanism. Our work paves the way towards novel routes of probing phonons and ultrafast material structural changes and provides a mechanism for controlling the high harmonic response.