Sensing coherent phonon dynamics in solids with delayed even harmonics

TL;DR

This paper theoretically explores how delayed even harmonics in high harmonic generation can serve as sensitive probes of coherent phonon dynamics and electron interactions in solids, especially when inversion symmetry is broken.

Contribution

It introduces a novel theoretical analysis of even harmonic yields in pump-probe setups, revealing their sensitivity to phonon dynamics and electron-electron interactions.

Findings

Odd harmonics oscillate in phase at phonon frequency.

Even harmonics show order-dependent phase shifts.

Sensitive range of even harmonics reveals microscopic effects.

Abstract

High harmonics have emerged as a powerful ultrafast probe of phonon dynamics and electron-phonon interactions in solids, with most studies focusing on odd harmonics. Here, in a pump-probe setup with variable delay, we theoretically investigate how even harmonics reveal coherent phonon dynamics. If pump and probe pulses overlap temporally, the spatial interference effect resulting from a non-coaxial pump-probe setup suppresses harmonic yields. At longer delays, odd-harmonic yields oscillate in phase at the optical phonon frequency, whereas even harmonics exhibit order-dependent phase-shifted oscillations. We identify a responsive range of even harmonic orders, in which the delay of yield oscillations is highly sensitive to subtle features of phonon dynamics and electron-electron interactions. Our findings highlight the potential of even harmonics to elucidate microscopic effects in systems with dynamically broken inversion symmetry.