# Spectral Interference in High Harmonic Generation from Solids

## TL;DR

This paper investigates quantum interference effects in high harmonic generation from solids, revealing how electron trajectory interference causes spectral modulations and broadening, with implications for controlling harmonic phase matching and probing crystal properties.

## Contribution

It provides the first experimental observation and theoretical interpretation of quantum path interference effects in HHG from solids, highlighting the role of long and short electron/hole trajectories.

## Key findings

- Spectral modulation and broadening of high harmonics in sapphire observed experimentally.
- Quantum path interference causes intensity-dependent spectral splitting and redshift.
- Potential for precise phase control and crystal probing in extreme ultraviolet and soft X-ray regimes.

## Abstract

Various interference effects are known to exist in the process of high harmonic generation (HHG) both at the single atom and macroscopic levels. In particular, the quantum path difference between the long and short trajectories of electron excursion causes the HHG yield to experience interference-based temporal and spectral modulations. In solids, due to additional phenomena such as multi-band superposition and crystal symmetry dependency, the HHG mechanism appears to be more complicated than in gaseous atoms in identifying accompanying interference phenomena. Here, we first report experimental data showing intensity-dependent spectral modulation and broadening of high harmonics observed from bulk sapphire. Then, by adopting theoretical simulation, the extraordinary observation is interpreted as a result of the quantum path interference between the long and short electron/hole trajectories. Specifically, the long trajectory undergoes an intensity-dependent redshift, which coherently combines with the short trajectory to exhibit spectral splitting in an anomalous way of inverse proportion to the driving laser intensity. This quantum interference may be extended to higher harmonics with increasing the laser intensity, underpinning the potential for precise control of the phase matching and modulation even in the extreme ultraviolet and soft X-ray regime. Further, this approach may act as a novel tool for probing arbitrary crystals so as to adjust the electron dynamics of higher harmonics for attosecond spectroscopy.

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Source: https://tomesphere.com/paper/1903.10749