Reciprocal-space-trajectory perspective on high harmonic generation in solids

TL;DR

This paper introduces a reciprocal-space-trajectory method to explain high harmonic generation in solids, revealing the significance of pre-acceleration and electron-hole polarization, and proposes a new four-step model for the process.

Contribution

It develops a novel reciprocal-space-trajectory approach and a four-step model to better understand HHG in solids, emphasizing the role of pre-acceleration and interband polarization.

Findings

HHG cutoff extends with circular polarization

Harmonic yield is high at large ellipticity

HHG mechanism involves electron-hole polarization, not just re-collision

Abstract

We revisit the mechanism of high harmonic generation (HHG) from solids by comparing HHG in laser fields with different ellipticities but constant maximum amplitude. It is shown that the cutoff of HHG is strongly extended in a circularly polarized field. Moreover, the harmonic yield with large ellipticity is comparable to or even higher than that in the linearly polarized field. To understand the underlying physics, we develop a reciprocal-space-trajectory method, which explains HHG in solids by a trajectory ensemble from different ionization times and different initial states in the reciprocal space. We show that the cutoff extension is related to an additional pre-acceleration step prior to ionization, which has been overlooked in solids. By analyzing the trajectories and the time-frequency spectrogram, we show that the HHG in solids cannot be interpreted in terms of the classical re-collision picture alone. Instead, the radiation should be described by the electronhole interband polarization, which leads to the unusual ellipticity dependence. We propose a new four-step model to understand the mechanism of HHG in solids.