VUV frequency combs from below-threshold harmonics

TL;DR

This paper investigates below-threshold harmonic generation in xenon gas for VUV frequency combs, revealing multiple generation pathways, coherence properties, and presenting the first explicit VUV comb structure beyond the 3rd harmonic.

Contribution

It provides a new quantitative understanding of below-threshold harmonic generation and demonstrates the coherence and structure of VUV frequency combs from these harmonics.

Findings

Multiple generation pathways identified for below-threshold harmonics

Temporal coherence of VUV combs is maintained under realistic conditions

First explicit VUV frequency comb structure beyond the 3rd harmonic demonstrated

Abstract

Recent demonstrations of high-harmonic generation (HHG) at very high repetition frequencies (~100 MHz) may allow for the revolutionary transfer of frequency combs to the vacuum ultraviolet (VUV). This advance necessitates unifying optical frequency comb technology with strong-field atomic physics. While strong-field studies of HHG have often focused on above-threshold harmonic generation (photon energy above the ionization potential), for VUV frequency combs an understanding of below-threshold harmonic orders and their generation process is crucial. Here we present a new and quantitative study of the harmonics 7-13 generated below and near the ionization threshold in xenon gas. We show multiple generation pathways for these harmonics that are manifested as on-axis interference in the harmonic yield. This discovery provides a new understanding of the strong-field, below-threshold dynamics under the influence of an atomic potential and allows us to quantitatively assess the achievable coherence of a VUV frequency comb generated through below threshold harmonics. We find that under reasonable experimental conditions temporal coherence is maintained. As evidence we present the first explicit VUV frequency comb structure beyond the 3rd harmonic.