Sub-cycle Electron Dynamics in the Generation of Below Threshold Harmonics

TL;DR

This paper investigates below threshold harmonic generation under various laser intensities, revealing that transient ac Stark shifts cause photon energy shifts and resonant enhancements, with sub-cycle electron dynamics playing a key role.

Contribution

It demonstrates that below threshold harmonic generation is governed by transient ac Stark shifts and sub-cycle electron dynamics, providing new insights into the underlying mechanisms.

Findings

Photon energy shifts linearly with laser intensity.

Resonant enhancement occurs around 18.6 TW/cm2.

Below threshold harmonics behave similarly to plateau harmonics.

Abstract

The generation of the below threshold harmonics (BTHs) under different driving laser intensities is investigated. The linearly shifting of photon energy and resonantly enhancement of photon yield of the harmonics from 23rd (H23) to 27th (H27) are found by changing the laser intensity around 18.6 TW/cm2. It is identified that this driving laser intensity dependence is due to the transient ac Stark-shifted resonance between the first excited state and the ground state. With this transient ac Stark, the linearly shifting of the photon energy can be interpreted very well by considering the sub-cycle electron dynamics for BTH generation, which shows that the generation of the BTHs is surprisingly similar to the plateau harmonics.