Femtosecond Single Cycle Pulses Enhanced the Efficiency of High Order Harmonic Generation.

Abdelmalek Taoutioui,Hicham Agueny

doi:10.3390/mi12060610

Abstract

High-order harmonic generation is a nonlinear process that converts the gained energy during light-matter interaction into high-frequency radiation, thus resulting in the generation of coherent attosecond pulses in the XUV and soft x-ray regions. Here, we propose a control scheme for enhancing the efficiency of HHG process induced by an intense near-infrared (NIR) multi-cycle laser pulse. The scheme is based on introducing an infrared (IR) single-cycle pulse and exploiting its characteristic feature that manifests by a non-zero displacement effect to generate high-photon energy. The proposed scenario is numerically implemented on the basis of the time-dependent Schrödinger equation. In particular, we show that the combined pulses allow one to produce high-energy plateaus and that the harmonic cutoff is extended by a factor of 3 compared to the case with the NIR pulse alone. The emerged high-energy plateaus is understood as a result of a vast momentum transfer from the single-cycle field to the ionized electrons while travelling in the NIR field, thus leading to high-momentum electron recollisions. We also identify the role of the IR single-cycle field for controlling the directionality of the emitted electrons via the IR-field induced electron displacement effect. We further show that the emerged plateaus can be controlled by varying the relative carrier-envelope phase between the two pulses as well as the wavelengths. Our findings pave the way for an efficient control of light-matter interaction with the use of assisting femtosecond single-cycle fields.

Highlights

High harmonic generation (HHG) is a coherent process that plays a key role in producing ultrashort coherent light in the extreme ultraviolet (XUV) and soft x-ray range [1,2], generating attosecond pulses [3,4], and has been growing rapidly for imaging molecular orbitals [5,6,7] and for temporal characterization of ultrafast processes [8,9].The underlying physics of HHG is well understood on the basis of the three-step model [10], in which tunnelling, acceleration and recombination of the electrons are the fundamental steps responsible of the generation of high-order harmonics
We show that varying the relative optical phase between the two pulses as well as the wavelength of the IR single-cycle pulse modifies dramatically the high-harmonic spectrum, and the extension of the plateaus can be controlled
The scheme consisted of an intense NIR multi-cycle pulse, which generated high-order harmonics and a weak IR single-cycle pulse introduced as an assisting field

Summary

Introduction

High harmonic generation (HHG) is a coherent process that plays a key role in producing ultrashort coherent light in the extreme ultraviolet (XUV) and soft x-ray range [1,2], generating attosecond pulses [3,4], and has been growing rapidly for imaging molecular orbitals [5,6,7] and for temporal characterization of ultrafast processes [8,9]. This has been discussed in the context of a THz single-cycle field interacting with Rydberg atoms [29,30] (see [31,32,33]) and its underlying physics has been shown to be valid in the ultrafast regime [34,35], in which a coherent displacement of the electron wavepacket was demonstrated [35] In this context, there has been recently a significant progress in developing schemes capable of producing single-cycle pulses in the infrared spectral range [36,37,38] for controlling ultrafast phenomena in gases and solids [38,39,40,41,42,43,44,45,46]. Atomic units (a.u.) are used throughout this paper unless otherwise specified

Theoretical and Computational Models

Results and Discussion

Conclusions