Effects of Laser Plasma Formation on Quasi-Phase Matching of High-Order Harmonics from Nanoparticles and Atoms.

Rashid Ganeev,Ganjaboy Boltaev,Chunlei Guo,Vyacheslav Kim

doi:10.3390/nano9040572

Abstract

The application of nanoparticles (NPs) and quasi-phase matching (QPM) each play an important role in the enhancement of high-order harmonics (HHG) of ultrashort laser pulses. We analyze various regimes of nanoparticle plasma formation for the creation conditions for maximal QPM-induced enhancement of the groups of harmonics in the extreme ultraviolet (XUV). Laser plasmas were formed on the surfaces of NPs- and microparticle (MPs)-contained targets using ablation by nanosecond, picosecond, and femtosecond pulses. Different conditions of laser plasma formation (extended and perforated plasma) and variable concentrations of free electrons in these three cases of laser ablation led to modifications of QPM conditions. We demonstrate novel approaches in the optimization of QPM at the conditions of laser ablation of NPs and MPs by pulses of different durations. The formation of QPM conditions using femtosecond and picosecond heating pulses during HHG in such plasmas allowed the growth of conversion efficiency of the groups of harmonics, with the enhancement factors exceeding 25× in different ranges of XUV, contrary to less efficient QPM in the case of nanosecond pulse-induced ablation.

Highlights

High-order harmonic generation (HHG) using ultrashort laser pulses remains a most reliable method of formation of the table-top coherent extreme ultraviolet (XUV) sources possessing unique temporal characteristics
Our experiments showed that the maintenance of the fluence of heating pulses in this range allows the formation of quasi-phase matching (QPM) conditions
We have demonstrated this process in the molecular (Ag2S), nanoparticle, and microparticle (B and Ag) plasmas

Summary

Introduction

High-order harmonic generation (HHG) using ultrashort laser pulses remains a most reliable method of formation of the table-top coherent extreme ultraviolet (XUV) sources possessing unique temporal characteristics. Among different methods of harmonic enhancement, one can admit the formation of the quasi-phase matching (QPM) conditions between the driving and generating waves during HHG [7,8,9] This mechanism has been originally demonstrated in the visible range using solid materials [10]. One of the approaches here is the division of an extended medium into groups of smaller media separated by a vacuum In these parts of the extended medium, the conditions of harmonics enhancement are maintained along the whole length (so-called coherence length) of divided plasmas and gas media until the phase difference ∆φ between converting and converted waves becomes equal to π

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Conclusion