Investigation Of Resonance Enhanced High-order Harmonic Generation From Laser-ablated Plasma Plume

High-order harmonic generation(HHG)is a powerful and well established technology to study the ultrafast process in atoms and molecules.It provides a compact and versatile coherent light source in the extreme ultraviolet and soft X-ray range.Due to the limitation of material state,the initial HHG research was mainly focused on atomic or small molecular gases.In the past decades,the method of laser-ablated plasma plume using as HHG medium has arroused researchs'interest.This method can be adopted to study HHG from almost any solid materials.Of particular interest is the characteristic of single-order resonance enhanced harmonics obtained from plumes of several metallic materials.This kind of enhancement is much stronger than the neighbors that is considered to be helpful to improve the harmonic conversion efficiency to a certain extent.This dissertation is carried out based on the single-order resonance enhancement of harmonic in the ablation plume,which can be mainly divided into the following aspects.1.Although there have been many researches about single-order resonance enhancement of harmonics,there are still macro(phase matching)or micro(single atom responce)disputes about the mechanism of the single-order resonance enhanced harmonic.In order to clarify whether the macro dispersion characteristic of plasma plume is the key factor for the enhancement,we carried out experiments through a two-component plasma medium.It is known that tin can produce 17^th resonance enhanced harmonic driven by 800 nm laser,and the other component is lead,which has no special properties under this laser wavelength.By comparing the harmonic spectra from pure tin plasma and tin&lead alloy plasma,the dependence of single-order resonance enhancement on phase matching conditions is analyzed.Further,by changing the driving laser ellipticity,we carried out the dependence experiment of the yield of resonant 17^th harmonic from tin ablation plume with the driving laser ellipticity.It is found that the dependence of the resonance enhanced harmonic with the driving laser ellipticity is independent from the variation trend of the overall harmonics,and it is relative less affected by the variation of ellipticity,meanwhile,this result is restructured by single-atomic quantum mechanical simulation.The experimental results from mixed plume and ellipticity dependence experiments both confirm that the resonance enhancement of 17^th harmonic of tin is more inclined to the single-atomic response mechanism.2.Among the HHG media,nano-clusters or nanoparticles combine the advantages of gas atomic medium(low average density)and solid medium(local high density),are very attractive in HHG.It is the most commonly used method to study HHG from nanoparticles by ablating samples composed of nanoparticles to produce nanoparticle-rich plume as HHG medium.Previous researchers believe that the harmonics from the plume under a low ablation intensity mainly dominated by nanoparticles,not isolated atoms or ions.However,this hypothesis has not been fully proved.We experiment on the relationship between harmonic yield and the distance between the driving laser and the sample surface(the interaction position of driving laser with the ablation plume)through the plasma plume produced by ablating the samples composed of indium nanoparticles.Results show that,both nanoparticle and monomer may play a significant role in the generation of harmonics in the same plume,which depends on the position where the plume interacting with the driving laser.The harmonic spectrum generated from nanoparticles is obviously different from that from monomers,mainly in the resonance enhancement and cut-off energy of harmonics.Our results demonstrate that accurately capturing the contribution of nanoparticles in HHG processes requires accurate selection of the laser-plasma interaction region,a factor that has been neglected or not carefully considered in previous studies.3.The composition of the plume produced by laser ablation is uncertain,which may contain atoms,ions or clusters,this may be a brake on the study of the harmonics from a single kind of particle.Therefore,we designed a new method to study HHG.Dense nanoparticles are used as the media,and the difference from the HHG experiment in the way of laser-ablated plasma plume is that driving pulse directly interacts with dense nanoparticles to produce HHG without using a pre-pulse,which eliminates the influence of other particle components in the plasma plume.Intriguingly,with this method,the resonance enhancement of single-order harmonics are also observed from medium like dense indium nanoparticle.Here,the appearance of enhancement of harmonic seems to be inconsistent with the explanation of enhancement by single-atomic mechanism.To this end,the ionization rate of particles is calculated and the possible reason of the enhancement is discussed.