Study Of Nonlinear Optical Effects Of Ultrafast Laser In Liquid Water

Ultrafast laser filamentation in liquid water involves a variety of physical effects,including conical emissions(CE),super-continuum white light,bubbles generation,ultrasonic shock wave,stimulated Raman scattering(SRS)and laser filamentation induced chemical reactions.These physical effects will deepen people's understanding of water,the simplest but also the most complicated liquid on the earth,and thus have important implications for life sciences,environmental monitoring,underwater communications,nuclear cooling,and many other fields that are closely related to human life.However,due to the complexity of laser filamentation in water and the lack of visual diagnostic methods,laser filamentation in water still remains many unknowns to be explored.In this dissertation,we studied nonlinear effects during 800 nm and 400 nm femtosecond lasers undergoing filamentation in water,and laser filaments were visualized directly with gold nanoparticles suspended in water as scattering media.Thus we carefully studied processes including fluid dynamics,super-continuum scattering,interference effect and SRS that are induced by laser filamentation in water.The main contents of this dissertation can be summarized as follows:1.By using 800 nm femtosecond(fs)laser to generate filaments in water,we found that bubbles induced by laser filamentation move directionally in water.The angles between the bubbles' moving directions and the laser propagation direction vary at different positions along the filament,which exhibits a fusiform distribution.This phenomenon reveals the convection processes caused by laser filamentation induced shock waves in water,implying that laser has the potential to be a non-contact,fixedpoint optical stirring tool for reactants in liquid medium.In pure water,the density of bubbles at different positions of the filament reflects the local optical intensity.As laser filamentation in pure water has weak side scattering and plasma fluorescence,bubbles generated by laser filamentation could be seen as visual marks for judgement of laser filamentation in pure water.2.By using a picosecond(ps)fiber laser to ablate gold nugget in liquid water,we produced a batch of gold nanoparticles with diameter of about 30 nm.The laser ablation method doesn't need additional chemical reagents and possesses unique advantages including green,purity and natural stability of the product.Then,with 800 nm fs laser to form filaments in the aqueous gold colloids,we obtained “colorful” filaments with scattering color changing from cyan to orange along the propagation direction.The evolution of the super-continuum in aqueous gold colloids and the Mie scattering by gold nanoparticles are responsible for it.3.With low concentration gold nanoparticles added into water as scattering media,we directly visualized filamentary plasma grating induced by interference of two 800 nm fs filaments in water.To verify the existence of laser filamentation induced plasma generation along the periodic interference fringes,we utilized a pair of platinum electrodes to measure the variation of local conductivity in the solution.Significant enhancement of local electron density was observed at the intersecting region as two laser beams formed plasma grating in water.The results indicated that the interference of optical fields resulted in ultrahigh local laser fields that breakthrough the clamped intensity of a conventional filament,which was similar to plasma grating in gases media.Our work extended the adaptation of plasma grating to liquid medium.4.Efficient forward SRS was observed along 400 nm fs laser filaments in water,and with two 400 nm fs laser beams to form plasma grating in water,significant enhancement of SRS conversion was observed.Plasma was shown to play an important role for efficient Raman conversion in compensating the normal group velocity dispersion(GVD)of pump pulse and Stokes Raman pulse in water.In plasma grating case,the GVD is further compensated by the ultra-high plasma density at the intersection region,which acted as a buffer area for pulse self-compression.The enhanced SRS emission mechanism revealed in this work may be of directive significance for applications in intense Raman lasers and fs Raman spectroscopy.