Studies On The Dynamics Of Femtosecond Laser Filamentation In Solids

When femtosecond laser pulses propagate in transparent optical media,a bright plasma channel can be generated due to the interplay of several linear and nonlinear optical effects.This phenomenon is termed as femtosecond laser filamentation.Due to its potential applications in remote sensing,lightening control,laser machining,few-cycle laser technology,ultrafast spectroscopy technique and many other areas,femtosecond laser filamentation has been a hot topic in recent 30 years.Up to now,the underlying physical mechanisms of femtosecond laser filamentation are still on debate.For example,how do the higher-order effect and the plasma effect affect the femtosecond laser filamentation.Therefore,a more comprehensive understanding of the dynamics of femtosecond laser filamentation is still in demand.Since direct measurements of the electrons,the ions and other products inside the filament in solids are basically impossible,other ways to research the dynamics of femtosecond laser filamentation have to be used.Fortunately,the supercontinuum,which is always generated accompanied by the generation of filament,contains the information of the dynamics of femtosecond laser filamentation.Thus,it is a feasible way to investigate the dynamics of femtosecond laser filamentation by studying the generated supercontinuum.In this dissertation,we develop all-optical detection methods,including the measurement of the supercontinuum and the changes of refractive index to investigate the dynamics of femtosecond laser filamentation in solids.The dissertation is organized as follows:In Chapter 1,we introduce the background and the research progress of femtosecond laser filamentation.In Chapter 2,we introduce the femtosecond laser systems and the experimental methods based on spectral measurement in our study of the dynamics of femtosecond laser filamentation.In Chapter 3,the dynamics of femtosecond laser filamentation in fused silica,generated by laser pulses with various beam profiles,are studied.We measure the angle-resolved supercontinuum generated in fused silica and find that the nonlinear X-wave model is most suitable for explaining the evolution of the supercontinuum.The experimental results suggest that pulses with various beam profiles all evolve into nonlinear X-waves during femtosecond laser filamentation.In Chapter 4,we focus on the dynamics of femtosecond laser filamentation generated in fused silica by using the Z-scan method.We measure a series of normalized transmittance under different incident laser energies and find that the higher-order Kerr effect is obvious even at a relatively low energy.The experimental results show that it is not complete to explain the mechanisms of femtosecond laser filamentation only by the "standard model".The higher-order Kerr effect plays a nonnegligible role in femtosecond laser filamentation in fused silica.In Chapter 5,we investigate the dynamics of femtosecond laser filamentation in BaF2 crystal by measuring the intensity and width of the supercontinuum.We find that the energies corresponding to the critical power of filamentation have a periodicity of ?/2 due to its fourfold symmetry.We also find that the cut-off of the blue-shifted wavelength of the supercontinuum keeps the same at different crystal orientations,which is not fitted for the "standard model".The experimental results also suggest that the "standard model" is not sufficient to explain the mechanisms of femtosecond laser filamentation.In Chapter 6,the depolarization properties of a femtosecond-laser-induced supercontinuum generated in BaF2 crystal at different incident laser energies and under various crystal orientations are investigated.We find the depolarization of the supercontinuum rises with the increase of the input laser energies and finally saturates at most crystal orientations.While at 45 degree,the depolarization of the supercontinuum is not changed and keeps nearly negligible with the increase of the input laser energies.These depolarization properties of the supercontinnum can be attributed to the interplay of the XPW generation and strong-field-ionization induced plasma scattering in BaF2 crystal.The experimental results reveal the critical role of the generated plasma in the depolarization properties of a supercontinuum and provide a more comprehensive understanding of the depolarization of the supercontinuum during femtosecond laser filamentation in transparent crystals.In Chapter 7,the conclusions and prospects are presented.