Dynamic Research Of High-energy Laser Ablation Of Target

Pulsed laser deposition (PLD) has received a great deal of attention nowadaysin the world as a promising and versatile technique, especially for growingthin films and preparing nanoparticle field. The research on the experiment isfar beyond the corresponding theoretical research. With the development ofchirp technology and mode-locking technique, high energy and short pulse arethe two main directions of laser development to the extreme conditions. Underthis condition, the rapid development trend of PLD calls for appearance ofcorresponding new mechanism.Our dissertation systematically and intensively focuses on the dynamic ofnanosecond and femtosecond PLD technology, specially, we discuss in detailthe dynamic caused by the the vaporization effect, plasma shielding effect, dynamicabsorption effects in nanosecond pulsed laser ablation (PLA) processing,and non-equilibrium conduction, electron-electron collisions, the effect causedby electron density of states (i.e., DOS effect), et al. We divide the thermophysicalprocessing into two subprocess which before and after the melting target toestablish more reasonable thermal conductivity dynamics model. More fruitfulresults in femtosecond PLA dynamic research are obtained: we establish a unifiedthermal model of thermophysical effects with pulse width from nanosecondto femtosecond, a new TTM equation includes electron-electron collisions andDOS effect which can be suitable for higher energy field. Using the correspondingnew model, take Ni, Au, Fe, Cu and superconducting YBCO as examples tosimulate their thermal conductivity properties, we find our simulation is moreprecise than the previous research. Our results will be valuable to understand widely the physical representation of PLA processing.The dissertation is organized as follows:The first chapter briefly introduces the PLD technique and correspondingdynamic mechanism, especially the ablation characteristics of nanosecond PLDand femtosecond PLD.The second and third chapters summarize our new results of thenanosecond pulsed laser (include ultraviolet and infrared one) ablationof one-component metal target and multi-elemental oxide superconductingYBa₂Cu₃O₇ (YBCO) target. Firstly, based on the superheat and phase transitionanalysis, two different heat flux equations for different ablation stages, namelybefore and after target melting are presented. Simultaneously considering influenceof vaporization effect, plasma shielding effect, dynamic absorption effects,the results obtained from the two heat flux equations are more reasonable.It must be mentioned that the mean ionization energy we introduced into theequation solve effectively the key part for the plasma shielding effect in multielementtarget. The dynamic development of space- and time-dependence oftemperature in the target is studied. Finally, the influence of vaporization effect,plasma shielding effect, dynamic absorption effects on whole ablation depth arestudied.In the fourth chapter, based on the study of equilibrium ablation fornanosecond laser ablation and non-equilibrium ablation for femtosecond laserablation, the ratio of the electron-phonon coupling timeÏ„_R and laser pulse widthÏ„_L is introduced as a smoothly transition parameter for making unified nonfourierthermal model. The space- and time-dependence of electron and latticetemperature of target, and the evolvement of vaporization threshold fluence withlaser pulse width are discussed. In the fifth chapter, the effect of pulse width and fluence of femtosecondlaser on the electron-phonon relaxation time is studied depending on twotemperaturemodel (TTM). For a certain laser fluence the shorter the pulsewidth, the shorter the electron-phonon relaxation time is. However, the electronphononrelaxation time becomes long for low laser fluence when the pulse widthis fixed.In the sixth and seventh chapters, the great improvement of femtosecondlaser ablation mechanism is studied following the increase of the laser energy.In the sixth chapter, when the electron temperature is higher than 4000K, bothelectron-phonon collisions and electron-electron collisions must be consideredfor describing the improved TTM. Utilizing the improved TTM equations, westudied the dynamic of the electrons and lattices on the target surface followingthe time and the three-dimensional development of the electrons and latticesinside the target following the time and the ablation depth. Moreover, in theseventh chapter, for electron temperature is higher than 10000K, the temperaturedependencies of thermophysical properties are studied for transition metalNi and noble metal Au based on electron density of states (DOS). The results ofthe analysis of the thermophysical properties at high electron temperatures areincorporated into TTM model and applied for simulations of laser melting ofthin films. The new calculated values of the threshold fluences for surface meltingare in a better agreement with the results of experimental measurements, sothe DOS effect can not be neglected when the electron temperature is higherthan 10000K.The main innovations of the dissertation are as follows:(1) Based on the consideration of vaporization effect, plasma shieldingeffect, dynamic absorption effect, two different heat flux equations for different ablation stages, namely before and after target melting, are presented.(2) A non-fourier unified thermal model is made for the first time, whichcan describe the thermophysical effects with laser pulse width ranges fromnanosecond to femtosecond. The space- and time-dependence of electron andlattice temperature of target, and the evolvement of vaporization threshold fluencewith laser pulse width are discussed in detail.(3) The improved TTM for high energy femtosecond ablation on the conditionthat temperature of electron is higher than 4000K is made by consideringelectron-phonon collisions and electron-electron collisions.(4) A new TTM is established by considering DOS effect under thecondition that temperature of electron is higher than 10000K.