Pulsed Laser Ablation And Target Optical Characteristic Investigation

Pulsed laser deposition method is a new thin-film preparation technology with many advantages, and it has been developing very quickly. The PLD technique has so many advantages, such as high pulsed laser energy density, similarity between target and prepared thin film components, the high deposition rate, and lower substrate temperature. The whole process can be divided into three stages: laser ablation, plasma expansion and film growth. Combined PLD technology with other techniques, high-quality KTN thin films are prepared by our group. Based on accumulated experimental experiences, our researches emphasis is laid on theoretical mechanism on PLD technology. Recently, a more integrated Zhang-Li (Z-L) model has been proposed. Every stage of PLD processes are considered as a uniform whole in this model. At the same time, the physics mechanism and image of every state of PLD processes is investigated detailed.The paper aims in the ablation process of PLD. Based on consulting other experiments, Zhang-Li model and local theoretical works of other researchers, physical phenomena in ablation stage are completely studied in detail, such as the target optical characteristic, and two new models are present to describe the heat conduction in ablation. Three works are fulfilled:Firstly, the physical picture of ablation stage is discussed. The dynamic absorptance and absorption coefficient as the function of irradiation time and incident laser intensity are derived from two points of view: law of conservation of energy, and Maxwell Equations with Lambert Beer's law respectively. By finite difference and analytical method, the effect of dynamic absorptance and absorption coefficient on target temperature and melt depth is studied. The calculated results demonstrate that the effect of the dynamic absorptance and absorption coefficient on PLA is notable:Secondly, a non-Fourier conduction model with heat source term is presented to study the target temperature evolvement when the target is radiated by picosecond pulsed laser. By Laplace transform, the analytical expression of the space- and time-dependence of temperature is derived. The corresponding physical mechanism is analyzed.Lastly, a new theoretical model of multipulse femtosecond laser ablation has been developed to describe the behavior of target temperature. The characteristic of this model is that, the femtosecond laser ablation is considered as a periodic process,and the periods is the laser pulse and the interval between two pulse. Every pulse is divided into three different phases (pulse laser irradiation stage, two temperature continue stage, Fourier heat conduction stage). In this model, the temperature dependence of the absorptance and the vaporization is considered.The results of above investigation shows that1. As the target is ablated by pulse laser,as the Gaussian distribution is introduced to describe the incident laser intensity,the dynamic absorptance and absorption coefficient show little effect on PLA due to the small intensity at the beginning and the end of a pulse,while in the longer middle period of a pulse, the dynamic absorptance and absorption coefficient exhibit their effect on the state of the system due to the steady incident laser intensity. Their effect on the temperature after melting is larger than that before melting.2. As the target is ablated by picosecond pulsed laser,a. The effect of non-Fourier conduction behaves mainly that there is a delay of heat conduction in the inner target, and the deeper the target is, the longer the delay time is. The non-Fourier temperature is distinctly higher than that of the Fourier heat conduction model.b. The effect of heat source term becomes more and more remarkable after the target is radiated for 10-11 s in the condition of this paper. Compared to the results of the model without heat source term, the increase of surface temperature obtained by non-Fourier model is quicker.3. As the target is ablated by femosecond pulsed laser,a. For single pulse laser irradiation, the target temperature will decrease for the heat conduction and vaporization after the coupling time of electron and lattice. With the increase of either laser intensity or laser width, the coupling time becomes longer.b. For multipulse laser irradiation, the target temperature after coupling time ascends with the pulse number. This proves that energy residua indeed play an important role. And the descending of target temperature will be rapid more and more due to the evaporation.