Analysis Of Plasma Property Induced By Pulsed-laser Ablation Of KTiOAsO₄,KTiOPO₄ And SiC Targets

Pulsed-laser ablation has been investigated extensively over the past several years because of the growing number of applications:(1) Pulsed laser plasma propulsion technology offers the prospect of delivering high thrust at high specific impulse (500-1000seconds) from a very simple thruster, using the energy of a remote ground-based laser to heat an inert propellant. This research addresses feasibility issues of a wide range of propulsion concepts, and may result in the development of the technologies that will enable exciting new missions within our solar system and beyond;(2) Pulsed-laser ablated plasma emission spectroscopy (PLAPES),which has long been used as a very promising technique for real-time detection of multiple elements and determining the composition of materials in any physical state, including solids, liquids and aerosols; (3) Pulsed-laser ablation of solids has been used as a unique technique to deposit high quality films for variety of applications, such as production of circuit components with superconducting or insulating properties, wearing resistance films, and biocompatible devices for medical application.The improvement of these applications is highly related to the fundamental mechanisms underlying laser ablation process, the aerosol transport and the vaporization, atomization and ionization within the plasma. As well-known, the electronic temperature (Te), density (Ne) in plasma and kinetic energy of the ablation plume are very important parameters to control the reactive plasmas. Therefore, in this dissertation, optical emission spectroscopy has been used to investigate the spatial and temporal evolution of these fundamental parameters, and the characteristics of dynamics distribution in the plume. The systematic investigations will be essential for understanding the mechanisms of plasma, and crucial for all aspects of laser beam applications. The plasmas were produced by ablation KTiOPO4(KTP), KTiOAsO4(KTA) and SiC crystal targets with the laser pulses from a Nd:YAG laser.The characteristics of the laser produced plasma strongly depend on the intensity of the irradiating laser beam. In this dissertation, optical emission studies are carried out on the spatial and temporal evolutions of the plasma over KTA samples produced by different laser radiations. The results show that the electron temperature and density at every delay time, and the lifetimes of plasma increase remarkably with increasing laser intensities for lower laser intensities and then come to saturation for higher laser intensities. It is noteworthy that the lifetime scales of plasma change substantially from 500 ns at 1.68 GW/cm2 to 700 ns at 2.54 GW/cm2, at 500ns delay time, the Te increase from 1.55eV tol.97eV as the laser intensities change froml.68-2.54 GW/cm2.The effect of laser wavelength is taken on the temporal evolution of plasma over KTP targets generated by 532 and 1064nm laser radiations. During the plasma expanding process, the electron temperature increases to a maximum of 2.2eV for 532 nm and 1.9eV for 1064 nm laser irradiations. The 532 nm laser beam can create hotter plasma than the 1064 nm laser beam. These results are expected be helpful for growing oxide thin films using shorter wavelength laser beam.Details of the experimental results on the spatial dependence of electronic temperature Te and density Ne in the plasma generated by ablation of the SiC samples using Nd:YAG laser pulses have been presented. From either the measurements of Te and Ne as a function of distance from the target or from the measurement of the core sizes of the plasma images recorded by a digital camera, it is found that in the intensity range used in this work, from 0.24 to 1.66 GW/cm2, the dimension of the core of the SiC plasma remains nearly constant, while the extension of the tail of the plasma increases remarkably with the laser intensity. It can be interpreted by absorption and self regulating of the plasma formed near the target surface. We have concentrated our attention on the fluctuation behavior of electron density in the tail region of the plasma at higher laser intensities. The electron density significantly changes in this region, which implies that the tail becomes more reactive and the structure of the plume becomes more instable. This behavior is attributed to the stronger plasma shielding and absorption of the laser energy in the case of irradiating at higher laser intensities. It reduces the laser irradiance reaching the target and also causes re-heating and re-ionization to form a higher density in the tail of the plasma. This behavior may be considered partly responsible for the complicated composition frequently observed near the surfaces of the films prepared by PLD.We also studied the temporal and spatial evolution of the well-isolated lines at 633.19 nm emitted from Si atoms,634.71 and 637.13 nm from Si ions in the plasma. In vacuum (～2.7 Pa), the spatial profile of 633.19 nm in the plasma extends from the target surface to a distance near the edge of the plume (16 mm).It is interesting that the time scales (～175 ns) of the line at 633.19 nm from Si atoms exists only in the early-stage of the plasma evolution in vacuum, are much shorter than those (～430 ns) for Si ions at 634.71 and 637.13 nm the spectral line. But the signal at 633.19 nm emitted from the Si atoms disappears in air environment (105 Pa) due to the gas phase collision effects in air background. While the signal intensity of spectral line at 637.13 nm originating from Si ions in air medium is almost a factor of 5 larger than that measured in vacuum, and the lifetimes are increase from 430 ns to 1.428×104ns. In order to give more explanations about these statements, a briefly theoretical estimation of the shock pressure induced by laser ablating SiC target were studied. It is deduced that by properly adjust the ambient gas pressure in the chamber during the deposition of SiC thin films, these incoming flux might be controlled effectively, then the quality of films may be improved.Time-Of-Flight studies are used to investigate the characteristics of dynamics distribution in the plume generated by ablation of SiC sample using Nd:YAG laser (laser irradiance 0.484 GW/cm2). The interesting result presented here is that the Si ions(634.71nm) spectra show diffident expansion dynamics regions:along the propagating direction of the plasma (perpendicular to the target surface), at short distance the time delay recorded shows a linear dependence on distance; at longer distance (5.5—7.5 mm) from the target surface, show two distinct expansion dynamics regions, which can be interpreted by drag force model and shock model.