Characteristics of ultrafast laser produced plasma and its application in thin film deposition.

摘要

The rapid development of ultrafast lasers offers tremendous research opportunities in the area of materials science. A femtosecond laser thin film deposition and plasma diagnostic system has been developed to study the interaction of femtosecond laser pulses with materials. Multiple diagnostic techniques are implemented on this system to provide a comprehensive picture of the plasma with temporal, spatial, and compositional information. Phenomena such as laser energy absorption, the plasma generation and expansion, the chemical composition and ionic fraction of the plasma and its kinetic energy distribution can be studied using this system.; In this thesis we compare the laser plasma generated by femtosecond and nanosecond lasers based on the similar ablation/deposition rate by optical emission spectroscopy, Langmuir probe, electrostatic energy analyzer, and HeNe deflection probe. Results show the difference in electron temperature, ionization state, and energy distributions, which lead to the different crystal structure of thin-films, deposited using femtosecond and nanosecond lasers.; Methods of using double-pulse ablation to enhance the ionic component in the femtosecond laser produced plasma are explored. The second pulse, delayed on a picosecond time scale, interacts with the plasma generated by the first pulse. The ion yield and average energy are enhanced by a factor of two with 5 to 10 ps delay between a double-pulse. The optimum enhancement occurs where the plasma density scale length kL is 1.5. The polarization dependent experiment indicates that the resonance absorption is a dominant absorption mechanism for the second pulse. Ionic mass separation effects are also enhanced by the double-pulse ablation by 25% overall and this suggests that self-generated electromagnetic fields which drive the mass separation process are enhanced in intensity or extended in time. Further enhancement can be realized by subsequently pumping the plasma with additional time-delayed pulses.; The cluster component formed in ultrafast laser ablation plumes is studied. The average size of the clusters is about 250 nm and the size distribution is about 150 nm for Nickel. The average size and the size distribution are directly related to characteristics of the ablation plasma and can be reduced by a factor of two by double-pulse ablation. The mechanisms of cluster formation are discussed including condensation, phase explosion, photomechanical effect, and nozzle effect.