Study Of Material Response Properties Under The Irridiation Of Shaped Femtosecond Laser Pulses

This thesis focuses on the study of material response under the irradiation of shapedpulse femtosecond laser field, including the ablation processes of silicon under theirradiation of femtosecond double pulse, the filament propagation of a shapedfemtosecond laser, the nonlinear effect of K9glass irradiated by shaped femtosecondlaser pulses. The thesis also uses genetic algorithm to optimize the electric field of areflectron time-of-flight mass spectrometer used to detect the product particles duringlaser ablation. Detailed research results are listed as follows:1. Femtosecond double-pulse laser is used to ablate silicon and generate plasmaspectroscopy. The evolution of spectral intensity is studied as functions of the sampleposition and the double-pulse time separation. Two laser energies are used in thisexperiment. The results show that the spectral intensity first increases, then drops withthe increase of double-pulse time separation when the sample position relative to thefocus is fixed. The same trend is observed under both energies. The plasma andmolten surface induced by first pulse can increase or decrease the absorption ofsecond pulse, and then the spectral intensity will be changed. There are two enhancedregions of the spectral intensity for the different sample position. It’s a dip, not a peakwhen the sample position is at the laser focal point. The spectral intensity at both sidesis bigger than that at the focal point. This is caused by the nonlinear Kerr effect offemtosecond laser pulse when femtoseocnd laser propagates in atmosphere. Theenergy distribution of laser in the propogation direction will be changed by theself-focusing and self-defocusing effects, which is the reason that leads to the twoenhanced regions. This study proves that the double-pulse is more effective for theablation of silicon than the single pulse. The ablation efficiency can be controlled bychanging the sample position and the pulse time separation. 2. The length of filament generated by shaped femtosecond pulse in atmosphere isoptimized by genetic algorithm. Three parts of jobs are done. First, the parameter Aand T of sine phase is scanned using open-loop control method, and the filamentlength is recorded by changing both parameters. It’s found that the length of filamentgenerated by all shaped pulses is shorter than that generated by the transform-limitedpulse. Then the length of filament generated by using sine phase is optimized usingclosed-loop control method. The length of filament generated by the optimally shapedpulse extends about8.2%compared with the length of filament generated by thetransform-limited pulse. At last, the length of filament generated by using randomphases is also optimized using closed-loop control method, and better results areobtained. The length of filament generated by the opitimally shaped pulse extendsalmost50%compared with the length of filament generated by the transform-limitedpulse. The above results prove that the filament is sensitive to the shape of laser pulse,a little change of which will result in bigger fluctuation of the filament length. Theclosed-loop control method is better than the open-loop control method for searchingthe good shaped pulse.3. The optical nonlinear effect of K9glass irradiated by shaping pulse is studied.Two parts of jobs are done. In the first part, the parameter A, T and of sinephase is scanned using open-loop control method, the chang of absorptivity of K9glass is recorded with the three parameters. It is found that: the change of parameterA changes the sub-pulse numbers in the pulse train, hence changes the energydistribution in the shaped pulse, hence affects the absorptivity; the anisotropy causedby the leading pulse can change the absorptivity of the trailing pulses, the change ofT changes the separation between sub-pulses, hence the influence of the leadingpulse to the trailing pulses also changes; the change of changes the position ofspectral phase in frequency domain, hence affects the multiphoton absorptionprocesses of the material. In the second part, the optimization of nonlinear absorptionof K9glass under the irradiation of sine phase modulated shaped pulse is carried outusing closed-loop control method. There is almost no absorption when the K9glass is irradiated by transform limited pulse, while the absorption increased to4%under theirradiation of the optimal shaped pulse.4. Genetic algorithm is used to optimize the resolution of reflectron time-of-flightmass spectrometer used to detect particles from laser ablation. And the experiment iscarried out to verify the simulation. The optimizations are carried out under differentinitial particle condition. Using the optimal electrode voltage combination, the electricfield of the spectrometer successfully compensates the difference of flight timescaused by different initial particle condition. For example, in the optimizationsimulating real experimental conditions, the initial particle condition is set to include45different mass ions with certain spatial distribution and initial velocity. The timeresolutions from the optimized result are different with different mass number, thelowest to be30512and the highest to be45635. In the experiment, the optimizedvoltage values from the simulation are applied to the electrodes, resulting in a highresolution Xe mass spectrum, in which the Xe132peak’s resolution is20968. Thisproves that the simulation results are correct.