Expansion dynamics and angular emission features of nanosecond and femtosecond laser produced plasmas.

摘要

Many applications that rely on the use of laser produced plasmas (LPP) require basic knowledge of the process in which the plasma cools down and emits particles. The goal of this work is to continue developing the understanding of the laser plume, which includes the light emission for determining temperature and density along with the ejection of atoms and ions from the plume across a wide spread of angles. This is accomplished through experimental work conducted employing a Nd:YAG laser at fundamental wavelength (1064 nm) and a Ti:Sapphire femtosecond laser at 800 nm in an evacuated chamber. A Faraday cup was positioned on a rotatable manipulator to measure the ion emission from LPP angularly from 0 to 65 degrees about the target normal. The atomic emission was also obtained using the same method by exchanging the Faraday cup with a quartz crystal microbalance which would measure material mass deposited. The angular emission of both the atomic and ionic species emanating from the LPP showed a strong dependence to target normal for the femtosecond laser while the nanosecond laser gave a much broader emission profile. The energy comparison of both lasers of the ion and atomic debris showed linear increases with increasing laser energy but the fs LPP had a higher dependence on the laser energy. The electron temperature and density was determined for ns and fs LPP using optical emission spectroscopy. Measurements of the plume were conducted temporally, spatially, and with energy dependence. The temporal studies showed an exponential decrease in temperature for the ns LPP while the fs LPP temperature decayed much slower; but for the density decay, both exhibited an exponential drop with the fs LPP occurring in a shorter time frame. When comparing the spatial dependence of temperature and density of both plumes, the ns and fs LPP dropped significantly with increasing distance from the target; but the fs LPP line emission was measureable at distances much further from the target than the ns emission. The last study completed using optical emission spectroscopy was laser energy dependence on electron temperature and density of LPPs. The fs LPP density was highly dependent with respect to the energy compared to the ns but in both cases the temperature and density both increased linearly.