Research On The Propagation Of Femtosecond Laser In Air,Water Solution And The Spectral Properties During Filamentation

With the rapid development of femtosecond laser technology,people have conducted extensive research on filamentation in different gases and condensed media for many years since the phenomenon of femtosecond laser filamentation was discovered in 1995,which has continuously opened up the process of the femtosecond laser filamentation.The laser intensity is continually increased due to Kerr self-focusing effect when femetosecond laser pulses propagating in the medium.The big amount of plasma created by multiphoton ionization has a defocusing effect on laser beam until the laser intensity is up to the ionization threshold.The long plasma channel can be realized when the two processes reach the dynamic equilibrium which can generate nonlinear phenomena like supercontinuum generation,fluorescence emission,conical emission and terahertz radiation,etc..These phenomena have important application prospects in remote sensing,rainmaking and lightning protection,etc..Although femtosecond laser filamentation in media has achieved a great of success over the last two decades,however,there are still many problems to be solved.For example,the factors such as humidity,temperature and airflow will be changed with the variation of weather when the femtosecond laser propagates in the real atmosphere.Besides,the variation of the content of impurities and temperature can influence the propagation in the liquid environment,which can affect the filamentation.In this paper,we studied the influence of different factors on fluorescence emission and supercontinuum generation induced by femtosecond laser filamentation in air and water.The main content of our research is as following:(1)During femtosecond laser filamentation in air,the fluorescence emission at different locations has been investigated,and the fluorescence spectra of nitrogen molecule can be used to characterize the process of laser filamentation.The length of the plasma filament can be effectively extended by increasing the laser pulse energy and the focal length of the focusing lens in our experiments,while the variation of spectral width is independent of the parameters mentioned above.By considering the group-velocity-dispersion,the collapse distances calculated by revised Marburger's law are in good accordance with the values of experimental measurements.The experiments demonstrate that each parameter plays a different role in controlling the laser filamentation,which in turn affects the fluorescence intensity,the position and length of the filament.(2)We study the effect of pulse energy and laser polarization state on the sidewise OH and N2 fluorescence induced by femtosecond laser filamentation with wavelength of 800 nm in different air humidity.The content of water molecules in filaments increases with the increase of pulse energy and relative humidity,which results in the enhancement of the OH fluorescence emission.However,the intensity of nitrogen fluorescence spectra weakens with the increase of the relative humidity.The intensity of OH fluorescence emission under linearly polarized laser pulses is stronger than that under circularly polarized laser pulses,which is due to the enhancement of dissociation effect under linearly polarized laser pulses.While,the intensity of nitrogen molecule fluorescence emission under circularly polarized laser pulses is stronger than that under linearly polarized laser pulses originating from the opening-up of impact excitation channel under circularly polarized laser pulses.The study of NH fluorescence induced by femtosecond laser filamentation with wavelength of 400 nm is also shown.The convection current caused by the increase of temperature and the decrease of nitrogen content lead to the variation of NH fluorescence emission,which increases firstly and then decreases with the increase of temperature.The results show that the intensities of OH,NH and N2 fluorescence emission are influenced respectively by different parameters.(3)We study the variation of supercontinuum generation with varying temperature caused by femtosecond laser filamentation in Na Cl solution.The temperature has little effect on supercontinuum generation under low pulse energy in Na Cl solution.However,it has the suppression tendency with increasing temperature under high pulse energy.The phenomenon is mainly attributed to the large amount of bubbles produced by multiphoton ionization which have the effect of absorption and scattering on supercontinuum with the increase of tempreture.(4)We also research the supercontinuum generation in different concentration of the solutions:it is found that the supercontinuum generation during filamentation is suppressed with the variation of concentrations of lactose solution and nitric acid solution.The supercontinuum suppression increases gradually with the increase of the concentration of lactose solution and nitric acid solution,which can be explained by the capture of electrons by solvents and the absorption of white light in solution.