Study On The Characteristic Of Femtosecond Laser Filamentation In Air

When the femtosecond laser pulse propagates in transparent media and its peak power is well above the critical power of Kerr self-focusing,then the self-focusing effect will occure.At the same time,the molecules along the propagating path will be ionized generating massive plasmas,the large amount of plasma generated by ionization play a role of defocusing on the incident laser pulse,and a plasma channel will be formed when these two processes reach a dynamic balance.It is also well known as femtosecond laser filament.A lot of nonlinear effects will occurre during the filamentation process,such as conical emission,supercontinuum generation,terahertz radiation,high order harmonic and self-phase modulation etc..Owing to these nonlinear effects,femtosecond laser filament has attracted considerable attention.In this paper,we studied the supercontinuum generation and third harmonic generation during the filamentation.Meanwhile,we also studied the filament process under different experiment conditions by using an axicon as optical focusing component and using different detection method.The main content of our researches is as followings:(1)Supercontinuum(SC)generation in ambient air using different focusing geometries and laser energies under linear and circular polarization was investigated experimentally.For same input laser energy,the SC spectra in the visible range became wider when smaller f-number convex lenes were used and the linearly polarized laser was used as the pump source.Under the same intensity of the optical field and the critical power for self-focusing of linearly and circularly polarized beams,the SC spectra range was wider when circularly polarized beams as the pump source.(2)We studied the influence of air molecular alignment on the modulation of third-harmonic generation by using pump-probe technique.By changing the delay and the incident power of the pump and probe beam,Not only the spectral intensity of the third harmonic,but also the line-width of the third harmonic can be changed under the influence of cross-phase modulations which originating from various molecular alignment revivals.(3)We experimentally investigated the formation of plasma channels with axicon-focused femtosecond laser pulse in air.Unlike the smooth on-aixs radial intensity profiles around the focal region generated by an ideal axicon,in practice,under the same incident beam size,the base angle of axicon played an important role on the on-axis intensity profile: when the axicon base angle is 0.5o,an evident periodic structure can be observed.However a more smooth on-axis intensity profile can be generated by an axicon with the base angle of 5o.In fact,the result showed that the axicon with smaller base angle caused strong oscillations occur on the on-aixs intensity profile.(4)The influence of the Bessel beam during the filament formation was analyzed by a time-resolved spectrum.The axicon with base angle of 0.5o and 5o were used as focusing component,the change in plasma filament over time were recorded at different delay times with an ICCD.At the same time,we also investigated the influence of laser polarization on the filament formation processes by using imaging method.