Research On Ultrafast UV Laser And UV Filamentation

The development of ultrafast intense laser technology shows broad application prospects in fields of high-energy physics,materials science,nonlinear optics,and inertial confinement fusion,etc.Nowadays,the most investigations of ultrafast laser are concentrated in visible and NIR range,while the UV pulse has a great value in practical applications due to its particular advantages of short wavelength,high-energy photons, and strong ability of focus,etc.Therefore,studying UV laser source has become one of hot spots with great attention at home and abroad.At present,there are two widespread approaches driven by NIR femtosecond laser to attain ultrafast UV pulses,one is nonlinear optical frequency conversion technology, which has been very mature for long pulses,however,in ultrafast field,thanks to the broadband feature of femtosecond pulses,it has to face some inevitable problems such as phase mismatch,group velocity mismatch,etc..Therefore,the high conversion efficiency and broadband spectrum come to be two breakthrough points of technical index for the generation of ultrafast and high-intensity UV laser.The other approach is using transformation of NIR ultrafast laser within plasma channel in gas medium to obtain the third harmonic with very broad frequency spectrum through nonlinear effect, which can be enough to support few cycle UV pulses,but generally only 10^-4frequency conversion efficiency,resulting in the application extremely restricted.The generation of ultrafast UV laser is regarded as the goal of this thesis,it is mainly investigated about obtaining efficient UV pulses with broadband spectrum in high-intensity femtosecond laser field.We designed and employed an approach of frequency doubling-compensation-tripling in collinear structure for femtosecond pulses, and solved the problems such as phase mismatch,group velocity mismatch etc.,attaining efficient frequency doubling process and mJ magnitude of tripling output;Moreover,we researched on filamentation in Ar gas of new band laser—UV,resulting in FWHM bandwidth broadened to 2.2 times of input spectrum;In addition,we used strong coupling effect between dual-plasma channels in two-color optical fields to broaden spectrum further,achieving a spectra bandwidth which can support sub-20fs pulse duration under transformation limit.My primary practical work can be summarized as follows:1.We investigated ultrashort pulse transmission through BBO crystal and mainly focused on the influence of phase matching on its mixing effect.It is found that under a certain matching condition,the phase and group velocity dispersion between excited wave and generated wave can be matched and compensated in a certain degree.2.An approach of frequency doubling-compensation-tripling in collinear structure for femtosecond pulses was designed and employed to fulfill efficient harmonic conversion:(1)In the research of frequency doubling process by using two anti-parallel crystal axes,not only the spatial walk-off between second harmonic and fundamental wave could be compensated,but also a small amount of positive and negative detuning of two crystals were beneficial to the phase matching of coupled waves.All these improved frequency doubling efficiency from 20.9%of a single block to 35.7%,an increase of 1.7 times.Moreover,when the strong part of fundamental wave was intercepted by a diaphragm,an efficiency of 42%was obtained.(2)A collinear structure of frequency doubling-compensation-tripling was designed and employed to fulfill efficient harmonic conversion.The time delay between second harmonic and fundamental wave was compensated by angle detuning of compensation crystal,and through cross-autocorrelation measurement,it was verified that time delay between injecting waves reduced nearly about 2 / 3;at the same time, the principal section of compensation plate was changed,which could realized type transformation of sum frequency fromⅡtoⅠ.When using a part fundamental beam of 6 mJ to experiment,the third-harmonic energy could be increased from 0.2mJ to 0.32mJ,an increase of 1.6 times.If 18mJ energy from laser system was totally employed and further optimized,the UV pulse centered at 270nm with an energy of 1.1 mJ and bandwidth of about 2 nm could be obtained.In addition,in subsequent work of our research group,the UV pulse duration has been already measured about 200 fs by two-photon fluorescence method.3.We investigated the nonlinear spectrum broadening effect of UV femtosecond pulses within plasma channel in argon gas.When UV pulses with bandwidth of 1.5 nm and single pulse energy of 0.58 mJ were focused into Ar gas by concave mirror,the filament was generated and the UV spectrum could be broadened in different degree in the case of different gas pressure,focal length and gas category.It is found that both the increase of gas pressure and focal length are beneficial to the increase of plasma channel length and spectrum broaden.With 2.2 atm Ar pressure and 1000mm focal length,the spectrum width of 3.3 nm could be obtained which was a factor-of-2.2 improvement compared with input spectrum.4.We investigated the UV laser spectrum broadening due to plasma channel coupling effect in two-color filed of UV and NIR lasers.Both UV and NIR lasers were collinearly focused into 2.0 atm Ar gas cell,when satisfying the conditions of space-time overlap,the full width at half maximum spectrum of UV could grow from 2.6 nm without interaction situation to 6.4 nm,a factor-of-2.5 improvement,which can provide a valid way for attaining sub-20fs ultrafast pulses in the range of UV.