Generation Of A Frequency Comb In The Extreme Ultraviolet

Extreme ultraviolet optical frequency comb(XUV comb)is an indispensable tool for high precision spectroscopy.It also brings many new opportunities to the field of strong field and ultrafast science.In the field of precision measurement,spectroscopic measurements by the aid of XUV comb on helium-like or hydrogen-like ions(e.g.,the 1S-2S transition of He+ at 60.8 nm and Li+at 41 nm)could provide the most stringent tests of bound state quantum electrodynamics yet;the development of high precision laser spectroscopy based on Th(at?150 nm)may prove crucial for the development of next-generation "nuclear" clocks.In the field of strong field and ultrafast science,XUV comb system can provide a new platform to study the atomic and molecular dynamics with an unprecedented high repetition rate,and accordingly,with a high signal-to-noise ratio and statistics.For example,the angle-resolved photoemission spectroscopy(ARPES)performed with XUV comb is beneficial for the improvement of the temporal resolution and the mitigation of the space charge effect;furthermore,the XUV attosecond bursts generated by XUV comb also help to achieve the attosecond transient absorption measurement at MHz or higher high repetition rate.To this end,we designed and developed an XUV comb,aimed at the precision measurements in the XUV region and investigations of strong field and ultrafast science at an unprecedented high repetition rate.The main works are listed as follows:1.The construction of a femtosecond enhancement cavity with kilowatt average power.Because of the lack of suitable laser medium in the solid or liquid phase,the method to generate an XUV comb is to utilize high harmonic generation(HHG)to convert the near infrared comb to the XUV.The requirement for HHG is a high peak intensity(>1013 W/cm2),which cannot be met by commercial laser systems with pulse energy typically less than 1 ?J.Thus,we build a femtosecond enhancement cavity(fsEC)to amplify the output of the driven laser.After the optimization of mode-matching of the fsEC,the cavity is locked by the Pound-Drever-Hall(PDH)technique for at least one hour.With a pump power of 27 W,we have achieved an intracavity power of 6.08 kW and the corresponding buildup is 225.The intracavity peak intensity is estimated to be about 4.8×1013 W/cm2,which has met the requirement for HHG.2.The generation of an XUV comb.By the combination of the fsEC and the HHG,we have achieved the generation of the XUV comb.With Xe gas as the working media of the intracavity HHG,harmonics up to the 19th order(?55 nm)are observed.Power measurement indicates that as much as 115.9 ?W are generated at?94 nm(11th harmonic),corresponding to the intracavity conversion efficiency is?2.5×10-8.The coherence of the generated light is also tested with an optical-heterodyne-based measurement of the third harmonic.These results suggest that the current system is ready for precision spectroscopic measurement.3.The investigation of the generation mechanism of the N2 emission at high repetition rate.We employ the XUV comb system to study the generation mechanism of the nitrogen molecular 337-nm emission at an unprecedented high repetition rate of 100 MHz by examining the dependence of the intensity of the 337-nm emission on the flow rate of the nitrogen gas and the polarization of the pump laser.The results eliminate the possibility that the dissociative-recombination is the dominant pathway,and suggest that the inelastic collision excitation mechanism is the major route to populate the C3?u state of N2.The role of the steady-state plasma which is generated under our experimental conditions is also discussed.In conclusion,this newly established XUV frequency comb system in our lab provides a versatile platform for the studies on precision spectroscopy measurements as well as strong field and ultrafast science with an unprecedented repetition rate of 100 MHz.