【正文】 re tunnel ionization. Note that the plasma density in this case is very high, more than mJ corresponds to a critical power of Fig. 1 shows a loglinear plot of the peak position of the fluorescence signal as a function of energy in the range of 3–45 mJ. The pressure was fixed at 1 atm. The peak positions were retrieved from Gaussian fittings of the onaxis fluorescence distributions versus distance in units of ICCD chip pixels [14]. Note that smaller pixel values correspond to distances closer to the focussing lens. It can be seen in Fig. 1 that the peak position moves closer towards the focussing lens as the energy is increased. This behaviour has been explained before [14] as a consequence of the power dependence of the selffocussing distance above the critical power [16]. [12] and [13]. Therefore, it is important to carry out a direct measurement of the critical power of helium. In this work, we have experimentally measured the critical power of helium using the moving focus method [14]. Also, the critical power has been obtained from the electron densities, based on the intensity clamping process, which have been measured as a function of energy and pressure. 2. ExperimentThe experiments were performed using 42 fs pulses (repetition rate: 10 Hz) with a center wavelength of about 800 nm. The pulses were focused ( ) into a gas chamber filled with pure helium gas whose pressure could be varied from 50 Torr to 1 atm. The energy could be varied in the range of 1–63 mJ. The pulses created a plasma filament inside the chamber. The fluorescence was collected from the side by imaging the length of the filament onto the entrance slit of a spectrometer (Acton Research Corp., Spectra Pro500i). The images of the filament were recorded in the spectrometer’s imaging mode (accumulations: 20) by using the zeroorder grating reflection with the slit widely opened. The presence of a single filament was verified. The spectra were taken with the 1200 grooves/mm grating. The spectral resolution of this grating was about nm (slit width: inferred from the literature value of the nonlinear refractive index 160 GW was well below the critical power , 6 m, mirror radii: each , 5 mJ [8]. Moreover, Nurhuda et al. have proposed a highly efficient pression scheme (conversion ratio: 10 fs with a peak power of more than 激光外文文獻(xiàn)翻譯+參考文獻(xiàn)100 GW [4]. It is important to note that this has been achieved without any dispersion pensation or pressure gradients. However, Skupin et al.’s theoretical analysis has revealed that this pression scheme is only optimum up to about five times the critical power [7]. Alternatively, a Japanese research group has used an argonfilled hollow fiber with a pressure gradient to produce sub 1021 cm2/W. In addition, the plots of the electron densities versus energy and pressure have also been used to determine the critical power of helium, based on the intensity clamping of the filamentation process. The value agrees well with the one by the moving focus method. Article Outline1. Introduction 2. Experiment 3. Results and discussion 4. Conclusion Acknowledgements References1. IntroductionRecently, there has been important progress in producing fewcycle pulses via filamentation of femtosecond laser pulses in gases [1], [2], [3], [4] and [5]. Couairon et al. have proposed that one could make use of the filamentation process in a noble gas with a pressure gradient to produce pressed pulses down to one optical cycle [6]. The perhaps most striking results are those obtained by Stibenz et al. who have reported highly efficient (conversion efficiency: 268 GW. Using this value, the nonlinear refractive index is inferred to be pulse in helium has been measured using the moving focus method. The experimental value is Available online 26 December 2007.AbstractThe critical power for selffocussing of a femtosecondbec, QC, Canada G1V 0A6bInstitut