【正文】 y is independent of pressure Icrit≠Icrit(p) [26], [27], [28] and [25]. Thus, the electron density is linearly proportional to pressure above the critical pressure (Eq. (3)). The critical point ( 35 mJ, 250 Torr) corresponds to a critical power of 274 GW (pulse duration: 42 fs). Note that the electron densities are in the order of 1017 cm?3 (Fig. 2 and Fig. 3). This corresponds to an ionization degree of 10?2, which is well below the depletion limit [29]. 激光外文文獻(xiàn)翻譯+參考文獻(xiàn)Fig. 3. Intensity clamping: electron density versus pressure. The data (black squares) shows the electron density as a function of pressure in the range of 50–760 Torr. The energy is fixed at 35 mJ. The electron density shows a linear dependence on pressure above a critical pressure of around 250 Torr (linear fit). This behaviour can be explained by intensity clamping (see text). The critical point ( 35 mJ, 250 Torr) corresponds to a critical power of (pulse duration: 42 fs). There is no saturation by depletion (ionization degree: 10?2). 4. ConclusionIn conclusion, the critical power of helium has been measured to be using the moving focus method. This value corresponds to a nonlinear refractive index of , which agrees well with the theoretical value in Ref. [11]. The critical power has also been measured using the plots of the electron densities versus energy and pressure, which give the same value (within the error bar) as the one by the moving focus method. AcknowledgmentsWe acknowledge the support in part by Natural Sciences and Engineering Research Council of Canada (NSERC), Defence Ramp。D Canada – Valcartier (DRDC Valcartier), Le Fonds Qu233。b233。cois de la Recherche sur la Nature et les Technologies (FQRNT), Canada Research Chairs, Canada Foundation for Innovation (CFI) and Canadian Institute for Photonic Innovations (CIPI). WL acknowledges the support of NSFC, NCET and Chinese National Major Basic Research Development Program. We appreciate the technical support by Mario Martin. SLC acknowledges the suggestion and encouragement by Justin Peatross of Brigham Young University, Utah, USA to carry out the current measurement of the critical power through their experience in high order harmonic generation. The discussion with him was very fruitful. References[1] . Hauri, W. Kornelis, . Helbing, A. Heinrich, A. Couairon, A. Mysyrowicz, J. Biegert and U. Keller, Appl. Phys. B 79 (2004), p. 673. [2] . Hauri, A. Guandalini, P. Eckle, W. Kornelis, J. Biegert and U. Keller, Opt. Express 13 (2005), p. 7541. [3] G. Steinmeyer and G. Stibenz, Appl. Phys. B 82 (2006), p. 175. [4] G. Stibenz, N. Zhavoronkov and G. Steinmeyer, Opt. Lett. 31 (2006), p. 274. [5] F. Th233。berge, N. Ak246。zbek, W. Liu, A. Becker and . Chin, Phys. Rev. Lett. 97 (2006), p. 023904. [6] A. Couairon, M. Franco, A. Mysyrowicz, J. Biegert and U. Keller, Opt. Lett. 30 (2005), p. 2657. [7] S. Skupin, G. Stibenz, L. Berg233。, F. Lederer, T. Sokollik, M. Schn252。rer, N. Zhavoronkov and G. Steinmeyer, Phys. Rev. E 74 (2006), p. 056604. [8] A. Suda, M. Hatayama, K. Nagasaka and K. Midorikawa, Appl. Phys. Lett. 86 (2005), p. 111116. [9] M. Nurhuda, A. Suda and K. Midorikawa, J. Opt. Soc. Am. B 23 (2006), p. 1946. [10] . Painter, M. Adams, N. Brimhall, E. Christensen, G. Giraud, N. Powers, M. Turner, M. Ware and J. Peatross, Opt. Lett. 31 (2006), p. 3471. [11] . Nibbering, G. Grillon, . Franco, . Prade and A. Mysyrowicz, J. Opt. Soc. Am. B 14 (1997), p. 650. [12] V. Tosa and . Nam, Opt. Lett. 32 (2007), p. 2707. [13] M. Turner, N. Brimhall, M. Ware and J. Peatross, Opt. Lett. 32 (2007), p. 2709. [14] W. Liu and . Chin, Opt. Express 13 (2005), p. 5750. [15] . Xu, J. Bernhardt, P. Mathieu, G. Roy and . Chin, J. Appl. Phys. 101 (2007), p. 033124. [16] . Marburger, Prog. Quantum Electron. 4 (1975), p. 35. [17] . Radziemski, . Loree, . Cremers and . Hoffman, Anal. Chem. 55 (1983), p. 1246. [18] V. Milosavljevi? and S. Djeniz?e, Astron. Astrophys. 393 (2002), p. 721. [19] NIST, (2007).