“…The process of up-converting the fundamental frequency can be well described with the semiclassical three-step model including tunneling, acceleration and recombination [2,3] and has been extensively studied during the past three decades [4,5,6,7,8,9]. Due to the rich information carried by HHG from the microcosm, it has been widely applied to the detection of structures and control of ultrafast dynamics with unprecedented spacial and temporal resolutions [10,11,12,13,14,15,16,17,18,19,20]. The cutoff in gases is theoretically and experimentally given by ω cutoff ≈ I p + 3.17U p ∝ Iλ 2 , illustrating the possibility of extending the energy of harmonic photons well into extreme ultraviolet and even "water window" region employing longer wavelengths [21,22,23,24].…”