Abstract
We theoretically investigate the broadband water-window supercontinuum generation from pre-excited medium with a mid-infrared pulse. We find that the wavelength scaling of the harmonic yield from near-visible (0.8 μm) to mid-infrared (1.8 μm) in single-atom level is λ(-2.7). Using an intense phase-stabilized few-cycle 1.6 μm laser pulse, a broadband water window supercontinuum with bandwidth of approximately 140 eV is obtained. We also investigate the macroscopic effects and find that large initial population of the excited state leads to the high-density of free electrons, which shift the carrier-envelop phase of the driving pulse and further diminish the water-window supercontinuum generation. The highly-ionized medium also results in poor temporal and spatial properties of the attosecond pulse. Instead, small initial population of the excited state can produce well phase-matched xuv supercontinuum in water-window region and an 100-as pulse with central wavelength of 2.8 nm and pulse energy of 0.15 nJ can be filtered out.
Highlights
High-order harmonic generation (HHG) is one of the major topics in the interaction of intense laser fields with atoms
It has provided an attractive way towards attosecond extreme ultraviolet (XUV) light source, making a breakthrough in attosecond science [1] and nonlinear optics in the extreme ultraviolet region [2]
The interference term < ψg(r,t)|x|ψe(r,t) >, where the excited state is responsible for the ionization and the dipole transition is between the continuum and the ground state, plays a more important role than other two terms in HHG process
Summary
High-order harmonic generation (HHG) is one of the major topics in the interaction of intense laser fields with atoms. The well-known Ip + 3.17Up law for the position of the HHG cutoff (where Ip is the atomic binding energy) shows that using mid-infrared lasers allows the generation of higher order harmonics [11] and offers the possibility for the generation of broader supercontinuum in waterwindow [the spectra range between the K-absorption edges of carbon (284 eV) and oxygen (543 eV)] region. The spreading of the electron wave packet leads to a decrease of the HHG yield in mid-infrared regime [15], which leads to low harmonic efficiency low energy of attosecond pulse. An effective way to increase the harmonic efficiency of harmonics driven by mid-infrared pulses is to populate part of the electron wave packet at the excited state. Small initial population of the excited state can produce well phase-matched water-window xuv supercontinuum and a 100 as attosecond pulse can be filtered out with pulse energy of 0.15 nJ and central wavelength of 2.8 nm
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