Abstract
We numerically demonstrate the generation of intense, low-divergence soft X-ray isolated attosecond pulses in a gas-filled hollow waveguide using synthesized few-cycle two-color laser waveforms. The waveform is a superposition of a fundamental and its second harmonic optimized such that highest harmonic yields are emitted from each atom. We then optimize the gas pressure and the length and radius of the waveguide such that bright coherent high-order harmonics with angular divergence smaller than 1 mrad are generated, for photon energy from the extreme ultraviolet to soft X-rays. By selecting a proper spectral range enhanced isolated attosecond pulses are generated. We study how dynamic phase matching caused by the interplay among waveguide mode, neutral atomic dispersion, and plasma effect is achieved at the optimal macroscopic conditions, by performing time-frequency analysis and by analyzing the evolution of the driving laser’s electric field during the propagation. Our results, when combined with the on-going push of high-repetition-rate lasers (sub- to few MHz’s) may eventually lead to the generation of high-flux, low-divergence soft X-ray tabletop isolated attosecond pulses for applications.
Highlights
Up to 180 eV by using a 2-μm, 10-cycle driving laser[20]
High harmonics generated with the optimized two-color waveform in the near and far field
We examine how the harmonic emission varies with the gas pressure
Summary
Up to 180 eV by using a 2-μm, 10-cycle driving laser[20]. Based on spatiotemporal isolation of wavefront rotation, Biegert’s group reported continuous photons up to the carbon K-edge of 284 eV with pulse duration below 400 as and with a bandwidth supporting a 30-as pulse duration[21]. We have demonstrated such optimized two-color waveform in a hollow waveguide can efficiently generate low-divergence soft X-ray harmonics when the gas pressure and waveguide parameters[45] were optimized. Our goal in this work is to demonstrate how spatially coherent soft X-ray IAPs are generated at the optimal macroscopic conditions by an optimized two-color waveform[42] in a hollow waveguide. We will show that high harmonics from XUV to soft X-rays have low divergence (smaller than 1 mrad) at the optimized gas pressure and waveguide parameters, where their yields are greatly enhanced in comparison with the single-color laser.
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