Optimal generation of spatially coherent soft X-ray isolated attosecond pulses in a gas-filled waveguide using two-color synthesized laser pulses

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.

Enhanced high-harmonic generation up to the soft X-ray region driven by mid-infrared pulses mixed with their third harmonic

“…In fact, both the strong-field ionization and the acceleration of the freed electron are controlled via shaping of the driving electric field on a sub-optical-cycle time scale [13][14][15][16][17][18]. Recent theoretical studies on two-color-driven HHG have concentrated on finding the optimal waveforms for either extending the cutoff energy [19][20][21][22][23][24] or improving the efficiency of HHG [19,[25][26][27][28][29][30]. The efficiency enhancement of two-color-driven HHG with the second harmonic pulse as a weaker "control" field, i.e., ω + 2ω mixing, has been extensively explored for the last two decades because it is relatively straightforward to add a 2ω beam via second-harmonic generation (SHG).…”

Control of soft X-ray high harmonic spectrum by using two-color laser pulses

“…It can be seen that harmonics covering from 70 to 250 eV are well localized along the propagation axis and their divergence is less than 2 mrad, indicating that good phase matching has been achieved. The behavior of harmonics obtained from the optimized two-color laser pulses in a gas cell is very similar to those generated in a hollow waveguide [48,49].…”

“…In recent years, it has become possible to control optical waveform of the driving lasers by synthesizing multi-color laser pulses [42][43][44][45] to optimize the yields of high harmonics (see the review [46]). In our recent works we have focused on the optimization of multi-color laser waveform for the enhancement of harmonic yields [47], the improvement of two-color harmonic divergence in soft X-rays [48,49], as well as the macroscopic scaling of HHG generated by a two-color laser in a hollow waveguide [50]. In this article, we address whether the optimized multi-color waveform can be employed to control the spectral structure of HHG by adjusting macroscopic phasematching conditions.…”

Effect of laser focus in two-color synthesized waveform on generation of soft x-ray high harmonics

“…[44][45][46][47][48][49][50][51] To take into account macroscopic conditions for HHG with optimized multi-color waveforms, for simplicity, it has been usually assumed that all colors are focused at the same plane. [42,43,[52][53][54] To the best of our knowledge, there are no studies to discuss how the macroscopic generation of HHG is affected if laser focuses are varied in the synthesis of multi-color waveforms.…”