Waveforms for optimal sub-keV high-order harmonics with synthesized two- or three-colour laser fields

Abstract

High-order harmonics extending to the X-ray region generated in a gas medium by intense lasers offer the potential for providing tabletop broadband light sources but so far are limited by their low conversion efficiency. Here we show that harmonics can be enhanced by one to two orders of magnitude without an increase in the total laser power if the laser's waveform is optimized by synthesizing two-or three-colour fields. The harmonics thus generated are also favourably phase-matched so that radiation is efficiently built up in the gas medium. Our results, combined with the emerging intense high-repetition MHz lasers, promise to increase harmonic yields by several orders to make harmonics feasible in the near future as general bright tabletop light sources, including intense attosecond pulses.

Generation of Bright, Spatially Coherent Soft X-Ray High Harmonics in a Hollow Waveguide Using Two-Color Synthesized Laser Pulses

“…As shown previously in Ref. [8], if the relative phase of a waveform is within AE0.2π from the optimized value, the short-trajectory harmonics still dominate and the harmonic cutoff and yield can be maintained (see supplementary Fig. 3 for Ref.…”

“…To improve harmonic efficiency, multicolor sinusoidal fields have been synthesized to modify the drive waveform in a way such that harmonic emission from each atom can be enhanced [2][3][4][5][6][7][8][9]. This approach has become very favorable recently because of technological advances in optics [10][11][12][13][14][15][16][17][18][19].…”

“…This approach has become very favorable recently because of technological advances in optics [10][11][12][13][14][15][16][17][18][19]. In our recent works [8,9], a general optimization scheme was proposed to obtain the best waveform such that the maximum HH yield from a single atom can be achieved by synthesizing two-or three-color fields. The harmonics generated from such an optimized waveform are better phase matched in the gas medium; thus, they would also exhibit better spatial coherence.…”

“…On the other hand, the optimized waveforms in Ref. [8], including WF1 used here, were obtained to enhance the emission from shorttrajectory electrons while suppressing the emission from long-trajectory electrons. Excellent phase matching with a waveform-optimized pulse like WF1 can be achieved when the following two conditions are satisfied: (i) the optimized waveform, especially the optimal relative phase between the two colors, needs to be maintained in the whole interaction volume; and (ii) HHs generated in the medium have to be phase matched over the entire interaction region.…”

“…Waveform WF1 is the on-axis initial electric field from Ref. [8]. WF2 mimics the on-axis electric field at the exit of the waveguide (5-mm long and 125-μm radius) filled with neon gas (50 torr Time shift of the two colors with respect to the reference frame at the exit of the waveguide (5-mm long and 125-μm radius) filled with neon gas (50 torr).…”