Improvement of harmonic spectra from superposition of initial state driven by homogeneous and inhomogeneous combined field

Abstract

A larger harmonic cutoff with a higher harmonic yield is a necessary requirement for producing intense ultrashort extreme ultraviolet (XUV) and X-ray sources. In this paper, we present a potential method to improve the harmonic cutoff and harmonic yield from the superposition of initial states in the He atom driven by the low-intensity homogeneous and inhomogeneous combined field. The results can be separated into two parts, that is, (i) the extension of harmonic cutoff via the optimization of three-color homogeneous and inhomogeneous field (covering the carrier envelope phase, delay time, intensity ratio, and inhomogeneous effect); and (ii) the enhancement of harmonic yield via the superposition of ground state and excited state as the initial state. Consequently, a water window harmonic plateau with an intensity enhancement of six orders of magnitude can be obtained. Moreover, the harmonic plateau is contributed by a single harmonic emission peak. Finally, through the Fourier transformation of some selected harmonics on the spectral continuum, a number of sub-50 as water window attosecond pulses with the intensity enhancement of six orders of magnitude can be obtained.

Half-cycle waveform control for producing a broad and intense harmonic spectral continuum and an isolated attosecond pulse

“…Many schemes have been proposed on the basis of this theory. For instance, waveform control by using a multi-color combined field [10][11][12][13], frequency-chirping technology [14][15][16] and polarization gating technology [17][18][19].…”

“…As can be seen, if one wants to obtain a broad and intense harmonic spectral region, proper control of the ionization and acceleration processes is necessary. However, in previous investigations [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28], control of the ionization and acceleration processes of the free electron has always been discussed separately. Thus, in this paper, the harmonic emission process in the half-cycle waveform has been well controlled by proper choice of the half-cycle waveform.…”

Waveform control in selective enhancement of single-order harmonic

“…If one wants to obtain the intense single attosecond pulses (SAPs) in XUV and x-ray spectral regions, two requirements are necessary, that is, the improvements of harmonic cutoff and harmonic yield. Thus, on the basis of the TSM, lots of schemes have been proposed to achieve the above requirements For instance, in the case of harmonic cutoff extension, by properly using the multi-color combined field [11][12][13][14][15][16][17]; the polarization gating technology [18][19][20][21][22]; the frequency-chirping technology [23][24][25] and the plasmonic field enhancement field [26][27][28][29][30], many ultrashort SAPs in XUV and x-ray regions can be produced. In the case of harmonic yield enhancement, the uses of excited initial state or superposition of initial state in atomic system [31][32][33]; and the uses of vibrational excited initial state or charge resonance enhanced ionization scheme in H + 2 molecule ion [34][35][36], the intensities of HHG yield and the produced SAPs can be enhanced by several orders of magnitudes.…”