Search

Fully coherent, soft X-ray attosecond pulses are now available through high-order harmonic generation (HHG); however, the output energy is insufficient for various applications, such as attosecond-scale soft X-ray nonlinear experiments, the seeding of soft X-ray free-electron lasers, attosecond-pump-attosecond-probe spectroscopies, and single-shot imaging. In this paper, we combine a newly developed TW class mid-infrared femtosecond laser and a loose focusing geometry for HHG. A soft X-ray harmonic beam up to the water window region is demonstrated, which is more than 100 times intense compared to previous works. We achieve a high conversion efficiency, low beam divergence and a significantly reduced medium gas pressure. As the first application, we demonstrate near edge X-ray absorption fine structure experiments with clear fine absorption spectra near the K- and L-edges observed. The robust energy scaling method on HHG opens the door for demonstrating single-shot absorption spectrum and live-cell imaging with a femtosecond time resolution. While high harmonic x-ray sources are well established sources for ultrafast spectroscopy, the decrease in efficiency towards higher energies limits their application. Here, a loose-focused, TW mid-IR laser is used to generate high harmonic soft x-rays with pulse energies of a few nJ in the water window and easy energy scaling.

We propose and theoretically demonstrate a scheme to generate elliptically polarized attosecond pulses with mixed gases. As the harmonic radiations from different gases with opposite orbital parity interfere destructively for one helicity component and constructively for the other, the high-order harmonics from the mixture exhibit large ellipticity with the same helicity in a wide spectral range. Hence, highly elliptically polarized attosecond pulses can be generated from the mixed gases and the ellipticity can be tuned by controlling the mixing ratio and the molecular alignment angle. Furthermore, this interference effect is only related to the features of the mixed gases such as the structure and orbital symmetry of the molecules and atoms. The polarization control is independent of the temporal profile manipulation of the attosecond pulses by shaping the driving field. Consequently, our work paves an effective and convenient way to produce elliptically polarized isolated attosecond pulses with mixed gases by optimizing the polarization and temporal profile individually.

We investigate the polarization properties of high harmonics generated from oriented carbon monoxide molecules driven by a linearly polarized laser field combined with a weaker circularly polarized high-frequency assistant laser field (LPCP). The results show that the harmonics with higher ellipticities are obtained close to the cutoff region. Further analysis shows that the results are due to the manipulation of the electron trajectory by the LPCP field and the molecular asymmetric structure. By taking advantage of the properties above, we propose and theoretically demonstrate a method to generate attosecond extreme ultraviolet pulses with large ellipticity. Because of the difference of harmonic ellipticities contributed from the long and short trajectories, an even higher harmonic ellipticity can be obtained by filtering the long trajectory. In addition, by adjusting the relative phase between linearly polarized and circularly polarized fields, the attosecond XUV pulse varies from being left-elliptically polarized to right-elliptically polarized.

Elliptically polarized high-order harmonics opens a path toward the study of ultrafast dynamics in circular dichroism of magnetic materials, chiral molecules, and electronic spin motion. Recently, efficient high-order harmonic generation with controllable ellipticity has attracted considerable attention in experiment and theory. Here, we experimentally demonstrate a scenario using cross-linearly-polarized two-color fields consisting of a fundamental field and its second harmonic (SH). Through adjusting the relative phase, crossing angle, and intensity ratio of the cross-linearly-polarized two-color field, we obtain ellipticity-tunable high-order harmonics, with ellipticity up to $\ensuremath{\varepsilon}=0.73$. Meanwhile, the elliptically polarized high-order harmonics preserve high efficiency. Furthermore, when the intensity of the SH field is stronger than that of the fundamental field, the even order harmonics will become several times stronger than the odd order harmonics with opposite helicity.

An all-optical measurement of high-order fractional molecular echoes is demonstrated by using high-order harmonic generation (HHG). Excited by a pair of time-delayed short laser pulses, the signatures of full and high order fractional (1/2 and 1/3) alignment echoes are observed in the HHG signals measured from CO2 molecules at various time delays of the probe pulse. By increasing the time delay of the pump pulses, much higher order fractional (1/4) alignment echo is also observed in N2O molecules. With an analytic model based on the impulsive approximation, the spatiotemporal dynamics of the echo process are retrieved from the experiment. Compared to the typical molecular alignment revivals, high-order fractional molecular echoes are demonstrated to dephase more rapidly, which will open a new route towards the ultrashort-time measurement. The proposed HHG method paves an efficient way for accessing the high-order fractional echoes in molecules., 8 pages, 4 figures

We theoretically investigate high-order harmonic generation (HHG) from a coherent superposition state of hydrogen atom in a circularly polarized (CP) laser field. The results show that, driven by the CP field, HHG from the coherent superposition state is enhanced by 4 to 5 orders of magnitude compared to that from the ground state. The enhanced intensity is comparable to that of the ground state in a linearly polarized laser field. Based on the classical and quantum analyses, we demonstrate that HHG in the CP laser field mainly arises from the recombination of electrons ionized at non-zero initial positions. The enhancement of HHG from the coherent superposition state is due to the higher ionization rate and also the larger probability density of electrons located far from the nucleus of the involved excited state. The obtained harmonic spectra support the generation of intense isolated attosecond pulses (IAPs) with the ellipticity as high as 0.93. Moreover, we show that the generation of IAPs with large ellipticity is robust against the variation of the carrier envelope phase (CEP) of the laser field. Our results provide a new route toward generating near-CP IAPs, which will be beneficial for ultrafast detection of chiral-sensitive light-matter interactions.

We theoretically investigate the high-order harmonic generation (HHG) driven by dual-pulse lasers. The first pulse of 800 nm can create plasma in the high-density gas target. Then, the second pulse of 1800 nm is spatiotemporally reshaped by the plasma and generates soft x-ray high-order harmonics in the overdriven regime. The simulation results show that adding the first pulse can promote the phase matching of HHG in the cutoff region. As a result, a cutoff-extended HHG is achieved, which is one order of magnitude more intense than that in one pulse scheme reported previously. In dual-pulse driving lasers, the harmonic yield and the cutoff photon energy can be optimized effectively by increasing the gas pressure inside the water window region.

We propose and theoretically demonstrate a method to generate the circularly polarized supercontinuum with three-color electric fields. The three-color field is synthesized from an orthogonally polarized two-color (OTC) laser field and an infrared gating field. All driving pulse durations are extended to 40 fs. We demonstrate that the three-color field imposes curved trajectories for ionized electrons and extends the time interval between each harmonic emitting. Through adjusting intensity ratios among three components of the driving field, a nearly circular isolated attosecond pulse can be generated.

A dynamic model based on the momentum distribution of the final state in the striped photoelectron energy spectra is constructed. An equal-value line reconstruction (ELR) algorithm is proposed, which includes the real momentum-time distribution of the electron wave-packet corresponding to the radiation pulse and is essentially different from the traditional attosecond streak measurement method of FROG-CRAB with a central momentum approximation. The research results show that, compared with those obtained by the FROG-CRAB method, the parameters of the pulse width and the chirp obtained by the method of ELR have a higher accuracy. © 2018, Chinese Lasers Press. All right reserved.