Generation of broad XUV continuous high harmonic spectra and isolated attosecond pulses with intense mid-infrared lasers

Abstract

We present experimental results showing the appearance of a near-continuum in the high-order harmonic generation (HHG) spectra of atomic and molecular species as the driving laser intensity of an infrared pulse increases. Detailed macroscopic simulations reveal that these near-continuum spectra are capable of producing IAPs in the far field if a proper spatial filter is applied. Further, our simulations show that the near-continuum spectra and the IAPs are a product of strong temporal and spatial reshaping (blue shift and defocusing) of the driving field. This offers a possibility of producing IAPs with a broad range of photon energy, including plateau harmonics, by mid-IR laser pulses even without carrier-envelope phase stabilization.It has been a decade since the first reported generation of isolated attosecond pulses (IAPs) [1] and the first full characterization of an attosecond pulse train (APT) [2], both seminal results that gave birth to the field of attosecond science [3][4][5][6][7]. While APTs can be now routinely generated based on high-order harmonic generation (HHG) in gases, production of IAPs is still a very active field of research. Common to all methods of generating IAPs is the concept of temporal gating, which is used to select a XUV burst from an APT. Current methods can be divided into three different categories. The most intuitive, but technically demanding, approach is based on XUV spectral filtering near the cutoff of the HHG spectra. This approach requires a short driving pulse (about two optical cycle) with a stabilized carrierenvelope phase. [6,8]. This approach could be called intensity gating, as it selects the harmonics emitted near the peak of the most intense half-cycle of the (short) laser pulse. The second approach is based on polarization gating [9][10][11], and the generalized double optical gating [12][13][14], in which the short driving pulse requirement is somewhat relaxed. While these two approaches have their origin already in the single-atom response, the third approach is based on the macroscopic propagation effects [15], which includes, ionization gating [16][17][18], phase matching [19], and ionization-driven reshaping Gaarde et al [15,20]. It has also been demonstrated recently that near-continuum spectra can be generated with two-color multicycle driving laser pulses by carefully adjusting the wavelength of the supplementary pulse [21].In order to produce a short IAP, XUV radiation with a broad frequency bandwidth needs to be generated. Since the HHG cutoff law is Ω cutof f ∼ I L λ 2 L , there are only two ways to extend Ω cutof f : increasing the laser intensity I L , or increasing the laser wavelength λ L . The first option is limited by ionization, which dramatically reduces HHG yield due to the depletion of neutral atoms and, more importantly, the phase mismatching due to free electrons in the medium. Using a short laser pulse does not help much, since the current technology nearly reaches the one-cycle limit already. The second option is to use lasers with a lo...

Quantitative rescattering theory of high-order harmonic generation for polyatomic molecules

“…This is probably due to the fact that the macroscopic propagation can suppress HHG yields near the cutoff. This is especially true for the case of the midinfrared laser of relatively high laser intensity used in the experiment due to the modification of the driving laser pulse in the medium [38]. This conclusion is further supported by the fact that the experimental laser intensity estimate is 9 × 10 13 W/cm 2 , significantly higher than the theoretical estimate of 0.55 × 10 14 W/cm 2 , based on the position of the cutoff.…”

Correlation enhancement of high-order harmonic generation in Xe

“…This latter approach includes macroscopic effects introducing dependencies on the focusing position and gas pressure. Of the QRS calculations presented [9,10] it is this which appears to have best agreement with the Shiner experiment. Our reproduction of the giant resonance is an improvement over the RMT method.…”

“…In which, the spectra is determined by the returning electron wavepacket multiplied by the corresponding dipole transition amplitude. For a comprehensive application of the QRS method to single-color HHG from Xe accounting for propagation effects and further experimental parameters the readers are referred to [9,10]. Under this same factorization, analytic expressions for the highenergy end of the HHG plateau were derived by Frolov et al [11,12,13].…”

Comparison of high-order harmonic generation of Ar using truncated Bessel and Gaussian beams

“…The harmonics in these studies were generated with multicycle (FWHM, ∼10 optical cycles) laser pulses. Later on, we simulated the HHG spectra of Xe by few-cycle (∼2 optical cycles) mid-infrared lasers [19,20] and we were able to reproduce the observed harmonic spectra. We also made an investigation of the harmonics generated in Ar and Ne atoms under the conditions of very high intensities and high pressures, where the nonlinear propagation of the laser field in the medium undergoes severe modifications [21].…”