High-order harmonic generation enhanced by XUV light

Abstract

The combination of high-order harmonic generation (HHG) with resonant xuv excitation of a core electron into the transient valence vacancy that is created in the course of the HHG process is investigated theoretically. In this setup, the first electron performs a HHG three-step process whereas, the second electron Rabi flops between the core and the valence vacancy. The modified HHG spectrum due to recombination with the valence and the core is determined and analyzed for krypton on the 3d → 4p resonance in the ion. We assume an 800 nm laser with an intensity of about 10 14 W cm 2 and xuv radiation from the Free Electron Laser in Hamburg (FLASH) with an intensity in the range 10 13 -10 16 W cm 2 . Our prediction opens perspectives for nonlinear xuv physics, attosecond x rays, and HHG-based spectroscopy involving core orbitals. c 2018 Optical Society of America OCIS codes: 190.2620, 140.2600 High-order harmonic generation (HHG) by atoms in intense optical laser fields is a fascinating phenomenon and a versatile tool; it has spawned the field of attoscience, is used for spectroscopy, and serves as a light source in many optical laboratories [1]. Present-day theory of HHG largely gravitates around the single-active electron (SAE) approximation and the restriction to HHG from valence electrons [1][2][3][4].Several extensions to the SAE view of HHG have been investigated previously. A two-electron scheme was considered that uses sequential double ionization by an optical laser with a subsequent nonsequential double recombination; in helium this leads to a second plateau with about 12 orders of magnitude lower yield than the primary HHG plateau [5]. Two-color HHG (optical plus xuv light) has been studied in a one-electron model [6] and with many-electron effects included by a frequency-dependent polarization [7]; the xuv radiation assists thereby in the ionization process leading to an overall increased yield [6] and the emergence of a new plateau [7], the latter, however, at a much lower yield. The above schemes suffer from tiny conversion efficiency beyond the conventional HHG cutoff (maximum photon energy).We propose an efficient two-electron scheme for a HHG process manipulated by intense xuv light from the newly constructed free electron lasers (FEL)-e.g., the Free Electron Laser in Hamburg (FLASH). Our principal idea is sketched in Fig. 1. In the parlance of the three-step model [2,3], HHG proceeds as follows: (a) the atomic valence is tunnel ionized; (b) the liberated electron propagates freely in the electric field of the optical laser; (c) the direction of the optical laser field is reversed and the electron is driven back to the ion and eventually recombines with it emitting HHG radiation. The excursion time of the electron from the ion is approxi- mately 1 fs for typical 800 nm optical laser light. During this time, one can manipulate the ion such that the returning electron sees the altered ion as depicted in Fig. 1. Then, the emitted HHG radiation bears the signature of the change. Perfectly suited for t...

Attosecond dynamics of light-induced resonant hole transfer in high-order-harmonic generation

“…For example, nonsequential double recombination, which is the inverse of single-photon double ionization, leads to a second plateau, but the probability for this process was found to be extremely small [32]. Here, we will study a two-electron scheme to generate a second plateau by inner-shell excitation with an assisting XUV pulse, as first proposed by Buth et al [33][34][35][36]: During the excursion of a valence electron in continuum, one can excite an inner-shell electron to the vacant valence state. Then, the returning electron recombines into the inner-shell hole, leading to an increase in energy of the emitted photon.…”

“…In both cases, the HHG process can be controlled by varying the time delay (or phase delay in the case of IR-APT setup) of IR and XUV field. A semiclassical model and stationary phase approximation beyond the strong field approximation (SFA) is presented to interpret the results, where the XUV field is treated as a perturbation to explain the hole dynamics on the subcycle time scale [33][34][35][36]. In the few cycle IR pulse + single XUV attopulse case, the temporal information of electron trajectories is exhibited.…”

High-order harmonic generation with resonant core excitation by ultraintense X-rays

“…Since the publication of the articles [80][81][82] and a preprint of this work [85] a few related or inspired articles have been published. Using the resonant interaction of xuv light with an atomic transition in hydrogenlike atoms that are strongly perturbed by optical light Antonov et al [86] predict attosecond pulse generation based on resonant interaction in a macroscopic medium for vuv or xuv light.…”

Enhancing the high-order harmonic generation yield within a specified spectral window via electron wave-packet engineering

“…Also the bound dynamics of the ion can be driven by the XUV field during the HHG process leading to the active influence of a core electron on the process. The XUV field has to be tuned to a resonance between a core and valence states resulting in a second plateau in the spectrum that is shifted to higher energies by the former resonance energy with respect to the first plateau [27]. The XUV assistance can be employed to enhance the efficiency for the generation of attosecond pulses [28].…”