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Laser Physics

The spectral minimum in the harmonics generated from xenon atoms driven by intense mid-infrared laser pulses

Jingtao Zhang, D-S Guo

Published Jun 13, 2013• 0 citations
Publisher

Abstract

The spectral minimum in the harmonics generated from driven xenon atoms is reproduced by a nonperturbative QED theory. We find that the spectral minimum is caused by the radial wavefunction of the Xe 5p subshell in momentum space, and thus reflects the inherent property of the field-free wavefunction. The harmonics between the minimum and the cutoff of the plateau form an island structure. The island structure is quite similar to the giant-resonant structure observed recently. Suggestions are proposed to identify whether a true resonance transition occurs or not between different atomic subshells.

DOI
10.1088/1054-660x/23/8/085302

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The spectral minimum in the harmonics generated from driven xenon atoms is reproduced by a nonperturbative QED theory. We find that the spectral minimum is caused by the radial wavefunction of the Xe 5p subshell in momentum space, and thus reflects the inherent property of the field-free wavefunction. The harmonics between the minimum and the cutoff of the plateau form an island structure. The island structure is quite similar to the giant-resonant structure observed recently. Suggestions are proposed to identify whether a true resonance transition occurs or not between different atomic subshells.
We show that a simple nonperturbative two-level model of an atom driven by a very strong periodic field, results in a rich picture of very high harmonic generation and related phenomena. It reproduces experimentally observed plateau, yields for the first time simple analytic formulas for the plateau cutoff frequency, critical driving intensity, and saturation, and predicts intensity-induced multi-resonances. One of the most fascinating phenomenon discovered recently in nonlinear interaction of light with atoms and ions, is very high-order (up to 135) odd harmonic generation (HHG) by intense (∼ 1013 W/cm2 and higher) optical laser radiation in rare gasses and some ions [1], The spectra of generated harmonics drasticly deviate from the perturbation theory predictions [2], In particular, intensity of harmonics, falling monotonically with their orders only up to a certain point, levels off forming a so-called “plateau", and falls monotonically again beyond it. Generally, harmonte generation depends on phase-matching conditions and nonlinear response of individual atoms. It has recently become clear however [3,4] that the major features of HHG, in particular the plateau, result mainly from general properties of atomic nonlinear response. The most direct and apparently successful way so far to approach the problem theoretically has been numerical simulation of the Schrödinger [2,5-7] (including an empiric rule [6]) for many-electron atoms using Hartry-Slater approximation. This approach requires, however, tremendous amount of calculations and involves many processes, making it difficult to gain simple insights. An interesting simplified model [8] based on 3-D delta-potential with a single (ground) level [9] produces results in integration form. The idea of retaining a single energy scale (ionization energy) brings one close to an even simpler system: a two-level model atom. A two-level model of HHG [10], however, due to various complications introduced into it, failed to generate simple results, whereas an analytic solution [11] holds for a virtually degenerate two-level model only (see below) which is unapplicable to the experimental conditions [1-4,7,9]; besides, no relaxation was considered in [10,11].
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Harmonic generation is studied in an idealized model medium in which the laser field essentially interacts only with a single atomic transition and where ionization can be neglected. Such two-level models are known to generate high harmonics with a plateau and a cutoff. We put forward a quantitative rule for the cutoff frequency in the harmonic spectrum in this model which we relate to the ``IP{\mathrm{I}}_{\mathrm{P}}IP​+3UP{3\mathrm{U}}_{\mathrm{P}}3UP​ rule'' in real systems with ionization. We examine the phase properties of the emitted harmonics and show that their coherence and thus phase matching is more favorable in the cutoff regime than in the plateau of the harmonic spectrum. This behavior is known to occur in realistic models of neon, and that this occurs also for the simple idealized two-level model strongly suggests that the sudden change of phase coherence from the cutoff to the plateau regime is as intrinsic a property of nonlinear systems as are the plateau and the cutoff.
We present photoelectron spectra measured by `short' pulse ( ps) MPI of xenon at 526.5 nm excitation wavelength in the intensity range to . Ionization into and channels is observed and resonance structures are found in both ionization channels. They are induced by the AC Stark shift of bound Xe states within the light pulse which brings them into transient n-photon resonance with the Xe ground state. The resonant states are tentatively identified as d- and s-angular momentum states having AC Stark shifts which differ from the shift of the xenon ionization threshold. In the high-intensity limit, ionization into the channel is completely suppressed.
We develop a Floquet based formalism which describes a process of the generation harmonics of in an atom driven by a laser field. Using this formalism, we show that several harmonics are strongly enhanced in the spectrum when the laser frequency is such that a multiphoton resonance occurs between the initial state and a quasienergy state. We illustrate these findings by a numerical calculation of the harmonics generation in the potassium and rubidium atoms driven by a laser in the near-infrared energy range. The present formalism is also applicable when the initial state is a coherent superposition of several field-free atomic states.