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Physical Review A

Watching the formation and reshaping of a Fano resonance in a macroscopic medium

Yu He, Zuoye Liu, Nan Xue, Christian Ott, Thomas Pfeifer, Adrian Nikolaus Pfeiffer, Bitao Hu

Published Apr 15, 2021• 12 citations
Publisher

Abstract

Time-resolved buildup of a Fano resonance in a macroscopic gaseous medium is theoretically studied by attosecond transient absorption spectroscopy. An intense near-infrared field is employed as an adjustable gate to confine the temporal dynamics, enabling one to resolve the line-shape formation in real time. Beyond the region where the single-atom approximation holds, we observe the emergence of spectral spikes with narrow linewidths for certain time delays and propagation distances, which represents that the medium can be delicately structured to be an efficient resonant absorber. Spectral substructures appear on the shoulder of the absorption lines after additional pulse propagation. The links between these features imprinted in spectral profiles and the formation of temporal characteristics in the excitation field are identified. The collective response of the medium versus time delay and propagation distance, in this laser-gated case, provides clues to the time-domain information on autoionization state and the microscopic dynamics occurring at each layer of the medium.

DOI
10.1103/physreva.103.l041102

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Time-resolved buildup of a Fano resonance in a macroscopic gaseous medium is theoretically studied by attosecond transient absorption spectroscopy. An intense near-infrared field is employed as an adjustable gate to confine the temporal dynamics, enabling one to resolve the line-shape formation in real time. Beyond the region where the single-atom approximation holds, we observe the emergence of spectral spikes with narrow linewidths for certain time delays and propagation distances, which represents that the medium can be delicately structured to be an efficient resonant absorber. Spectral substructures appear on the shoulder of the absorption lines after additional pulse propagation. The links between these features imprinted in spectral profiles and the formation of temporal characteristics in the excitation field are identified. The collective response of the medium versus time delay and propagation distance, in this laser-gated case, provides clues to the time-domain information on autoionization state and the microscopic dynamics occurring at each layer of the medium.
We investigate the evolution of extreme ultraviolet (XUV) spectral line shapes in an optically thick helium gas under near-infrared (IR) perturbation. In our experimental and theoretical work, we systematically vary the IR intensity, time-delay, gas density, and IR polarization parameters to study line-shape modifications induced by collective interactions in a regime beyond the single-atom response of a thin, dilute gas. In both experiment and theory, we find that specific features in the frequency-domain absorption profile, and their evolution with propagation distance, can be attributed to the interplay between resonant attosecond pulse propagation and IR-induced phase shifts. Our calculations show that this interplay also manifests itself in the time domain, with the IR pulse influencing the reshaping of the XUV pulse propagating in the resonant medium.
Recording the transmitted spectrum of a weak attosecond pulse through a medium, while a strong femtosecond pulse copropagates at variable delay, probes the strong-field dynamics of atoms, molecules, and solids. Usually, the interpretation of these measurements is based on the assumption of a thin medium. Here, the propagation through a macroscopic medium of helium atoms in the region of fully allowed resonances is investigated both theoretically and experimentally. The propagation has dramatic effects on the transient spectrum even at relatively low pressures (50 mbar) and short propagation lengths (1 mm). The absorption does not evolve monotonically with the product of propagation distance and pressure, but regions with characteristics of Lorentz line shapes and characteristics of Fano line shapes alternate. Criteria are deduced to estimate whether macroscopic effects can be neglected or not in a transient absorption experiment. Furthermore, the theory in the limit of single-atom response yields a general equation for Lorentz- and Fano-type line shapes at variable pulse delay.
Autoionization of argon atoms was studied experimentally by transient absorption spectroscopy with isolated attosecond pulses. The peak position, intensity, linewidth, and shape of the 3s3p⁶np ¹P Fano resonance series (26.6-29.2 eV) were modified by intense few-cycle near infrared laser pulses, while the delay between the attosecond pulse and the laser pulse was changed by a few femtoseconds. Numerical simulations revealed that the experimentally observed splitting of the 3s3p⁶4p ¹P line is caused by the coupling between two short-lived highly excited states in the strong laser field.
We report on the experimental observation of the buildup of the 2s2p Fano resonance in helium in the time domain, which has been under theoretical investigation for more than a decade. The emergence of the absorption line is temporally resolved by interrupting the natural decay of the excited state via saturated strong-field ionization at a variable time delay. We compare the experimental data with full ab-initio simulations to validate the time-gating by strong-field ionization and thereby confirm the recently developed theory for the formation of Fano line-profiles.
We investigate the absorption line shapes of laser-dressed atoms beyond the single-atom response, by using extreme ultraviolet (XUV) attosecond pulse trains to probe an optically thick helium target under the influence of a strong infrared (IR) field. We study the interplay between the IR-induced phase shift of the microscopic time-dependent dipole moment and the resonant-propagation-induced reshaping of the macroscopic XUV pulse. Our experimental and theoretical results show that as the optical depth increases, this interplay leads initially to a broadening of the IR-modified line shape, and subsequently, to the appearance of new, narrow features in the absorption line.
An extreme ultraviolet (EUV) single attosecond pulse passing through a laser-dressed dense gas is studied theoretically. The weak EUV pulse pumps the helium gas from the ground state to the 2s2p(1P)2s2p({}^{1}P)2s2p(1P) autoionizing state, which is coupled to the 2s2(1S)2{s}^{2}{(}^{1}S)2s2(1S) autoionizing state by a femtosecond infrared laser with the intensity in the order of 1012{}^{12}12 W/cm2{}^{2}2. The simulation shows how the transient absorption and emission of the EUV are modified by the coupling laser. A simple analytical expression for the atomic response derived for $\ensuremath{\delta}$-function pulses reveals the strong modification of the Fano lineshape in the spectra, where these features are quite universal and remain valid for realistic pulse conditions. We further account for the propagation of pulses in the medium and show that the EUV signal at the atomic resonance can be enhanced in the gaseous medium by more than 50% for specifically adjusted laser parameters, and that this enhancement persists as the EUV propagates in the gaseous medium. Our result demonstrates the high-level control of nonlinear optical effects that are achievable with attosecond pulses.