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Physical Review A

Signatures of self-modulation effects during pulse propagation in single-pulse absorption spectra

Yu He, Zuoye Liu, Zuwen Cui, Yuxuan Zhang, Adrian Nikolaus Pfeiffer, Thomas Pfeifer, J. J. Ding, Bitao Hu

Published May 20, 2019• 11 citations
Publisher

Abstract

The absorption spectrum of a bound state with a finite lifetime gives rise to the well-known Lorentzian line shape. This fundamental mechanism breaks down at high intensities. Here we demonstrate how even a single intense multicycle excitation pulse can result in Fano absorption profiles in the frequency domain. The intensity-dependent line shapes indicate the dipole responses are phase modulated by the single excitation pulse itself. Further, as the atomic density increases, we observe broader Fano-like shapes and the appearance of novel substructures in absorption lines. These are clear signatures of propagation effects in which collective interactions need to be taken into account. An analytical model is applied, which describes both effects observed in the experiment. Understanding the transition from optically dilute to dense medium is crucial to distinguish between single-atom and macroscopic effects.

DOI
10.1103/physreva.99.053418

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The absorption spectrum of a bound state with a finite lifetime gives rise to the well-known Lorentzian line shape. This fundamental mechanism breaks down at high intensities. Here we demonstrate how even a single intense multicycle excitation pulse can result in Fano absorption profiles in the frequency domain. The intensity-dependent line shapes indicate the dipole responses are phase modulated by the single excitation pulse itself. Further, as the atomic density increases, we observe broader Fano-like shapes and the appearance of novel substructures in absorption lines. These are clear signatures of propagation effects in which collective interactions need to be taken into account. An analytical model is applied, which describes both effects observed in the experiment. Understanding the transition from optically dilute to dense medium is crucial to distinguish between single-atom and macroscopic effects.
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.
Symmetric Lorentzian and asymmetric Fano line shapes are fundamental spectroscopic signatures that quantify the structural and dynamical properties of nuclei, atoms, molecules, and solids. This study introduces a universal temporal-phase formalism, mapping the Fano asymmetry parameter q to a phase φ of the time-dependent dipole response function. The formalism is confirmed experimentally by laser-transforming Fano absorption lines of autoionizing helium into Lorentzian lines after attosecond-pulsed excitation. We also demonstrate the inverse, the transformation of a naturally Lorentzian line into a Fano profile. A further application of this formalism uses quantum-phase control to amplify extreme-ultraviolet light resonantly interacting with He atoms. The quantum phase of excited states and its response to interactions can thus be extracted from line-shape analysis, with applications in many branches of spectroscopy.
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.
Ultrafast laser pulses with broad bandwidth can simultaneously excite multiple quantum states, forming coherent superpositions. The phases of the associated optical transitions evolve in time, giving rise to spectral line profiles that range from symmetric Lorentzian to asymmetric Fano-like forms. Yet, the line shape relations between different transitions within such superpositions remain largely unexplored. Here, we use transient absorption spectroscopy to probe transitions from coherent superpositions. While each transition’s line profile evolves over time, the relative line shape relations between different transitions remain fixed. We identify 4 fundamental types of line shape relations, each producing distinct spectral signatures. These relations naturally arise from second-order perturbation processes and can be actively tuned via the laser–matter interaction strength. Our findings establish a pathway for line shape-engineered control across atomic and molecular systems.
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.
We show that by combining strong-field dressing and resonant propagation of XUV pulses, the transition of absorption lines from their natural Lorentzian profiles through Fano and complex multipeak shapes all the way back to broadened near-Lorentzian profiles can be achieved, in the limit of optically thick samples. The final stage of this spectral modification can be understood in terms of a significant temporal stretching and delay of the resonant XUV pulse as it propagates through the dense gas, which alters the ultrafast absorption that is modified by the time-synchronized few-femtosecond laser pulse. We first demonstrate this concept in numerical calculations, both using a model system and through a fully coupled solution of the time-dependent Schrödinger equation and the Maxwell wave equation. The applicability and generality of the underlying mechanism is then illustrated in proof-of-principle attosecond transient absorption measurements in a helium gas with a widely varying atomic density. These results provide insights into the interaction of ultrashort laser pulses with dense media and its coherent control.