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Ultrafast Science

Line Shape Relations in Ultrafast Transient Absorption from a Coherent Superposition

Chunyuan Bai, Chun Hui Huang, Chun Hui Huang, Mathew A. Britton, Marianna Lytova, Zhongyao Yue, Xiaojing Liu, Ruxin Li, David M. Villeneuve, Peng Peng

Published Jan 1, 2026• 0 citations
Publisher

Abstract

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.

DOI
10.34133/ultrafastscience.0165

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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.
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.
Line shapes observed in attosecond transient absorption spectra (ATAS) encode information on the phase of the optical dipole response. Understanding and controlling the phase evolution of interference fringes is among the fundamental goals of exploring coherent interaction. Here, we simulate the attosecond transient absorption in the near-infrared (NIR) dressing field by considering a three-level model in the adiabatic approximation. We find that the absorption line shape can be changed from Lorentzian to Fano to inverted Lorentzian and back again at two-photon dressed energy levels “ω ± 2ωι”, and line shapes evolve in half-cycle periodicity (T/2). Moreover, the direction of phase evolution of “ω ± 2ωι” is opposite, while the phases of “ω ± 2ωι” are roughly the same at certain delays with an interval of approximately a quarter-cycle periodicity (T/4). These results demonstrate that absorbance of two-photon dressed energy levels “ω ± 2ωι” can qualitatively map the AC Stark shift. These observations provide new insights into the control of spectral line shapes and open the way for the modulation of quantum optical interference in the laser-driven atom.
The attosecond pump-femtosecond probe technique is widely used in electron dynamics research, with temporal resolution reaching tens of attoseconds. Here, we propose a scheme for controlling the Fano line shapes of helium doubly excited states by replacing the pump source with a pair of time-delayed attosecond pulses. We show that the interference induced by this delay provides an additional, highly effective means of controlling electron dynamics, enabling temporal resolution on the zeptosecond scale. We demonstrate that the corresponding line shapes in transient absorption spectra can shift dramatically within a time-delay variation of just a few hundred zeptoseconds. This extreme spectral modulation occurs under two key conditions: (i) when the time delay aligns with the spectral minimum of the coherently combined attosecond pulses in the frequency domain, and (ii) when the delay between the infrared probe pulse and the first attosecond pulse is held constant. This approach opens a new pathway to zeptosecond-scale control in quantum systems and demonstrates the powerful synergy between phase manipulation in both light and electron waves for achieving ultrafast spectral transformations.
Photon absorption spectroscopy is a powerful tool for uncovering the structure of atoms, molecules, and solids. Symmetric Lorentzian and asymmetric Fano line shapes are fundamental spectroscopic signatures related to the structural and dynamical properties. Recently, Ott et al. [Science 340, 716 (2013)] successfully transferred Fano profile into Lorentzian line shape using an intense infrared laser, after excitation of autoionizing states in helium by attosecond XUV pulse. This is a very important step forward in quantum phase control. However, here we show experimentally that an autoionizing state can have both Fano and Lorentzian behavior naturally, depending on the process involved. This study utilized the inverse process of photon absorption ionization, i.e., electron ion recombination with photon emission, making sure the resonant autoionizing state is not modified or decorated by the laser fields. Our result implies that excitation of the state through different paths---for example, one photon versus multiphoton excitation, or even one step versus multistep excitation---can lead to different Fano profiles for the same resonant state. We also report an experimental determination of the energy shifts in the recombination photon-intensity peaks due to the interference between the resonant and nonresonant processes.
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.