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Physical Review Research

Local enhancement in transient absorption spectroscopy by gating the resonance in the time domain

Yu He, Christian Ott, Thomas Pfeifer, Mette B. Gaarde

Published Jan 26, 2024• 7 citations
Publisher

Abstract

The concept of resonant perfect absorption, enabled by the combined action of pulse propagation and an auxiliary gate pulse, was recently proposed and demonstrated in a group of two-level systems [Y. He , ]. Here we exploit this method in a more realistic scenario by solving the coupled time-dependent Schrödinger equation and the Maxwell wave equation in helium. Through emptying the population of the 1s2p excited state after its excitation, we explore the evolution of the spectral profile with time delay and propagation distance and link the observations to the controlled interference between the original field and the gated new field. We find that resonant absorption for higher-lying states can also be strongly enhanced, in spite of the congestion of multiple resonances and the presence of complex laser-induced couplings. Our results show that interferometric control of absorption using intense laser fields can be applied selectively in both the temporal and spatial domains. Published by the American Physical Society 2024

DOI
10.1103/physrevresearch.6.013103

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The concept of resonant perfect absorption, enabled by the combined action of pulse propagation and an auxiliary gate pulse, was recently proposed and demonstrated in a group of two-level systems [Y. He , ]. Here we exploit this method in a more realistic scenario by solving the coupled time-dependent Schrödinger equation and the Maxwell wave equation in helium. Through emptying the population of the 1s2p excited state after its excitation, we explore the evolution of the spectral profile with time delay and propagation distance and link the observations to the controlled interference between the original field and the gated new field. We find that resonant absorption for higher-lying states can also be strongly enhanced, in spite of the congestion of multiple resonances and the presence of complex laser-induced couplings. Our results show that interferometric control of absorption using intense laser fields can be applied selectively in both the temporal and spatial domains. Published by the American Physical Society 2024
We propose and study the manipulation of the macroscopic transient absorption of an ensemble of open two-level systems via temporal engineering. The key idea is to impose an ultrashort temporal gate on the polarization decay of the system by transient absorption spectroscopy, thus confining its free evolution and the natural reshaping of the excitation pulse. The numerical and analytical results demonstrate that even at moderate optical depths, the resonant absorption of light can be reduced or significantly enhanced by more than 5 orders of magnitude relative to that without laser manipulation. The achievement of the quasicomplete extinction of light at the resonant frequency, here referred to as resonant perfect absorption, arises from the full destructive interference between the excitation pulse and its subpulses developed and tailored during propagation, and is revealed to be connected with the formation of zero-area pulses in the time domain.
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.
We present theoretical simulations of the attosecond transient absorption of singly-excited states of helium atoms in the presence of a dressing near-infrared or infrared laser. In particular, we aim to address several unresolved questions in the transient absorption of helium and to resolve the remaining discrepancies between theory and experiment. We initially focus on the forklike structures in the Autler–Townes splitting of the 1s2p state and the effects of resonant coupling to the 1s2s and 1s3s states. We find that the delay-dependent features of the Autler–Townes doublet depend strongly on both the laser frequency detuning from resonance and on the laser pulse duration, and explain the lack of such structures in current experimental data. Next, we identify the interference mechanism which causes the half-cycle oscillations in the absorption spectrum below the excited state manifold. Finally, we observe for the first time the presence of quantum beating in the simulated transient absorption spectrogram, and discuss the conditions under which such wavepacket dynamics could be observed experimentally.
The goal of this work is to study and understand dynamics of atomic systems interacting with strong fields. To this end, a method referred to as strong-field spectroscopy is used, which measures the absorption spectrum of atoms interacting with such fields. In the scope of this work, a technique which allows the simultaneous measurement of absorption spectra of attosecond pulses and their reference signal is developed, which significantly increases the sensitivity to absorption changes. Additionally, a reconstruction method is introduced which retrieves the time-dependent evolution of the dipole response of an atomic system directly from a single absorption spectrum. Strong-field spectroscopy combines these techniques to examine the nonlinear strong-field dynamics in atomic and molecular systems and is applied to study the strongly correlated doubly excited states in helium. Furthermore, a measurement approach is introduced which uses the interaction with a high-intensity laser pulse to impose a time gate on the evolution of the coherent response of a system, which has previously been excited by an ultrashort laser pulse. With this approach the buildup of resonance phenomena in a wide range of physical systems can be studied. As a first application, the time-dependent buildup of a Fano resonance as well as the buildup of a whole Rydberg series are investigated.
In the present paper, the time-resolved transient absorption spectroscopy of helium atoms is investigated based on the three-level modeling. The helium atoms are subjected to an extreme ultraviolet (XUV) attosecond pulse and a time-delayed infrared (IR) few-cycle laser field. The odd excited state are populated from the ground state by the XUV pulse due to the dipole selection rule, and probed by the time-delayed IR laser. The time-resolved transient absorption spectroscopy based on the different coupling mechanism demonstrate some different features, the photoabsorption spectrum based on three-level model with rotating wave approximation (RWA) cannot repeat the fast oscillation and the sideband structure which have been observed in the previous experimental investigation. The dressing effect of IR laser pulse on the ground state can contribute new interference structures in the photoabsorption spectrum.
We present a theoretical study of the delay-dependent Autler-Townes (AT) splitting in transient absorption spectroscopy of an isolated attosecond pulse in helium atoms subject to a delayed infrared (IR) pulse. We concentrate on cases in which the IR pulse is resonant with the helium $1s2p\text{\ensuremath{-}}1s2s$ transition and provide a time-domain perspective of the dynamics in the delay-dependent pump-probe system. We identify several interesting delay-dependent features in the transient absorption spectrum such as AT splitting, oscillation between absorption and emission at the resonant absorption frequency, and sub-IR-cycles oscillations. We then explain the origins of these features in the time domain in terms of a strongly driven two-level system, in the language of population transfer and coherent control.