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arXiv:1706.07218

Real-time reconstruction of complex non-equilibrium quantum dynamics of matter

Veit Stooß, Stefano M. Cavaletto, Alexander Blättermann, Paul Birk, Christoph H. Keitel, Christian Ott, Thomas Pfeifer

Published Jun 22, 2017• 0 citations
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Abstract

Causality implies that by measuring an absorption spectrum, the time-dependent linear response function can be retrieved. Recent experiments suggest a link between the shape of spectral lines observed in absorption spectroscopy with the amplitude and phase of the systems response function. This has even been observed in the presence of strong, nonlinear interactions, which promote the observed system out of equilibrium, making it explicitly time dependent. Thus far, however, only the special case of a sudden modification of the response function was understood analytically, leaving the general case of the dynamical response to arbitrary interactions open to interpretation. Here, we demonstrate that even for the case of a strongly driven, time-dependent system, one can reconstruct the full temporal response information from a single spectrum if a sufficiently short signal is used to trigger the absorption process. This finding is directly applied to a time-domain observation of Rabi cycling between doubly-excited atomic states in the few-femtosecond regime. This general approach unlocks single-shot real-time-resolved signal reconstruction across time scales down to attoseconds for non-equilibrium states of matter. In contrast to available pump-probe schemes, there is no need for scanning time delays in order to access real-time information. The scientific applications of this technique range from testing fundamental quantum dynamics, to measuring and controlling ultrafast, chemical and biological reaction processes.

DOI
10.48550/arxiv.1706.07218

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Causality implies that by measuring an absorption spectrum, the time-dependent linear response function can be retrieved. Recent experiments suggest a link between the shape of spectral lines observed in absorption spectroscopy with the amplitude and phase of the systems response function. This has even been observed in the presence of strong, nonlinear interactions, which promote the observed system out of equilibrium, making it explicitly time dependent. Thus far, however, only the special case of a sudden modification of the response function was understood analytically, leaving the general case of the dynamical response to arbitrary interactions open to interpretation. Here, we demonstrate that even for the case of a strongly driven, time-dependent system, one can reconstruct the full temporal response information from a single spectrum if a sufficiently short signal is used to trigger the absorption process. This finding is directly applied to a time-domain observation of Rabi cycling between doubly-excited atomic states in the few-femtosecond regime. This general approach unlocks single-shot real-time-resolved signal reconstruction across time scales down to attoseconds for non-equilibrium states of matter. In contrast to available pump-probe schemes, there is no need for scanning time delays in order to access real-time information. The scientific applications of this technique range from testing fundamental quantum dynamics, to measuring and controlling ultrafast, chemical and biological reaction processes.
The reconstruction of the full temporal dipole response of a strongly driven time-dependent system from a single absorption spectrum is demonstrated, only requiring that a sufficiently short pulse is employed to initialize the coherent excitation of the system. We apply this finding to the time-domain observation of Rabi cycling between doubly excited atomic states in the few-femtosecond regime. This allows us to pinpoint the breakdown of few-level quantum dynamics at the critical laser intensity near 2 TW/cm^{2} in doubly excited helium. The present approach unlocks single-shot real-time-resolved signal reconstruction across timescales down to attoseconds for nonequilibrium states of matter. In contrast to conventional pump-probe schemes, there is no need for scanning time delays in order to access real-time information. The potential future applications of this technique range from testing fundamental quantum dynamics in strong fields to measuring and controlling ultrafast chemical and biological reaction processes when applied to traditional transient-absorption spectroscopy.
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.
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A nonlinear interferometry scheme is described theoretically to induce and resolve electron wavefunction beating on time scales shorter than the optical cycle of the time-delayed pump and probe pulses.By employing two moderately intense few-cycle laser fields with a stable carrier-envelope phase, a large range of the entire electronic level structure of a quantum system can be retrieved.In contrast to single-photon excitation schemes, the retrieved electronic states include levels that are both dipole-and non-dipole-accessible from the ground electronic state.The results show that strong-field interferometry can reveal both high-resolution and broad-band spectral information at the same time with important consequences for quantum-beat spectroscopy on attosecond or even shorter time scales.
We demonstrate a two-dimensional time-domain spectroscopy method to extract amplitude and phase modifications of excited atomic states caused by the interaction with ultrashort laser pulses. The technique is based on Fourier analysis of the absorption spectrum of perturbed polarization decay. An analytical description of the method reveals how amplitude and phase information can be directly obtained from measurements. We apply the method experimentally to the helium atom, which is excited by attosecond-pulsed extreme ultraviolet light, to characterize laser-induced couplings of doubly excited states.