Session N7: Focus Session: Attosecond Transient Absorption

Understanding and Laser Control of Fano Resonances and Absorption in the Time Domain

Quantum states and resonances associated with transitions between them are among the most fundamental elements of physics and chemistry. It is thus crucial to comprehensively understand states and resonances, not only in their (traditional) energy representation, but also from a time-domain perspective. This is especially important for time-dependent interactions such as atoms exposed to weak or intense laser fields. Here, I will present an experiment on the time-domain measurement and laser control of atomic resonances in helium atoms. Using extreme-ultraviolet (XUV) attosecond-pulsed light from high-harmonic generation (HHG), we were able to study Fano resonances arising in the double-excitation of both He electrons after absorption of a single XUV photon. The XUV absorption spectrum was recorded after the passage of the broadband HHG pulse through a He medium, as a function of time delay and intensity of a synchronized and coherently-locked near-visible (VIS) laser pulse. The modification of the original Fano absorption profiles during the interaction with the VIS laser pulse could be understood by considering a single time-domain phase control operation. As a result, the Fano resonance and the defining asymmetry (q) parameter could be mapped into a phase of the dipole response. This understanding also allows to create resonant amplification of light without inversion by a short-pulsed (impulsive) control operation, thus complementing the traditional dressed-state picture of electromagnetically-induced transparency. The same physical mechanism not only allows to measure quantum-state resolved phase changes of individual doubly-excited states in the control of two-electron wave packets, but also paves a route towards frequency combs at x-ray frequencies for precision spectroscopy of highly-charged ions.   

Transient absorption lineshapes in a dense, laser-dressed Helium target probed by attosecond pulse trains

Attosecond transient absorption is an emerging time-resolved spectroscopic technique to explore electron dynamics in atoms and molecules. In this experimental study, we used extreme ultraviolet (XUV) attosecond pulse trains (APTs) in energy range of 20-25 eV to probe the transient excited-state absorption of an optically thick Helium gas sample under the influence of moderately strong (1-3 TW/cm$^{\mathrm{2}})$, infrared (IR), femtosecond pump pulse. We found that the resonant absorption lineshapes for Helium 1s2p, 1snp, and continuum states show rich dynamics, evolving between Lorenzian and Fano profiles with phases imposed by IR laser pulse and multi-channel quantum-path interference. Both AC Stark shifts and light-induced states were studied as a function of pump-probe delay and IR intensity. By changing the Helium gas density, we observed the lineshape modification due to the macroscopic propagation effects, which is usually not included in the single-atom response model. We found that the 13th and 15th high harmonics of XUV produce two coupled polarizations, and the relative coherence between these two polarizations changes the absorption even when the IR pulse arrives after a long time (about 500 fs) after the XUV.   

Hole burning and higher order photon effects in attosecond light-atom interaction

We have performed calculations of attosecond laser-atom interactions for laser intensities where interesting two and three photon effects become relevant. In particular, we examine the case of ``hole burning" in the initial orbital. Hole burning is present when the laser pulse duration is shorter than the classical radial period because the electron preferentially absorbs the photon near the nucleus. We also examine how 3 photon Raman process can lead to a time delay in the outgoing electron for the energy near one photon absorption. For excitation out of the hydrogen $2s$ state, an intensity of $2.2\times 10^{16}$~W/cm$^2$ leads to a 6 attosecond delay of the outgoing electron. We argue that this delay is due to the hole burning in the initial state.   

Theory of strong-field attosecond transient absorption

Attosecond transient absorption (ATA) has recently received a lot of attention as a powerful approach to probing ultrafast electron dynamics. In particular, a number of experimental and theoretical works have investigated the transient absorption of attosecond extreme ultraviolet (XUV) pulses in systems exposed to a moderately strong, ultrafast, infrared (IR) laser pulse. ATA is based on the inherent synchronization between an attosecond pulse and the few-cycle IR pulse used to produce it, and provides insight into the energy transfer between electromagnetic fields and matter even at the sub-femtosecond time scale. In this talk, I will first briefly present an overview of the theoretical framework we have developed to describe the IR-assisted attosecond absorption, both at the single-atom level and at the level of a macroscopic number of atoms in a gas. This framework is based on a time-dependent approach to absorption; a process which is most often described in the frequency domain. In the second part of the talk, I will present recent results, several of which were obtained in collaboration with different experimental groups: (i) the appearance of light-induced structures in the absorption spectrum of an isolated attosecond pulse, (ii) the evolution of a resonant absorption line shape that can be caused by either microscopic or macroscopic effects, and (iii) the use of a fourth-order nonlinear coupling - in the absorption of the harmonics that constitute an attosecond pulse train -- to define delay zero of the overlap between the XUV and the IR pulses.   

Nonperturbative calculation of multidimensional spectra using the Multiconfiguration Time-Dependent Hartree Fock method

A nonperturbative approach to multidimensional spectroscopy is demonstrated which makes use of a recent implementation of the Multiconfiguration Time-Dependent Hartree Fock (MCTDHF) method to describe the correlated many-electron response of atoms and molecules to VUV or X-ray pulses. A multidimensional spectrum, for example a three pulse photon echo spectrum, is determined by the dipole polarization $\mathbf{P}(t) = \langle \Psi(t) \vert \hat{\mu} \vert \Psi(t) \rangle$ of the system following a series of ultrafast laser pulses. Phase matched components of the polarization can be found using established techniques by calculating polarization corresponding to pulse sequences which vary only in their carrier envelope phases, then solving a linear system of equations for the phase matched components. An atomic test case making use of subfemtosecond VUV pulses is presented.   

Investigation of coupling mechanisms in attosecond transient absorption of auto-ionizing states: comparison of theory and experiment in xenon

Attosecond transient absorption spectra near the energies of autoionizing states are analyzed in terms of the photon coupling to other states. A simple expression, which was used to determine the autoionization lifetimes of highly excited states of xenon in a recent transient absorption experiment, is shown to be more general and can be used to describe cases involving resonant or nonresonant coupling of the attosecond-probed autoionizing state to either continua or discrete states by a time delayed near IR pulse. Additional fast oscillations versus delay-time in the measured spectrum are shown to reveal the energies of other states, both encompassed by the radiation field of the attosecond pulse and coupled by the NIR probe pulse.   

Attosecond Control of Photoabsorption Through Manipulating the Electron--Electron Correlation

This talk reports on studies of photoabsorption control by manipulating the electron$-$electron correlation in a double-ionization process with an attosecond extreme ultraviolet (EUV) pulse. Electron correlation plays an essential role in a wide range of fundamentally important many-body phenomena in modern physics and chemistry. An example is the importance of electron--electron correlation in multiple ionization of multielectron atoms and molecules exposed to intense laser pulses. Manipulating the dynamic electron correlation in such photoinduced processes is a crucial step toward the coherent control of chemical reactions and photobiological processes. We will show for the first time, from full-dimensional \textit{ab initio} calculations of double ionization of helium in intense laser pulses ($\lambda =$ 780 nm), that the electron--electron interactions can be instantaneously tuned using a time-delayed attosecond EUV pulse.\footnote{S. X. Hu, Phys. Rev. Lett. \textbf{111}, 123003 (2013).} Consequently, the probability of producing energetic electrons from excessive photoabsorption can be enhanced by an order of magnitude through the attosecond control of electron--electron~correlation. This work was partially supported by the Department of Energy National Nuclear Security Administration under Award No. DE-NA0001944, the University of Rochester, and the New York State Energy Research and Development Authority. Computations have been conducted utilizing the ``Kraken'' Supercomputer at NICS.