28 research outputs found
Fano Resonances in the Time Domain - understanding and controlling the absorption and emission of light
Fano Resonances in the Time Domain - understanding and controlling the absorption
and emission of light—Within this work, starting from the well known theory for
photoabsorption cross sections by Ugo Fano, a new formalism is developed, which enables
measurement and control of phase and amplitude changes in an emitting dipole.
This is shown by experimental measurements in singly and doubly excited helium that
confirm the formalism. An application in time-domain quantum holography of excited
states, allowing to separate laser-induced coupling pathways, is also presented
Lorentz meets Fano spectral line shapes: A universal phase and its laser control
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
{\phi} 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 prove the inverse, the transformation of a naturally Lorentzian line
into a Fano profile. A further application of this formalism amplifies
resonantly interacting extreme-ultraviolet light by quantum-phase control. The
quantum phase of excited states and its response to interactions can thus be
extracted from line-shape analysis, with scientific applications in many
branches of spectroscopy.Comment: 11 pages, 4 figure
Scientific Opportunities with an X-ray Free-Electron Laser Oscillator
An X-ray free-electron laser oscillator (XFELO) is a new type of hard X-ray
source that would produce fully coherent pulses with meV bandwidth and stable
intensity. The XFELO complements existing sources based on self-amplified
spontaneous emission (SASE) from high-gain X-ray free-electron lasers (XFEL)
that produce ultra-short pulses with broad-band chaotic spectra. This report is
based on discussions of scientific opportunities enabled by an XFELO during a
workshop held at SLAC on June 29 - July 1, 2016Comment: 21 pages, 12 figure
Reconstruction and control of a time-dependent two-electron wave packet
The concerted motion of two or more bound electrons governs atomic1 and molecular2,3 non-equilibrium processes including chemical reactions, and hence there is much interest in developing a detailed understanding of such electron dynamics in the quantum regime. However, there is no exact solution for the quantumthree-body problem, and as a result even the minimal system of two active electrons and a nucleus is analytically intractable4. This makes experimental measurements of the dynamics of two bound and correlated electrons, as found in the helium atom, an attractive prospect.However, although the motion of single active electrons and holes has been observed with attosecond time resolution5-7, comparable experiments on two-electron motion have so far remained out of reach. Here we showthat a correlated two-electron wave packet can be reconstructed froma 1.2-femtosecondquantumbeatamong low-lying doubly excited states in helium.The beat appears in attosecond transient-absorption spectra5,7-9 measured with unprecedentedly high spectral resolution and in the presence of an intensity-tunable visible laser field.Wetune the coupling10-12 between the two low-lying quantum states by adjusting the visible laser intensity, and use the Fano resonance as a phase-sensitive quantum interferometer13 to achieve coherent control of the two correlated electrons. Given the excellent agreement with large-scalequantum-mechanical calculations for thehelium atom, we anticipate thatmultidimensional spectroscopy experiments of the type we report here will provide benchmark data for testing fundamental few-body quantumdynamics theory in more complex systems. Theymight also provide a route to the site-specificmeasurement and control of metastable electronic transition states that are at the heart of fundamental chemical reactionsWe thank E. Lindroth for calculating the dipole moment (2p2|r|sp2,3+), and also A. Voitkiv, Z.-H. Loh, and R. Moshammer for helpful discussions. We acknowledge financial support by the Max-Planck Research Group Program of the Max-Planck Gesellschaft (MPG) and the European COST Action CM1204 XLIC. L. A. and F. M. acknowledge computer time from the CCC-UAM and Mare Nostrum supercomputer centers and financial support by the European Research Council under the ERC Advanced Grant no. 290853 XCHEM, the Ministerio de Economía y Competitividad projects FIS2010-15127, FIS2013-42002-R and ERA-Chemistry PIM2010EEC-00751, and the European grant MC-ITN CORIN