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Photoelectron Momentum Distributions of Single-Photon Ionization under a Pair of Elliptically Polarized Attosecond Laser Pulses
摘要： The attosecond ionization dynamics of atoms has attracted extensive attention during these days. However, the role of the initial state is not clearly understood. To address this question, we perform simulations on the neon atom and its model atom with different initial states by numerically solving the corresponding two-dimensional time-dependent Schr?dinger equations. We theoretically investigate atomic photoelectron momentum distributions (PMDs) by a pair of elliptically polarized attosecond laser pulses. We find that the PMD is sensitive not only to the ellipticities of the pulses, the relative helicity and time delay of the pulses, but also to the symmetry of the initial electronic states. Results are analyzed by the first-order time-dependent perturbation theory (TDPT) and offer a new tool for detecting the rotation direction of the ring currents.
关键词： photoelectron momentum distributions,elliptically polarized attosecond laser pulses,magnetic quantum numbers,the rotation direction of the ring currents
更新于2025-09-23 15:21:01
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - All-Optical Background-Free Detection of Ring Currents by Dynamical Symmetry Breaking High Harmonic Spectroscopy
摘要： Excited atoms and molecules can carry long-lived currents that circulate in the microscopic media. From a quantum mechanical perspective, these currents can be understood as a coherent wave-packet comprising a superposition of bound- states that oscillates in time [1–3]. When the wave-packet has a non-zero angular momentum expectation value, ring-currents circulate in the medium. For instance, a hydrogen atom excited to a 2p-state with non-zero magnetic quantum number m (e.g. by interaction with circularly polarized light) carries a steady-state ring current [2]. More complex systems can also carry persistent ring currents, e.g. spin-orbit wave-packets in Xenon [4], or multi-electron wave-packets in larger molecules [1]. This phenomenon is general to any quantum system and is especially interesting because it occurs on the natural time-scale of electronic motion – attoseconds to femtoseconds. Understanding ring currents is thus fundamentally important for manipulating and controlling ultrafast processes on the nanoscale, including chemical bond formation and topologically protected surface currents [5], as well as for the generation of intense attosecond-duration magnetic fields [1,6]. However, ring currents are very difficult to detect, particularly in a time-resolved manner. Only very recently were ring currents directly experimentally resolved in Argon through pump-probe angularly-resolved incidence photoelectron spectrum measurements [3]. Here we propose and theoretically explore an all-optical technique for ultrafast time-resolved detection of ring currents in atoms, molecules and solids, based on high harmonic generation (HHG). In this technique a microscopic medium interacts with a bi-chromatic (ω-2ω) bi-elliptical laser pulse, generating high harmonic photons. We show that the harmonic spectra emitted from current-carrying media differs from that of current-free media. We use dynamical symmetry (DS) considerations [7] to derive conditions for a maximal (background-free) signal in the harmonic ellipticity, which occurs when the pump beams are cross-linearly polarized [8]. In this configuration the bi-chromatic laser field exhibits a dynamical reflection symmetry that leads to linearly polarized harmonic selection rules [7]; however, this selection rule is broken when the medium carries a current, because ring-currents are not reflection-symmetric (similar to chiral systems [9], but where the current can be described in 2D). Thus, current-carrying media emit elliptically polarized harmonics, where the harmonic ellipticity is correlated to the intensity and sign of the current in the system. We numerically demonstrate the approach by pump-probe HHG calculations from Neon noble gas atoms, and from both aligned and un-oriented aromatic molecules (benzene and furan), using a non-interacting electron quantum model, and time-dependent density functional theory calculations. The presented work could be useful for ultrafast spectroscopy of current-carrying processes (chemical reactions, topological currents, etc.), as well as for manipulation and control of ring currents, paving the way for their table-top all- optical detection.
关键词： high harmonic generation,ultrafast spectroscopy,ring currents,dynamical symmetry breaking
更新于2025-09-11 14:15:04