title: Lightwave-driven quasiparticle collisions on a subcycle timescale.
abstract: Ever since Ernest Rutherford scattered α-particles from gold foils, collision experiments have revealed insights into atoms, nuclei and elementary particles. In solids, many-body correlations lead to characteristic resonances--called quasiparticles--such as excitons, dropletons, polarons and Cooper pairs. The structure and dynamics of quasiparticles are important because they define macroscopic phenomena such as Mott insulating states, spontaneous spin- and charge-order, and high-temperature superconductivity. However, the extremely short lifetimes of these entities make practical implementations of a suitable collider challenging. Here we exploit lightwave-driven charge transport, the foundation of attosecond science, to explore ultrafast quasiparticle collisions directly in the time domain: a femtosecond optical pulse creates excitonic electron-hole pairs in the layered dichalcogenide tungsten diselenide while a strong terahertz field accelerates and collides the electrons with the holes. The underlying dynamics of the wave packets, including collision, pair annihilation, quantum interference and dephasing, are detected as light emission in high-order spectral sidebands of the optical excitation. A full quantum theory explains our observations microscopically. This approach enables collision experiments with various complex quasiparticles and suggests a promising new way of generating sub-femtosecond pulses.
authors.affiliation: Department of Physics, University of Regensburg, 93040 Regensburg, Germany.
authors.forename: F
authors.initials: F
authors.lastname: Langer
authors.affiliation: Department of Physics, University of Regensburg, 93040 Regensburg, Germany.
authors.forename: M
authors.initials: M
authors.lastname: Hohenleutner
authors.affiliation: Department of Physics, University of Regensburg, 93040 Regensburg, Germany.
authors.forename: C P
authors.initials: CP
authors.lastname: Schmid
authors.affiliation: Department of Physics, University of Regensburg, 93040 Regensburg, Germany.
authors.forename: C
authors.initials: C
authors.lastname: Poellmann
authors.affiliation: Department of Physics, University of Regensburg, 93040 Regensburg, Germany.
authors.forename: P
authors.initials: P
authors.lastname: Nagler
authors.affiliation: Department of Physics, University of Regensburg, 93040 Regensburg, Germany.
authors.forename: T
authors.initials: T
authors.lastname: Korn
authors.affiliation: Department of Physics, University of Regensburg, 93040 Regensburg, Germany.
authors.forename: C
authors.initials: C
authors.lastname: Schüller
authors.affiliation: Department of Physics and the Institute for Terahertz Science and Technology, University of California at Santa Barbara, Santa Barbara, California 93106, USA.
authors.forename: M S
authors.initials: MS
authors.lastname: Sherwin
authors.affiliation: Department of Physics, University of Marburg, 35032 Marburg, Germany.
authors.forename: U
authors.initials: U
authors.lastname: Huttner
authors.affiliation: Department of Physics, University of Marburg, 35032 Marburg, Germany.
authors.forename: J T
authors.initials: JT
authors.lastname: Steiner
authors.affiliation: Department of Physics, University of Marburg, 35032 Marburg, Germany.
authors.forename: S W
authors.initials: SW
authors.lastname: Koch
authors.affiliation: Department of Physics, University of Marburg, 35032 Marburg, Germany.
authors.forename: M
authors.initials: M
authors.lastname: Kira
authors.affiliation: Department of Physics, University of Regensburg, 93040 Regensburg, Germany.
authors.forename: R
authors.initials: R
authors.lastname: Huber