Lightwave-driven quasiparticle collisions on a sub-cycle timescale

Ever since Ernest Rutherford first scattered α-particles from gold foils 1 , collision experiments have revealed unique insights into atoms, nuclei, and elementary particles 2 . In solids, many-body correlations also lead to characteristic resonances 3 , called quasiparticles, such as excitons, dropletons 4 , polarons, or Cooper pairs. Their structure and dynamics define spectacular macroscopic phenomena, ranging from Mott insulating states via spontaneous spin and charge order to high-temperature superconductivity 5 . Fundamental research would immensely benefit from quasiparticle colliders, but the notoriously short lifetimes of quasiparticles 6 have challenged practical solutions. Here we exploit lightwave-driven charge transport 7 – 24 , the backbone of attosecond science 9 – 13 , 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 wave packet dynamics, including collision, pair annihilation, quantum interference and dephasing, are detected as light emission in high-order spectral sidebands 17 – 19 of the optical excitation. A full quantum theory explains our observations microscopically. This approach opens the door to collision experiments with a broad variety of complex quasiparticles and suggests a promising new way of sub-femtosecond pulse generation.