Lightwave-driven quasiparticle collisions on a subcycle timescale

Abstract

Ever since Ernest Rutherford first scattered α-particles from gold foils1, collision experiments have revealed unique insights into atoms, nuclei, and elementary particles2. In solids, many-body correlations also lead to characteristic resonances3, called quasiparticles, such as excitons, dropletons4, 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 superconductivity5. Fundamental research would immensely benefit from quasiparticle colliders, but the notoriously short lifetimes of quasiparticles6 have challenged practical solutions. Here we exploit lightwave-driven charge transport7–24, the backbone of attosecond science9–13, 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 sidebands17–19 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.

Quasi-classical analysis of the dynamics of the high-order harmonic generation from solids

“…The HHG has resulted in the birth of attosecond (1 as = 10 −18 s) pulses [4,5] and new imaging tools, such as molecular tomography [6] and spectroscopy [7]. Recent experiments have demonstrated that light-solid interactions offer a wide range of other phenomena and applications to be explored [8][9][10], including HHG from solid-state materials [11][12][13][14][15].Experimental results present a recollision feature in two-color laser fields [16], which is similar to HHG from the gas phase. However, it is also found that the cutoff of HHG from the solid phase depends on the strength E 0 of the laser fields linearly [11], rather than quadratically in the gas phase.…”

Ultrafast Dynamics of High-Harmonic Generation in Terms of Complex Floquet Spectral Analysis

“…(Please note that the radiative lifetime is considered to be much longer than this value [62]). This is much shorter than the pulse width of the driving laser used in the experiments ( 100 fs) [27]. As long as the pulse width of the driving field is longer than the lifetime of the excited state, the HHG spectrum does not depend on its pulse width.…”

“…We take these parameters to approximately represent the experiments of WSe 2 , such as the driving laser field frequency 2π/ω 20 THz, the band gap between the valence and the conduction bands ∆ 0 400 THz [27]. The driving laser amplitude a ≡ A/ω is determined so as to agree with the cutoff energy of the experiments n cutoff /hω 10.…”

“…Some have used two-color light beams, such as near infrared and far infrared, to clearly distinguish the different electronic excitations, which are sometimes called high-order sideband generation (HSG) [26][27][28][29]. Even though coherent electronic motion in solids is different from that of atoms and molecules, characteristic features of the HHG spectrum, such as the plateau and cutoff, are common to those from atoms and molecules.…”

“…Most conventional theories, however, have employed phenomenological treatments that may terminate the quantum coherence in the HHG process: for example, the HHG emission spectrum is calculated classically based on Maxwell's equation [2,6,[33][34][35], or the Markov approximation is used in a master equation with a phenomenological parameter [18,27,30,36]. In fact, how to interpret the dissipative spontaneous photon emission within the framework of quantum mechanics has been a debate since the early days of the theory [37][38][39][40], as an irreversible decay process contradicts the time-reversible quantum dynamics.…”