Ever since Ernest Rutherford scattered α-particles from gold foils1, collision experiments have revealed insights into atoms, nuclei and elementary particles2. In solids, many-body correlations lead to characteristic resonances3—called quasiparticles—such as excitons, dropletons4, 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 superconductivity5. However, the extremely short lifetimes of these entities6 make practical implementations of a suitable collider challenging. Here we exploit lightwave-driven charge transport7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24, the foundation of attosecond science9,10,11,12,13, 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 dynamics of the wave packets, including collision, pair annihilation, quantum interference and dephasing, are detected as light emission in high-order spectral sidebands17,18,19 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.
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We thank H. Banks for critical reading of the manuscript. The work in Regensburg was supported by the European Research Council through grant number 305003 (QUANTUMsubCYCLE) and the Deutsche Forschungsgemeinschaft (through grant number HU 1598/2-1 and GRK 1570), and the work in Marburg by the Deutsche Forschungsgemeinschaft (through SFB 1083, SPP 1840 and grant numbers KI 917/2-2, KI 917/3-1). The work in Santa Barbara was supported by the National Science Foundation (through grant number DMR 1405964).
The authors declare no competing financial interests.
Extended data figures and tables
a, b, False-colour plot of the spectral intensity of high-order sidebands (generated under resonant, spectrally narrow, optical excitation) of orders two to eight as a function of their polarization for horizontal (a) and vertical (b) polarization of the interband excitation pulse. The polarization angle is defined such that 0° corresponds to a horizontally polarized excitation. c, Spectrally integrated sideband intensity as a function of the polarization angle.
a, The measured high-order sideband intensity IHSG (spectrally and temporally integrated, red spheres) scales linearly with the pump fluence Φ, as indicated by a guide to the eye (black dashed line). b, The modulation depth of the spectrally integrated temporal trace of IHSG (red spheres, see Fig. 2b) decreases with increasing pump fluence, closely following a fit (black curve) proportional to the inverse square-root of the pump fluence. c, By contrast, the relative subcycle delay δscT−1 does not substantially change with increasing pump fluence. Red spheres represent the average of δscT−1 over the nine dominant, consecutive, high-order sideband peaks (see Fig. 2d) for different pump fluences; error bars indicate their standard deviation. The black dashed line marks the mean value of the displayed data points.
a, Computed high-order sideband intensity IHSG (red shaded area) and driving waveform (blue). Vertical black dotted lines highlight characteristic delay times tex at extrema of IHSG. b–d, Coherent interband polarization |p|2 (colour scale) as a function of time t and reciprocal space coordinate k (in units of the wave vector kBZ limiting the first Brillouin zone) for distinct delay times tex according to maximum (b, tex = 7 fs; d, tex = 28 fs) and minimum (c, tex = 18 fs) HSG emission. Horizontal white lines mark k = 0. The driving field is depicted as grey curves and the recollision times (see Fig. 3d) are highlighted by black arrows.
a, b, The measured sideband spectrum (shaded area) is compared with computations using constant dephasing times of T2 = 3.2 fs (a, red curve), T2 = 4 fs (b, blue curve) and a momentum-dependent dephasing model as presented in Fig. 1 (black curves; EID, excitation-induced dephasing). The sideband orders n are indicated above the relevant peaks. All spectra are normalized to the sideband peak corresponding to n = 2. The inset in a depicts the corresponding dephasing times T2(k) as a function of the wave vector k; the red (blue) horizontal line indicates a constant decay level T2 = 3.2 fs (T2 = 4 fs).
a, Scaling of the modulation depth (see discussion in Methods section ‘Influence of excitation fluence’) of temporal traces of high-order sideband intensity with the driving peak field strength. An increase in the modulation depth suggests a faster dephasing T2. b, Subcycle delay δsc in units of the driving period T averaged over the eight most-dominant sideband peaks as a function of the peak field. The error bars represent the standard deviation of the eight peaks and the dashed line depicts a linear fit to the data points.
a, Terahertz waveform used in the computations. b, c, Mean electron excursion calculated with a constant dephasing T2 = 12 fs (b) and excitation-induced dephasing (c) for a delay of tex = 7 fs (see vertical dotted lines in a). The shaded areas indicate the intensity envelopes of the excitation pulse with a full-width at half-maximum of 10 fs (yellow) and 100 fs (grey). The corresponding mean momenta are shown as red and black curves, respectively. The red dashed curves show after the electron–hole collision. The dashed horizontal lines in c mark the positions in k space where the scattering time T2(k) switches from slow to fast dephasing.
An intensity spectrum (blue shaded area) of THz-driven high-order harmonic generation in 60-nm-thick WSe2 shows distinct peaks at odd orders n′ = 13 to n′ = 47 (indicated by numerals) of the fundamental frequency νTHz = 22.3 THz (peak electric field in air, 21 MV cm−1; φ = 90°). An intensity spectrum for φ = 120° is also shown as a black curve. The spectral intensity has been corrected for the grating efficiency and for the quantum efficiency of the spectrograph used.
a, b, Contours of differential spectra ΔI(ν, νopt) (colour scale) as a function of h(ν − ν2SB) and detuning Δopt computed for an excitonic binding energy of EB = 60 meV (a) and EB = 600 meV (b). The black and grey vertical lines mark the positions of the slices shown in c and d, respectively. c, d, Snapshots of ΔI(ν, νopt) at fixed values of |h(ν − ν2SB)| = 16 meV below (c) and above (d) the second-order sideband peak for three different binding energies EB = 60 meV (black curves), EB = 240 meV (red curves) and EB = 600 meV (blue curves).
a, Measured differential spectra ΔI(ν, νopt) for three different detunings Δopt = −75 meV (shaded area), Δopt = −29 meV (black curve) and Δopt = 27 meV (red curve) as functions of ν, centred at the position of the second sideband ν2SB. b, d, f, Computed differential spectra ΔI(ν, νopt) for binding energies of EB = 60 meV (b), EB = 240 meV (d) and EB = 600 meV (f) and detunings Δopt as in the experiment shown in a. c, e, Measured (c) and calculated (e) HSG intensities (colour scale) for Δopt = 0 as functions of νHSG and delay tex.
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Langer, F., Hohenleutner, M., Schmid, C. et al. Lightwave-driven quasiparticle collisions on a subcycle timescale. Nature 533, 225–229 (2016) doi:10.1038/nature17958
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