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Description
Undulating surface currents that charged particles induce in a metal grating as they pass over its facets at close range emit broadband Smith-Purcell radiation (SPR) into an angle-encoded fan of wavelengths 𝜆(𝜃), where the angle 𝜃 is measured from the particle propagation direction [1, 2]. If the length l = c𝜏 of a charged particle bunch of duration 𝜏 is shorter than the grating period d, then SPR at 𝜆 > d becomes coherent, and the energy of the coherent portion of the radiated pulse is proportional to N^2, where N is the number of particles in the bunch. Thus, few-femtosecond (c𝜏 ≈ few micrometers) electron bunches from laser-wakefield accelerators (LWFAs) can generate strong coherent SPR (CSPR) at much shorter wavelengths than was possible with few-picosecond (c𝜏 ≈ few millimeters) bunches typical of RF accelerators [2]. We experimentally confirmed that SPR remains strongly coherent down to the shortest THz wavelengths (𝜆 ≈ 10 µm) when generated by few-fs LWFA electron bunches [3], as described in separate workshop contributions.
Here, we address the question of whether the radiated CSPR "fan" can be tailored into strong-field (≳ GV/m) pulses that are useful for ultrafast nonlinear THz spectroscopy in applications such as nonlinear phononics, solid-state high-order harmonic generation, and ultrafast magnetic switching. Through simulations, we show that it is possible to remove the inherent spatial and angular chirp of a selected angular cone of the THz CSPR fan, compress the remaining longitudinally-chirped waveform to sub-ps duration, and focus the compressed pulses to field strengths of several GV/m using standard commercially-available optical components from chirped-pulse amplification (CPA) technology. The unique advantage of LWFA-driven CSPR over existing strong-field THz sources based on optical rectification of sub-ps infrared laser pulses in 𝜒(2) crystals [4] is the perfect synchronization of the CSPR pulses with the fs e-bunches that generated them. Thus, they are also perfectly synchronized with secondary betatron or inverse-Compton-scattered hard X-ray pulses of fs duration that the e-bunches generate. This opens the possibility for jitter-free probing of THz-driven structural dynamics via fs X-ray diffraction or fs X-ray absorption spectroscopy. The principal advantages of LWFA-driven CSPR over RF-accelerator-driven THz sources [5] are its compactness, low cost, mobility, and configurability. Any existing LWFA laboratory can home-build the CSPR generation, detection, de-chirping, and focusing systems presented here at low marginal cost. Thus, LWFA-driven CSPR offers unique capabilities for ultrafast strong-field THz spectroscopy that complement existing laser-based and RF-accelerator-based THz sources.
[1] S. J. Smith and E. M. Purcell, Phys. Rev. 92, 1069 (1953).
[2] G. Doucas, Smith-Purcell Radiation: Basic Theory and Applications (Oxford U. Press, 2025).
[3] R. Rudzinsky et al., Optica 13 (5), 810-821 (2026).
[4] C. Vicario et al., Phys. Rev. Lett. 112, 213901 (2014); G. Toth et al., Light Sci. Appl. 12, 256 (2023).
[5] B. Green et al., Sci. Rep. 6, 22256 (2016); M. Helm et al., Eur. J. Phys. Plus 138, 158 (2023).
| Working group | WG6 |
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