Wakefield excited by intense lasers or charged particle beams in plasmas has made great strides recent years in demonstrating high-gradient acceleration of electron and positron beams, showing its tremendous potential in revolutionizing the design of next-generation compact light sources and colliders. In a plasma wakefield accelerator, the wakefield essentially serves as an "accelerator" for the witness beam, at the same time a "decelerator" for the driver. For a beam driver, the deceleration is simply the effect of the deceleration field. But for a laser driver, photons do not feel a deceleration field directly, instead they deplete their energy via frequency downshifting on a refractive index gradient, leading to a slowing down of their group velocity, literally behaving like “a photon decelerator”. In fact, theory and simulations suggest that such a photon decelerator with a properly designed plasma structure could serve as an ideal nonlinear optical device for the generation of intense single-cycle broadband long-wavelength infrared (IR) pulses. Here we successfully demonstrate this novel scheme in experiments. An intense single-cycle IR pulse with a central wavelength of 9.4 µm and energy of 3.4 mJ is generated using a ~580 mJ, 36 fs, 810 nm drive laser. Furthermore, the tunability of the IR wavelength in the range of 4-15µm is also successfully demonstrated through simple adjustment of the plasma structure. This relativistically intense, ultra-broadband infrared pulse opens up many opportunities for relativistic-infrared nonlinear optics, attosecond X-ray pulses via high-harmonic generation, and pump-probe experiments in the “molecular fingerprint” region.
 Zan Nie, Chih-Hao Pai, Jianfei Hua, et al., “Relativistic single-cycle tunable infrared pulses generated from a tailored plasma density structure”, Nature Photon., 12: 489, 2018
 Zan Nie, et al., to be submitted
Jianfei Hua, Zan Nie, Chih-Hao Pai, and Wei Lu, "Relativistic single-cycle tunable infrared pulses generated from a tailored plasma density structure (Conference Presentation)," Proc. SPIE 11036, Relativistic Plasma Waves and Particle Beams as Coherent and Incoherent Radiation Sources III, 110360L (Presented at SPIE Optics + Optoelectronics: April 04, 2019; Published: 14 May 2019); https://doi.org/10.1117/12.2520712.6035323241001.
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