Tunable high-power light sources are highly desired for exploring the unknown and revolutionising technologies, as near-infrared and x-ray lasers have demonstrated in last decades. However, the realization of such a source at the long-wavelength terahertz (THz) band has long been a formidable challenge. Here, Dr Guoqian and his team demonstrate a spectrally-tunable THz source delivering a ~1012-watt peak power, around 100 times as high as state-of-the-art accelerator- and crystal-based THz sources.
The team focused a high-intensity picosecond laser pulse onto a metal foil, and a high-brightness THz burst is observed at the foil rear side. Via theoretical modeling and experimental demonstration, the physical scenarios for THz generation are clarified. First, the laser pulse accelerates a large number of energetic electrons. Then, some of them escape from the foil target, and the rest are trapped around the target surface, accelerating ions. All of these processes can induce broadband THz radiation but with distinctly different spectra. This enables us to manipulate the global THz spectra by controlling the relative contributions of various radiation mechanisms, which is confirmed experimentally by varying the laser pulse duration or target size. The tunable THz spectra also reflect some dynamical change in laser-plasma interactions.
The unprecedented THz source demonstrated in our work may open up new avenues for the emerging extremely nonlinear THz wave-matter interactions, likely enabling a new relativistic paradigm which is currently accessible only at the infrared wavelengths. The THz radiation also offers a novel versatile laser-plasma diagnostic.
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