Abstract
The production of coherent radiation at ever shorter wavelengths has been a long-standing challenge since the invention of lasers1,2 and the subsequent demonstration of frequency doubling3. Modern X-ray free-electron lasers (FELs) use relativistic electrons to produce intense X-ray pulses on few-femtosecond timescales4,5,6. However, the shot noise that seeds the amplification produces pulses with a noisy spectrum and limited temporal coherence. To produce stable transform-limited pulses, a seeding scheme called echo-enabled harmonic generation (EEHG) has been proposed7,8, which harnesses the highly nonlinear phase mixing of the celebrated echo phenomenon9 to generate coherent harmonic density modulations in the electron beam with conventional lasers. Here, we report on a demonstration of EEHG up to the 75th harmonic, where 32 nm light is produced from a 2,400 nm laser. We also demonstrate that individual harmonic amplitudes are controlled by simple adjustment of the phase mixing. Results show the potential of laser-based manipulations to achieve precise control over the coherent spectrum in future X-ray FELs for new science10,11.
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Acknowledgements
The authors thank D. McCormick, K. Jobe, I. Makasyuk, T. Beukers, C. Hudspeth, J. Cruz, V. Yakimenko and the rest of the Test Facilities Group for their support. The authors also thank M. Guehr for his guidance with the design and construction of the EUV spectrometer and E. Johnson for the VISA undulator. This work was supported by the US DOE Office of Basic Energy Sciences under award no. 2012-SLAC-10032 using the SLAC NLCTA facility, which is partly supported by US DOE Office of High Energy Physics under contract no. DE-AC02-76SF00515. The work of D.X. was supported by the Major State Basic Research Development Program of China (grant no. 2015CB859700) and by the National Natural Science Foundation of China (grant no. 11327902).
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G.S. conceived the original EEHG theoretical concept, which, with D.X., was later developed further. E.H., B.G. and M.D. carried out the experiments. B.G. and M.D. designed and built the EUV spectrometer. E.H. and B.G. performed the data analysis and performed simulations. D.X., C.H. and T.R. provided guidance on the experiments and on potential applications. E.H. and D.X. wrote the paper, with contributions from all other authors.
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Hemsing, E., Dunning, M., Garcia, B. et al. Echo-enabled harmonics up to the 75th order from precisely tailored electron beams. Nature Photon 10, 512–515 (2016). https://doi.org/10.1038/nphoton.2016.101
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DOI: https://doi.org/10.1038/nphoton.2016.101
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