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Specifically, we leverage the presence of high-order spatial harmonics in metasurface-enhanced plasmon polaritons that are in turn excited by ultrashort laser pulses to generate multiple higher-order X-ray harmonics via electron–polariton scattering. More importantly, we show how the concept is not specific to plasmonic designs and generally can be achieved by shaping the near-field of nanopatterned surfaces of any kind. Here, we present a novel plasmonic design that can further reduce the required electron energy as well as generate multi-harmonic X-ray radiation with spatiotemporal profiles that can be directly shaped at the source by the user.
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These plasmon-based free-electron light sources have the potential to access extremely high photon energies (e.g., hard X-ray photons) without using highly relativistic electrons or high-intensity lasers.
#Harmonic convergence 2020 free
Recently, active graphene plasmon-based light sources 13, 14 have been considered, exploiting the strongly confined plasmonic field sustained by graphene to generate high-frequency radiation by scattering free electrons. Passive compact radiation sources based on free electrons have been studied in schemes such as Smith–Purcell emission 6, 7, 8, 9, collective-mode metamaterial light sources 10, and Cherenkov radiation 11, 12, but are limited by the material response at X-ray frequencies. The development of compact, tunable light sources is an extremely coveted goal: on-chip infrared and optical sources are sought after for photonic integration 1, while powerful tabletop UV to X-ray sources are potentially useful in applications ranging from medical therapy and diagnostics to industrial quality control, security scans, and the fundamental sciences 2, 3, 4, 5. Our developments could help pave the way for compact multi-harmonic sources of high-energy photons, which have potential applications in industry, medicine, and the fundamental sciences. As an example, we present the design of a four-color X-ray source, a potential candidate for tabletop multicolor hard X-ray spectroscopy. Using ab initio simulations, we predict bright and monoenergetic X-rays, achieving energies of 30 keV (with harmonics spaced by 3 keV) from 5-MeV electrons using 3.4-eV plasmon polaritons on a metasurface with a period of 85 nm. We show that the X-ray spectral profile can be arbitrarily shaped by controlling the metasurface geometry, the electron energy, and the incidence angle of the laser input.
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In particular, the metasurface near-field contains higher-order spatial harmonics that can be leveraged to generate multiple higher-harmonic X-ray frequency peaks. Here, we investigate the potential of the metasurface near-field profile, generated by an incident few-cycle pulse laser, to facilitate the generation of high-frequency light from free electrons. Metasurfaces are subwavelength spatial variations in geometry and material where the structures are of negligible thickness compared to the wavelength of light and are optimized for far-field applications, such as controlling the wavefronts of electromagnetic waves.