Terahertz (THz)-based electron acceleration has potential as a technology for next-generation cost-efficient compact electron sources. Here we present a novel millimeter-scale multicell waveguide-based THz-driven photogun that exploits field enhancement to boost the electron energy, a movable cathode to achieve precise control over the accelerating phase as well as multiple cells for exquisite beam control. The short driving wavelength enables a peak acceleration gradient as high as ~3 GV m−1. Using microjoule-level single-cycle THz pulses, we demonstrate electron beams with up to ~14 keV electron energy, 1% energy spread and ~0.015 mm mrad transverse emittance. With a highly integrated rebunching cell, the bunch is further compressed by about ten times to 167 fs with ~10 fC charge. High-quality diffraction patterns of single-crystal silicon and projection microscopy images of the copper mesh are achieved. We are able to reveal the transient radial electric field developed from the charged particles on a copper mesh after photoexcitation with high spatio-temporal resolution, providing a potential scheme for plasma-based beam manipulation. Overall, these results represent a new record in energy, field gradient, beam quality and control for a THz-driven electron gun, enabling real applications in electron projection microscopy and diffraction. This is therefore a critical step and milestone in the development of all-optical THz-driven electron devices, validating the maturity of the technology and its use in precision applications.
Despite the popularity and ubiquitousness of the tilted-pulse-front technique for single-cycle terahertz (THz) generation, optimization of the experimental setup remains complex and difficult due to the sensitive dependence on and coupling between the optical pulse parameters, including fluence, beam size, angular dispersion and temporal compression. Here we present a systematic and robust method to tune the tilted pulse-front setup, based on use of selected multi-dimensional scans, which enables a straight-forward and accurate determination of optimum parameter values. Our methodology not only allows us to determine parameter sensitivities and achieve a robust optimum in the performance, but also enables a verification of certain physical properties of the lithium niobate prism, including the THz refractive index. The detailed step-by-step procedure is discussed and applied to a tilted-pulse-front THz setup at both room temperature and cryogenic temperatures. The procedure can be applied to any setup based on the tilted-pulse-front geometry and is important for the construction of high energy THz sources required for strong field terahertz applications such as novel particle acceleration schemes or beam manipulators.
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