High-Energy Subcycle Electron Emission Driven by Spatiotemporally Confined THz Fields

Tip-based structures offer a pathway to strong-field, optical-driven field emission, holding the potential for generating high-brightness ultrashort electron sources. While direct optical-field-driven electron emission and acceleration within the single optical subcycle provides a straightforward solution for ultrafast electron generation, achieving precise control over the emission process to generate high-quality electron beams suitable for practical applications remains challenging. Here, we present THz-driven subcycle electron field emission in a ridged waveguide cavity, achieving the first practical high-energy femtosecond field-emission source. This ridged cavity is engineered to spatially and temporally confine the electric field, effectively minimizing dispersion and suppressing field leakage. Through carrier-envelope phase control of the THz field, we achieve subcycle field emission, generating, for the first time, isolated electron bunches with a 940-fs (FWHM) duration. Using dual <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:mn>2</a:mn><a:mn>4</a:mn><a:mtext> </a:mtext><a:mtext> </a:mtext><a:mi mathvariant="normal">μ</a:mi><a:mi mathvariant="normal">J</a:mi></a:mrow></a:math> (<e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:mrow><e:mn>2</e:mn><e:mo>×</e:mo><e:mn>2</e:mn><e:mn>4</e:mn><e:mtext> </e:mtext><e:mtext> </e:mtext><e:mi mathvariant="normal">μ</e:mi><e:mi mathvariant="normal">J</e:mi></e:mrow></e:math>) of THz energy, we achieve a peak field strength of approximately <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"><i:mrow><i:mn>23</i:mn><i:mtext> </i:mtext><i:mtext> </i:mtext><i:mi>GV</i:mi><i:mo>/</i:mo><i:mi mathvariant="normal">m</i:mi></i:mrow></i:math>, accelerating electrons to a record energy of 46 keV with a charge of 2 pC per bunch, thereby enabling single-shot imaging. The high-field strength suppresses emittance blowup due to space-charge effects, ensuring excellent beam quality. The generated electron pulse exhibits high stability. As an initial application, we investigate transient electric-field dynamics in real time and space using time-resolved schlieren radiography with this field-emission electron source. These proof-of-principle results signify a critical advancement in developing compact, high-performance electron sources for user-oriented applications.