  Ultrashort photoemitted electron bunches can provide high electron currents
within sub-picosecond timeframes, enabling time-resolved investigations of
ultrafast physical processes with nanoscale resolution. Non-resonant conductive
nanotips are typically employed to realize nanoscale photoelectron sources with
high brightness. However, such emitters require complex non-scalable
fabrication procedures featuring poor reproducibility. Planar resonant antennas
fabricated via photolithography have been recently investigated, also because
of their superior field enhancement properties. Nevertheless, the electron
emission from these structures is parallel to the substrate plane, which limits
their practical use as electron sources. In this work, we present an innovative
out-of-plane, resonant nanoantenna design for field-driven photoemission
enabled by high-resolution 3D printing. Numerical and experimental evidences
demonstrate that gold-coated, terahertz resonant nanocones provide large local
electric fields at their apex, automatically ensuring out-of-plane coherent
electron emission and acceleration. We show that the resonant structures can be
conveniently arranged in an array form, for a further significant electron
extraction enhancement via a collective terahertz response. Remarkably, such
collective behaviour can also be harvested to boost photoemission from an
individual nano-source. Our approach opens the path for a new generation of
photocathodes that can be reproducibly fabricated and designed at will,
significantly relaxing the requirement for intense terahertz drivers.
