  Plasma waves generated in the wake of intense, relativistic laser or particle
beams can accelerate electron bunches to giga-electronvolt (GeV) energies in
centimetre-scale distances. This allows the realization of compact accelerators
having emerging applications, ranging from modern light sources such as the
free-electron laser (FEL) to energy frontier lepton colliders. In a plasma
wakefield accelerator, such multi-gigavolt-per-metre (GV m$^{-1}$) wakefields
can accelerate witness electron bunches that are either externally injected or
captured from the background plasma. Here we demonstrate optically triggered
injection and acceleration of electron bunches, generated in a multi-component
hydrogen and helium plasma employing a spatially aligned and synchronized laser
pulse. This ''plasma photocathode'' decouples injection from wake excitation by
liberating tunnel-ionized helium electrons directly inside the plasma cavity,
where these cold electrons are then rapidly boosted to relativistic velocities.
The injection regime can be accessed via optical density down-ramp injection,
is highly tunable and paves the way to generation of electron beams with
unprecedented low transverse emittance, high current and 6D-brightness. This
experimental path opens numerous prospects for transformative plasma wakefield
accelerator applications based on ultra-high brightness beams.
