  Plasma photocathodes open a path towards tunable production of well-defined,
compact electron beams with normalized emittance and brightness many orders of
magnitude better than state-of-the-art. Such beams could have a far-reaching
impact on applications such as light sources, but also open up new vistas on
high energy physics and high field physics. We report on challenges and details
of the proof-of-concept demonstration of a plasma photocathode in 90$^\circ$
geometry at SLAC FACET within the "E-210: Trojan Horse" program. Using this
experience, alongside theoretical and simulation-supported advances, we discuss
the upcoming "E-310: Trojan Horse-II" program at FACET-II.
