  The production of ultra-bright electron bunches using ionization injection
triggered by two transversely colliding laser pulses inside a beam-driven
plasma wake is examined via three-dimensional (3D) particle-in-cell (PIC)
simulations. The relatively low intensity lasers are polarized along the wake
axis and overlap with the wake for a very short time. The result is that the
residual momentum of the ionized electrons in the transverse plane of the wake
is much reduced and the injection is localized along the propagation axis of
the wake. This minimizes both the initial 'thermal' emittance and the emittance
growth due to transverse phase mixing. 3D PIC simulations show that ultra-short
(around 8 fs) high-current (0.4 kA) electron bunches with a normalized
emittance of 8.5 and 6 nm in the two planes respectively and a brightness
greater than 1.7*10e19 A rad-2 m-2 can be obtained for realistic parameters.
