  Intense lasers can accelerate electrons to very high energy over a short
distance. Such compact accelerators have several potential applications
including fast ignition, high energy physics, and radiography. Among the
various schemes of laser-based electron acceleration, vacuum laser acceleration
has the merits of super-high acceleration gradient and great simplicity. Yet
its realization has been difficult because injecting free electrons into the
fast-oscillating laser field is not trivial. Here we demonstrate free-electron
injection and subsequent vacuum laser acceleration of electrons up to 20 MeV
using the relativistic transparency effect. When a high-contrast intense laser
drives a thin solid foil, electrons from the dense opaque plasma are first
accelerated to near-light speed by the standing laser wave in front of the
solid foil and subsequently injected into the transmitted laser field as the
opaque plasma becomes relativistically transparent. It is possible to further
optimize the electron injection/acceleration by manipulating the laser
polarization, incident angle, and temporal pulse shaping. Our result also sheds
new light on the fundamental relativistic transparency process, crucial for
producing secondary particle and light sources.
