  A new method capable of capturing coherent electric field structures
propagating at nearly the speed of light in plasma with a time resolution as
small as a few femtoseconds is proposed. This method uses a few femtoseconds
long relativistic electron bunch to probe the wake produced in a plasma by an
intense laser pulse or an ultra-short relativistic charged particle beam. As
the probe bunch traverses the wake, its momentum is modulated by the electric
field of the wake, leading to a density variation of the probe after free-space
propagation. This variation of probe density produces a snapshot of the wake
that can directly give many useful information of the wake structure and its
evolution. Furthermore, this snapshot allows detailed mapping of the
longitudinal and transverse components of the wakefield. We develop a
theoretical model for field reconstruction and verify it using 3-dimensional
particle-in-cell (PIC) simulations. This model can accurately reconstruct the
wakefield structure in the linear regime, and it can also qualitatively map the
major features of nonlinear wakes. The capturing of the injection in a
nonlinear wake is demonstrated through 3D PIC simulations as an example of the
applications of this new method.
