  We describe a simple technique to passively bunch non-ultrarelativistics
($\lesssim 10$~MeV) electron bunches produced in conventional photoinjectors.
The scheme employs a dielectric-lined waveguide located downstream of the
electron source to impress an energy modulation on a picosecond bunch. The
energy modulation is then converted into a density modulation via ballistic
bunching. The method is shown to support the generation of sub-picosecond bunch
train with multi-kA peak currents. The relatively simple technique is expected
to find applications in compact, accelerator-based, light sources and advanced
beam-driven accelerator methods.
