  After the successful fusion ignition at National Ignition Facility, seeking
for a high-gain fusion scheme becomes the next hot-spot in inertial confinement
fusion community. Fast ignition provides an alternative due to its potential to
reduce the energy of driven lasers and achieve higher target gain, whose core
step is generating fast electron beam using picosecond lasers. The properties
of the electron beam, such as energy spectra, determines the succeed heating
process and the yield. In this study, with 2D particle-in-cell simulations, we
find that the energy transfer ratio would significantly increase, and the
energy spectra would be lifted up in the dual-color injecting scheme, where an
extra weak low-frequency laser is injected with the main pulse. These phenomena
are attributed to the longitudinal electric field modulation in low-density
region and electromagnetically-induced-transparency process in the
microchannels formed near the critical surface. Our results can be applied to
fast ignition schemes and experimental designs of super-hot electron generation
with different pulse durations. For example, in the DCI project with a fast
ignition scheme, such a dual-color intense picosecond laser facility is under
construction to control the fast electron generation and enhance the laser
transparency.
