  The explosion in attosecond technology has opened the gate to investigating
many unexplored areas which require ultrahigh spatial and temporal resolution.
In the area of nuclear physics, using gamma-rays with ultrahigh resolution in
time and space will help to investigate intra-nuclear dynamics in an
unprecedentedly explicit way. However, the generation of ultrahigh brilliance
attosecond gamma-ray pulses with current-generation laser facilities has not
been reported. In this letter, we propose a novel method to generate high
charge (~1nC) attosecond (<200 attosecond) electron bunch by the near-threshold
self-injection in a wakefield accelerator. We demonstrate the ability to
generate an ultrahigh-brilliance (> 2*1024 photons s-1mm-2mrad-2 per 0.1%BW)
attosecond (<200 attosecond) gamma-ray (Emax > 3 MeV) pulse via nonlinear
Compton scattering. To the best of our knowledge, this is the first method
reported to generate attosecond gamma-ray photon source using
current-generation laser. This is the shortest gamma-ray photon and the highest
brilliance photon source in MeV range (orders higher than the results
reported). This method can be widely applied for experimental generation of 100
keV to several MeV high brilliance attosecond gamma-ray sources with current
~100 TW laser facilities, which will benefit basic science such as application
in ultra-high resolution radiography.
