  The recent advancement of high-intensity lasers has made all-optical Compton
scattering become a promising way to produce ultra-short brilliant
$\gamma$-rays in an ultra-compact system. However, so far achieved Compton
$\gamma$-ray sources are severely limited by low conversion efficiency (lower
than $10^{-5}$) and spectral intensity ($\sim10^{4}$ ${\rm photons/0.1\%BW}$).
Here we present a highly efficient gamma photon emitter obtained by irradiating
a high-intensity laser pulse on a miniature plasma device consisting of a
plasma lens and a plasma mirror. This concept exploits strong spatiotemporal
laser-shaping process and high-charge electron acceleration process in the
plasma lens, as well as an efficient nonlinear Compton scattering process
enabled by the plasma mirror. Our particle-in-cell simulations demonstrate that
in this novel scheme, brilliant $\gamma$-rays with very high conversion
efficiency (higher than $10^{-2}$) and spectral intensity ($\sim10^{9}$ ${\rm
photons/0.1\%BW}$) can be achieved by employing currently available
petawatt-class lasers with intensity of $10^{21}$ ${\rm W/cm^2}$. Such
efficient and intense $\gamma$-ray sources would find applications in
wide-ranging areas.
