  We investigate a novel scheme for brilliant attosecond {\gamma}-ray emission
and high-yield positron production, which is accomplished with an ultra-intense
laser pulse incident upon a Nano-Micro array (NMA) with substrate incorporated.
This scheme is able to realize effectively electron acceleration and colliding
geometry. Both the {\gamma}-ray flash and positron bunch are then generated
with high conversion efficiency. At laser intensity of I_0 = 8 times 10^{23}
W/cm^2, ~27% of the laser energy is transferred successfully into the
{\gamma}-rays, and ~0.7% of the laser energy into the positrons. As a
consequence, ultra-short (~440 as) and ultra-brilliant (~10^{24} photons s^{-1}
mm^{-2} mrad^{-2} per 0.1%BW @ 15 MeV) {\gamma}-ray burst, and high-yield (1.48
times 10^{11}) and overdense (~10^{22} cm^{-3}) positron bunch are generated.
We found a sub-linear scaling of laser-to-photon conversion efficiency
(proportional to I_0^{0.75}) and a super-linear scaling of laser-to-positron
conversion efficiency (proportional to I_0^{2.5}) with the laser intensity.
Multi-dimensional particle-in-cell simulations show that particle ({\gamma}
photon and positron) generation can be manipulated by laser-focusing position,
and NMA's length and spacing. Optimal conditions for particle generation in
NMAs are obtained, indicating that microwire array has the advantage over
nanowire array in particle generation in the extreme laser fields. Furthermore,
positron annihilation effect in high-energy-density (HED) environment is
discussed. The scheme using NMAs would provide effective avenues toward
investigating attosecond nuclear science and HED physics with the coming 10 PW
laser facilities.
