Electron pulses down to the femtoseconds duration can be generated by exciting metallic nanotips by ultrafast laser pulses. Using an array of metal nanotips, one can generate high-bunch charge pulses with ∼10⁷ electrons. Double-gate field emitter arrays with sub-micron pitch nano-tips combined with the surface plasmon polariton resonance of the gate electrode and the beam collimation performance of the stacked-double-gate structure have been proposed recently to further enhance the electron yield and reduce the beam emittance aiming at realizing a ultrabright cathode for the X-ray free-electron lasers and THz vacuum electronic amplifiers, In this work, we present the detailed numerical studies of the proposed structure to elucidate the physical process that enables the efficient tip-laser couplings.