We design and fabricate a low-loss silicon photonic two-dimensional grating coupler that serves to couple light between standard single-mode fibers and single-mode waveguides in the silicon-on-insulator platform and to split both orthogonal polarization states. The efficiency of the fabricated device is enhanced by a backside metal mirror and reaches a record value of -1.8 dB with a 1 dB bandwidth of 32 nm around 1550 nm. The demonstrated coupling efficiency is 2.2 dB better than the conventional two-dimensional grating coupler without a metal mirror.