||Ordered three dimensional (3-D) nanophotonic structures have been extensively explored in recent photovoltaics research, since they provide the possibility to bring down the cost of solar cells via boosting light absorption efficiency while using even less material. Up until now, most efforts are devoted to 3-D nanostructure synthesis and fabrication. Even though efficient light absorption has been experimentally demonstrated using different nanostructures, including nanopillar arrays, nanohole arrays, nanocone arrays and etc., the mechanisms leading to their enhanced light absorption have rarely been systematically discussed. Since it becomes clear that the geometrical factors of ordered nanostructures greatly affect their optical properties, a systematic study on this aspect is worthy of conducting. In this study, geometrical dependent light absorption efficiency of nanowell and nanopillar array structures has been systematically analyzed by finitedifference time-domain (FDTD) simulation. The mechanisms that lead to efficient light absorption are found to be the combination of optical grating and cavity resonance coupling effects for nanowell arrays, and guided mode for nanopillar arrays. In experiment, a set of nanowell arrays with different geometrical parameters have been fabricated by using a novel nanoimprint assisted anodization method in conjunction with low pressure chemical vapor deposition method. Consistent with simulations, experimental measurements demonstrate an optimal geometrical design for efficient light harvesting, and as high as 94% broad band light absorption has been achieved with the fabricated structures. These results not only greatly substantiate the understanding of the interplay between photons and nanophotonic structures but also serve as solid stepping stones toward the implementation of novel-structured optoelectronic devices, such as solar cells and photodetectors.