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This paper presents a detailed numerical electromagnetic characterization of GaAs photovoltaic (PV) nanopillar array solar cells recently developed for solar energy harvesting. Through electromagnetic theory, full-wave simulations, and an optical measurement, a deeper understanding of the electromagnetic operation of these nanostructure arrays is achieved by revealing the mechanisms that allow for its inherent improvement of optical absorption over conventional PV solar cells. Initial investigations include incorporating and verifying material optical properties through measurements of bulk GaAs samples, simulating the effects of nanopillar geometry and configuration, and an analysis of the optical absorption mechanism of the nanopillar arrays through the graphical visualization of the electric fields in the vicinity of the nanopillars. These investigations will offer critical insights into the effects of pillar dimensions and configuration that can significantly increase optical solar energy absorption approximately 1.5 times that of conventional solar cells spanning the entire visible spectrum and for angles of incidence up to 60 °. Furthermore, comparisons between nanopillar arrays with and without substrates will demonstrate the mechanism that drives the efficient optical absorption. In addition, the importance of the nanopillar structure (i.e., dimensions) and reflecting substrate in providing energy coupling and improving the air-to-array interface will be discussed.