We present a bias-free photoconductive terahertz generation scheme based on band-engineered photoconductive layers, enabling completely passive optical-to-terahertz conversion with zero dark current. By engineering the band structure of the photoconductive layers, a built-in electric field is induced, leading to efficient collection of almost all optically generated electrons. Additionally, a large area plasmonic nanoantenna array is utilized to enhance the optical absorption near each nanoantenna to reduce the average electron transit time to the radiating elements. Using this scheme, we demonstrate record-high optical-to-terahertz conversion efficiencies compared to previously demonstrated passive techniques utilizing nonlinear optical processes, spintronics, and photo-Dember effect.
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