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Although hot-carrier-based photodetection using plasmonic effects has been widely investigated, photodetectors of this type with an external quantum efficiency (EQE) > 1 % and an active area of < 1 mm2 remain out of reach even in the visible frequencies. In this work, a novel hot-electron-based, non-trench-type photodetector exploiting pure photoexcitation in a thin aluminum (Al) film and leaky plasmonic modes at and between its heterojunctions is proposed, analyzed, and experimentally demonstrated. Combining diffracted-order-resolved analytical analysis and numerical computations unravels the optical absorption mechanism of the innovative design. Leaky surface plasmon resonance (with leakage radiation into the air) produced by a propagating diffracted order and quasibound supermodes (with power leakage via coupled gap plasmon polariton and bound surface plasmon polariton modes) excited by evanescent diffracted orders are shown to significantly contribute to the absorptance in the preferred thin Al film where hot electrons are generated. At 638.9 nm and electric bias −0.9951 V, the measured per-unit-area responsivity, detectivity, and the external quantum efficiency reach 298.1444 μA/mW/mm2, 4.3809 × 109 cm Hz1/2/W, and 2.6878%, respectively, from an active area of 4.6457 × 10−2 mm2. The performance is among the best of those previously reported operating at similar wavelengths and biases. The RC time constant is estimated to be about 1.673 μs from the current-voltage measurements. The physical insight into the innovative, experimentally demonstrated device could lay the groundwork for the practical use of low-voltage, metal-based photodetection. [ABSTRACT FROM AUTHOR]