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A generic bifunction GaN light-emitting diode (LED) structure, which has a 300 nm n⁺-ZnO epitaxial layer grown on a standard GaN LED epistack and a mesa size of 200 μm × 600 μm, exhibits a peak responsivity of 450 mA/W to purple lights (380-400 nm) under zero bias and a narrow bandpass of around 60 nm. The corresponding product of quantum efficiency and gain is estimated as 140%. The purple light (380-400 nm) to blue light (>460 nm) rejection ratio can reach two orders of magnitude. The generic bifunction LED does not sacrifice its optical and modulation performances as a light transmitter. The optical power reaches 100 mW with a peak wavelength around 447 nm. A new proposition for duplex free-space visible light communications is depicted.