Free-space circularly polarized light (CPL) detection, requiring polarizers
and waveplates, has been well established, while such spatial degree of freedom
is unfortunately absent in integrated on-chip optoelectronics. So far, those
reported filterless CPL photodetectors suffer from the intrinsic small
discrimination ratio, vulnerability to the non-CPL field components, and low
responsivity. Here, we report a distinct paradigm of geometric photodetectors
in mid-infrared exhibiting colossal discrimination ratio, close-to-perfect
CPL-specific response, a zero-bias responsivity of 392 V/W at room temperature,
and a detectivity of ellipticity down to 0.03o Hz−1/2. Our approach
employs plasmonic nanostructures array with judiciously designed symmetry,
assisted by graphene ribbons to electrically read their near-field optical
information. This geometry-empowered recipe for infrared photodetectors
provides a robust, direct, strict, and high-quality solution to on-chip
filterless CPL detection and unlocks new opportunities for integrated
functional optoelectronic devices