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Graphene, as an optically transparent material, typically defies any attempt for mid-infrared (mid-IR) absorption, which limits its applications in mid-IR biosensing. Although remarkable evidence for mid-IR nanopatterned graphene plasmons has been reported via the induction of free charge carriers, no study so far has investigated plasmonic excitation in nanopatterned graphene without employing induced voltage, high chemical doping, or metallic reflectors. In this work, we show that localized plasmon resonance (LSPR) can be probed in metal-free, naturally doped, nanomembrane graphene (NMG) without induced voltage or using metallic layers. We rely on facile, lithography-free, fabrication methodology to pattern nanoscale holes in a single sheet of graphene using Au nanoislands with hole dimensions as small as 10 nm. We image the LSPR at the graphene membrane edges via scanning near-field optical microscopy. Our experimental findings are confirmed by theoretical electromagnetic field mapping at the graphene membrane edges leading to noticeable absorption. We demonstrate the dependence of this absorption wavelength on the hole diameter and interhole distance; hence, we present a new avenue to fundamentally boost light harvesting with naturally doped NMG which is pivotal for mid-IR sensors. We show that our designed NMG can be used as a mid-IR biosensor with theoretically calculated sensitivity of 825 nm/RIU.

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