Strong mid-infrared photoresponse in small-twist-angle bilayer graphene

Small-twist-angle (<2°) bilayer graphene has received extraordinary attention recently due to its exciting physical properties1–11. Compared with monolayer graphene, the Brillouin zone folding in twisted bilayer graphene (TBG) leads to the formation of a superlattice bandgap and substantial modification to the density of states4,6,7,12,13. However, these emerging properties have rarely been leveraged to realize new optoelectronic devices. Here, we demonstrate the strong, gate-tunable photoresponse in the mid-infrared wavelength range of 5 to 12 μm. A maximum extrinsic photoresponsivity of 26 mA W−1 has been achieved at 12 μm when the Fermi level in 1.81° TBG was tuned to its superlattice bandgap. Moreover, the strong photoresponse critically depends on the formation of a superlattice bandgap, and it vanishes in the gapless case with an ultrasmall twist angle (<0.5°). Our demonstration reveals the promising optical properties of TBG and provides an alternative material platform for tunable mid-infrared optoelectronics. Owing to the superlattice-induced bandgap and superlattice-enhanced density of states, small-twist-angled (<2°) bilayer graphene exhibits a strong gate-tunable photoresponse in the mid-infrared regime of 5 to 12 μm, reaching an extrinsic peak responsivity of 26 mA W−1 at 12 μm.