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

Owing to the superlattice-induced bandgap and superlattice-enhanced density of states, small-twist-angled (<2 degrees) bilayer graphene exhibits a strong gate-tunable photoresponse in the mid-infrared regime of 5 to 12 mu m, reaching an extrinsic peak responsivity of 26 mA W-1 at 12 mu m. Small-twist-angle (<2 degrees) bilayer graphene has received extraordinary attention recently due to its exciting physical properties(1-11). 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 states(4,6,7,12,13). 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 mu m. A maximum extrinsic photoresponsivity of 26 mA W-1 has been achieved at 12 mu m when the Fermi level in 1.81 degrees 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 degrees). Our demonstration reveals the promising optical properties of TBG and provides an alternative material platform for tunable mid-infrared optoelectronics.