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

Small-twist-angle bilayer graphene has drawn much attention recently due to its exotic transport properties such as superconductivity and correlated insulating phase. The Brillouin zone folding and the formation of flat moiré bands significantly modify the density of states (DOS). Here we demonstrate the colossal and gate-tunable mid-infrared (5 ~ 12 μm) photoresponse leveraging the enhanced DOS and the emergence of superlattice-induced bandgaps. At liquid nitrogen temperature, we achieve a maximum extrinsic responsivity of 26 mA W^-1 at 12 μm in a 1.81° twist angle bilayer graphene, when the Fermi level is tuned to the middle of the superlattice gap in the electron branch. Together with transport study, we reveal the bolometric origin of the photoresponse. Moreover, the colossal photoresponse critically relies on the formation of superlattice gaps as it is vanishingly small in ultrasmall twist angle (< 0.5°) device, where the gaps are absent in our measurement range. Our results show the desirable properties and promising applications of twisted bilayer graphene for mid-infrared optoelectronics.