  Recently the small-twist-angle (< 2{\deg}) bilayer graphene has received
extraordinary attentions due to its exciting physical properties. Compared with
monolayer graphene, the Brillouin zone folding in twisted bilayer graphene
(TBG) leads to the formation of superlattice bandgap and significant
modification of density of states. However, these emerging properties have
rarely been leveraged for the realization of new optoelectronic devices. Here
we demonstrate the strong, gate-tunable photoresponse in mid-infrared
wavelength range of 5 to 12 um. A maximum extrinsic photoresponsivity of 26
mA/W has been achieved at 12 um when the Fermi level in a 1.81{\deg} TBG is
tuned to its superlattice bandgap. Moreover, the strong photoresponse
critically depends on the formation of superlattice bandgap, and it vanishes in
the gapless case with ultrasmall twist angle (< 0.5{\deg}). Our demonstration
reveals the promising optical properties of TBG and provides an alternative
material platform for tunable mid-infrared optoelectronics.
