  Detection of individual molecules is the ultimate goal of any chemical
sensor. In the case of gas detection, such resolution has been achieved in
advanced nanoscale electronic solid-state sensors, but it has not been possible
so far in integrated photonic devices, where the weak light-molecule
interaction is typically hidden by noise. Here, we demonstrate a scheme to
generate ultrasensitive down-conversion four-wave-mixing (FWM) in a graphene
bipolar-junction-transistor heterogeneous D-shaped fiber. In the communication
band, the FWM conversion efficiency can change steeply when the graphene Fermi
level approaches 0.4 eV. In this condition, we exploit our unique two-step
optoelectronic heterodyne detection scheme, and we achieve real-time individual
gas molecule detection in vacuum. Such combination of graphene strong
nonlinearities, electrical tunability, and all-fiber integration paves the way
toward the design of versatile high-performance graphene photonic devices.
