  Optical microcavity enhanced light-matter interaction offers a powerful tool
to develop fast and precise sensing techniques, spurring applications in the
detection of biochemical targets ranging from cells, nanoparticles, and large
molecules. However, the intrinsic inertness of such pristine microresonators
limits their spread in new fields such as gas detection. Here, a functionalized
microlaser sensor is realized by depositing graphene in an erbium-doped
over-modal microsphere. By using a 980 nm pump, multiple laser lines excited in
different mode families of the microresonator are co-generated in a single
device. The interference between these splitting mode lasers produce beat notes
in the electrical domain (0.2-1.1 MHz) with sub-kHz accuracy, thanks to the
graphene-induced intracavity backward scattering. This allows for multispecies
gas identification from a mixture, and ultrasensitive gas detection down to
individual molecule.
