  Field-enhanced infrared molecular spectroscopy has been widely applied in
chemical analysis, environment monitoring, and food and drug safety. The
sensitivity of molecular spectroscopy critically depends on the electromagnetic
field confinement and enhancement in the sensing elements. Here we propose a
concept for sensing, consisting of a graphene plasmonic nanoresonator separated
from a metallic film by a nanometric spacer. Such a resonator can support
acoustic graphene plasmons, AGPs; that provide ultra-confined electromagnetic
fields and strong field enhancement. Compared to conventional plasmons in
graphene, AGPs exhibit a much higher spontaneous emission rate, higher
sensitivity to the dielectric permittivity inside the AGP nano resonator, and
remarkable capability to enhance molecular vibrational fingerprints, of
nanoscale analyte samples. Our work opens novel avenues for sensing of
ultra-small volume of molecules, as well as for studying enhanced light-matter
interaction, e.g. strong coupling applications.
