  Resonant coupling between plasmonic nanoantennas and molecular vibrational
excitations is employed to amplify the weak overtone transitions that reside in
the near-infrared. We explore for the first time the differential extinction of
forbidden molecular overtone transitions coupled to the localized surface
plasmons. We show that a non-trivial interplay between the molecular absorption
enhancement and suppression of plasmonic absorption in a coupled system gives
rise to orders of magnitude enhancement of the probe molecule differential
extinction. Our results pave a road toward a new class of surface-enhanced
near-infrared absorption-based sensors.
