  Room temperature detection of molecular vibrations in the mid-infrared (MIR,
$\lambda$ =3-30$\mu$m) has numerous applications including real-time gas
sensing, chemical reactivity, medical imaging, astronomical surveys, and
quantum communication [1,2]. However, MIR detection is severely hindered by
thermal noise, hence current technologies rely on energy-intensive cooled
semiconductor detectors (mercury cadmium telluride, MCT) [3,4,5]. One way to
overcome this challenge is to upconvert the low-energy MIR light into
high-energy visible wavelengths ($\lambda$ =500-800nm) where detection of
single photons is easily achieved using silicon technologies [6,7]. This
process suffers from weak cross sections and the mismatch between MIR and
visible wavelengths, limiting its efficiency. Here, we exploit molecular
emitters possessing both MIR and visible transitions from molecular vibrations
and electronic states, coupled through Frank-Condon factors. By assembling
molecules into a nanoscale cavity and continuously optically pumping them below
the electronic absorption band, we show the transduction of MIR light absorbed
by the molecular vibrations. The upconverted signal is observed as enhanced
high-energy luminescence. Combining Purcell-enhanced visible luminescence with
enhanced rates of vibrational pumping gives transduction efficiencies exceeding
10%. By down-scaling the cavity volume below $1nm^3$, we show MIR detection of
single-molecular bonds, inaccessible to any previous detector.
