  Frequency upconversion is a cornerstone of electromagnetic signal processing,
analysis and detection. It is used to transfer energy and information from one
frequency domain to another where transmission, modulation or detection is
technically easier or more efficient. Optomechanical transduction is emerging
as a flexible approach to coherent frequency upconversion; it has been
successfully demonstrated for conversion from radio- and microwaves (kHz to
GHz) to optical fields. Nevertheless, optomechanical transduction of multi-THz
and mid-infrared signals remains an open challenge. Here, we utilize molecular
cavity optomechanics to demonstrate upconversion of sub-microwatt
continuous-wave signals at $\sim$32~THz into the visible domain at ambient
conditions. The device consists in a plasmonic nanocavity hosting a small
number of molecules. The incoming field resonantly drives a collective
molecular vibration, which imprints an optomechanical modulation on a visible
pump laser and results in Stokes and anti-Stokes upconverted Raman sidebands
with sub-natural linewidth, indicating a coherent process. The nanocavity
offers 13 orders of magnitude enhancement of upconversion efficiency per
molecule compared to free space, with a measured phonon-to-photon internal
conversion efficiency larger than $10^{-4}$ per milliwatt of pump power. Our
results establish a flexible paradigm for optomechanical frequency conversion
using molecular oscillators coupled to plasmonic nanocavities, whose
vibrational and electromagnetic properties can be tailored at will using
chemical engineering and nanofabrication.
