Molecules have rich signatures in their spectra at infrared wavelengths and are typically accessed with dedicated spectroscopic instrumentation. Researchers have discovered optomechanical frequency upconversion from the mid-infrared to the visible domain using molecular vibrations coupled to a plasmonic nanocavity at ambient conditions.
Using different nanoantenna designs, one with a nanoparticle-on-resonator and the other with nanoparticle-in-groove, both approaches show the ability to upconvert the mid-infrared vibrations of the molecules in the nanocavity to visible light wavelengths. The effect could be used to simplify infrared spectroscopy, possibly with single-molecule sensitivity.
Coherent frequency upconversion of terahertz and mid-infrared signals into visible light opens new horizons for spectroscopy, imaging, and sensing but represents a challenge for conventional nonlinear optics. Here, the researchers used a plasmonic nanocavity hosting a few hundred molecules to demonstrate optomechanical transduction of submicrowatt continuous-wave signals from the mid-infrared (32 terahertz) onto the visible domain at ambient conditions.