Near-Field Vibrational Energy Transfer for Mid-Infrared Upconversion in Plasmonic Nanogaps

TL;DR

This paper demonstrates MIR vibrational energy transfer and upconversion to visible light using plasmonic nanogaps, revealing a new pathway for vibrational nanophotonics and bioimaging applications.

Contribution

It introduces a method for MIR vibrational energy transfer and upconversion via sub-2 nm plasmonic nanogaps, enabling efficient energy transfer and emission.

Findings

Achieved upconversion efficiencies exceeding 0.3%.

Demonstrated selective excitation of vibrational donors and energy transfer to electronic acceptors.

Showed plasmonic confinement can redirect vibrational relaxation pathways.

Abstract

F\"{o}rster energy transfer underpins modern photonics, yet establishing an analogous vibrational pathway in the mid-infrared (MIR) remains highly challenging, as sub-picosecond intramolecular vibrational redistribution (IVR) suppresses intermolecular coupling. Here we demonstrate vibrational donor--acceptor transfer in the MIR and subsequent upconversion to visible luminescence enabled by sub-2 nm plasmonic nanogaps. The extreme lateral field confinement in metal--molecule--metal ring cavities defined by self-assembled molecular spacers couples efficiently to in-plane molecular dipoles. Continuous-wave MIR excitation selectively populates $-\mathrm{C}\equiv\mathrm{N}$ vibrational donors, and plasmon-enhanced near-field coupling transfers this energy to nearby electronic acceptors, generating anti-Stokes visible emission under low power densities. Upconversion efficiencies exceeding $0.3\%$ are observed, limited by competition between the plasmon-mediated transfer rate and IVR. These results show that extreme plasmonic confinement can redirect molecular vibrational relaxation pathways, opening a route toward vibrational nanophotonics, intermolecular interactions for bioimaging, and room-temperature MIR detection based on molecular degrees of freedom.