Optically-driven collapse of a plasmonic nanogap self-monitored by optical frequency mixing

A nanoparticle separated from a metallic surface by a few-nanometer thick polymer layer forms a nanoscale junction, or nanogap. Illuminating this structure with ultrashort optical pulses, exciting the plasmon resonance, results in a continuous, monitorable collapse of the nanogap. The four-wave mixing signal generated by this illumination of the nanogap provides a simultaneous monitoring of the collapse, increasing dramatically upon gap closure. Collapse is irreversible, occurring with simultaneous ablation of the dielectric from the junction.