Infrared imaging of single nanoparticles via strong field enhancement in a scanning nanogap.

We demonstrate nanoscale resolved infrared imaging of single nanoparticles employing near-field coupling in the nanoscopic gap between the metal tip of a scattering-type near-field optical microscope and the substrate supporting the particles. Experimental and theoretical evidence is provided that highly reflecting or polariton-resonant substrates strongly enhance the near-field optical particle contrast. Using Si substrates we succeeded in detecting Au particles as small as 8 nm (<lambda/1000) at midinfrared wavelengths of about lambda=10 microm. Our results open the door to infrared spectroscopy of individual nanoparticles, nanocrystals, or macromolecules.