A. Cvitkovic, N. Ocelic, J. Aizpurua, R. Guckenberger, and R. Hillenbrand
Physical Review Letters 97, p.60801 (2006)
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 (<λ/1000) at midinfrared wavelengths of about λ=10 μm. Our results open the door to infrared spectroscopy of individual nanoparticles, nanocrystals, or macromolecules.