Strong Field Enhancement in a Scanning Nanogap for Infrared Imaging of Single Nanoparticles (lambda/1000)

Far-field infrared analysis of individual nanoparticles has not been possible so far due to the extremely weak scattering cross section of nanosize objects at infrared wavelengths, which is 5 orders of magnitude smaller than at visible wavelengths. Scattering type near-field optical microscopy (s-SNOM) offers nanoscale spatial resolution at IR wavelengths and background- free imaging [1,2]. We investigate theoretically and experimentally the use of strong optical field enhancement in the nanogap formed between the s-SNOM tip and the substrate supporting the particles. We show the key role of the substrate response to obtain strong field enhancement at the scanning gap, and therefore spatially resolve the nanoparticles. We provide clear experimental evidence that for highly reflective substrates, single particles as small as 8 nm can be detected with a mid-IR s-SNOM operating at 10 micrometer wavelength. [1] F Keilmann and R. Hillenbrand, Phil. Trans. Roy. Soc. A 362, 787 (2004). [2] A. Cvitkovic et al., Phys. Rev. Lett. 97, 060801 (2006).