1. Max-Planck-Institut für Biochemie, Abteilung Molekulare Strukturbiologie, 82152 Martinsried & Center for NanoScience, Ludwig-Maximilians-Universität, 80799 München, Germany

Correspondence to: F. Keilmann Correspondence and requests for materials should be addressed to F.K. (e-mail: Email: keilmann@biochem.mpg.de).

Abstract

Optical near fields exist close to any illuminated object. They account for interesting effects such as enhanced pinhole transmission¹ or enhanced Raman scattering enabling single-molecule spectroscopy². Also, they enable high-resolution (below 10 nm) optical microscopy^{3, 4, 5, 6}. The plasmon-enhanced near-field coupling between metallic nanostructures^{7, 8, 9} opens new ways of designing optical properties^{10, 11, 12} and of controlling light on the nanometre scale^{13, 14}. Here we study the strong enhancement of optical near-field coupling in the infrared by lattice vibrations (phonons) of polar dielectrics. We combine infrared spectroscopy with a near-field microscope that provides a confined field to probe the local interaction with a SiC sample. The phonon resonance occurs at 920 cm^-1. Within 20 cm^-1 of the resonance, the near-field signal increases 200-fold; on resonance, the signal exceeds by 20 times the value obtained with a gold sample. We find that phonon-enhanced near-field coupling is extremely sensitive to chemical and structural composition of polar samples, permitting nanometre-scale analysis of semiconductors and minerals. The excellent physical and chemical stability of SiC in particular may allow the design of nanometre-scale optical circuits for high-temperature and high-power operation.