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Phonon-enhanced light-matter interaction at the nanometre scale.

R. Hillenbrand, T. Taubner, and F. Keilmann

Nature 418, p.159 (2002)
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 microscopy3, 4, 5, 6. The plasmon-enhanced near-field coupling between metallic nanostructures7,8, 9 opens new ways of designing optical properties10, 11, 12 and of controlling light on the nanometre scale13, 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.