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Abstract

Tip-enhanced Raman spectroscopy is a vibrational spectroscopy with hitherto unprecedented sensitivity and spatial resolution. Since the enhancement is mainly provided by the near-field excited at the apex of a suitable tip, TERS appears to be a widely applicable spectroscopy and microscopy tool, in contrast to its parents, surface-enhanced Raman spectroscopy (SERS) and scanning near-field optical microscopy (SNOM). TER scattering has been observed for a number of molecules adsorbed at various substrates, including single-crystalline metal surfaces, showing thereby a more than million-fold enhancement of the Raman scattering. It is important to note that the field enhancement provides, beyond TERS, promising avenues for applications to other optical techniques, such as tip-enhanced CARS, two-photon fluorescence and infrared scattering-type near-field microscopy. Common to all these approaches is the high spatial resolution that is by far better than Abbe’s diffraction limit of λ/2. The lateral resolution achieved today is in the range of 10 nm to 20 nm. Optical microscopy with such an excellent resolution has a very promising future. The keys for further advances in the application of enhanced near-fields to scientific and technological (analytical) tasks include the optimization of tips, excitation and collection optics as well as of imaging techniques. Last but not least, it is necessary to achieve a deeper theoretical understanding of the optical properties of the cavity formed between tip and substrate as well as of the influence of (light-absorbing) adsorbates on the optical resonances of this cavity.