Spectroscopic near-field microscopy using frequency combs in the mid-infrared

\newcommand{\wn}{cm\textsuperscript{-1}} We introduce a new concept of spectroscopic scattering-type near- field optical microscopy that records 200 \wn \ broad infrared spectra at each pixel during scanning. Two coherent beams with harmonic frequency-comb spectra are employed, one for illuminating the scanning tip, the other as reference for multi- heterodyne detection of the scattered light. Our implementation yields amplitude and phase spectra centered at 950 \wn (this band can be tuned between 700 and 1400 \wn). A new technique of background suppression is introduced which is enabled by the short, 10 $\mu$s ``snapshot'' acquisition of infrared spectra which allows time-resolving the tapping motion. Thus we demonstrate broad-band mid-infrared near-field imaging that is essentially free of background artefacts.\\ (1) A. Schliesser, M. Brehm, F. Keilmann \& D. W. van der Weide \emph{Frequency-comb infrared spectrometer for rapid, remote chemical sensing} Optics Express, 13, 9029-9038 (2005)\\ (2) M. Brehm, A. Schliesser \& F. Keilmann \emph{Spectroscopic near-field microscopy using frequency combs in the mid-infrared} Optics Express, 14 ,11222-11233 (2006)