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

We introduce a new concept of spectroscopic near-field optical microscopy that records 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 200 cm(-1) wide amplitude and phase spectra centered at 950 cm(-1) (this band can be tuned between 700 and 1400 cm(-1)). We introduce a new technique of background suppression enabled by the short, 10 mus "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.