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Summary form only given. We describe a coherent mid-infrared continuum source with 700 cm<sup>-1</sup> usable bandwidth, readily tuned within 600-2500 cm<sup>-1</sup> (18-75 THz) and thus covering much of the infrared "fingerprint" molecular vibration region [1]. It is based on nonlinear frequency conversion in GaSe using a compact commercial 100-fs-pulsed Er fiber laser system providing two amplified near-infrared beams, one of them broadened by a nonlinear optical fiber. The resulting collimated mid-infrared continuum beam of 1 mW quasi-cw power represents a coherent infrared frequency comb with zero carrier-envelope phase, containing about 500,000 modes that are exact multiples of the pulse repetition rate of 40 MHz. The beam's diffraction-limited performance enables long-distance spectroscopic probing as well as maximal focusability for classical and ultraresolving near-field microscopies. Applications are foreseen also in studies of transient chemical phenomena even at ultrafast pump-probe scale, and in high-resolution gas spectroscopy for e.g. breath analysis.One application area is spectroscopic near-field microscopy where near-field spectra were obtained in exploring polymer fingerprints at 20 nm spatial resolution [2-7]. For the first time, a multitude of resonances between 750 and 1730 cm-1 could be determined with the help of this novel coherent continuum source (Fig. 1).A final positive aspect of mid-infrared dual-comb spectroscopy [5,6] is the ability to record spectra in rapid sequence, thus allowing to follow chemical reactions in real time. Even ultrafast phenomena could be probed with our coherent mid-infrared source. To understand this recall it emits repetitive pulses of sub 100 fs duration which can be systematically delayed o r advanced in respect to a "pump" pulse which could induce specific excitations of a sample.

Mid-infrared frequency comb spanning an octave based on an Er fiber laser and difference-frequency generation