Mid-infrared Fourier-transform spectroscopy with a high-brilliance tunable laser source: investigating sample areas down to 5 µm diameter.

ABSTRACT

We demonstrate highly sensitive infrared spectroscopy of sample volumes close to the diffraction limit by coupling a femtosecond fiber-feedback optical parametric oscillator (OPO) to a conventional Fourier-transform infrared (FTIR) spectrometer. The high brilliance and long-term stable infrared radiation with 1e(2)-bandwidths up to 125 nm is easily tunable between 1.4 µm and 4.2 µm at 43 MHz repetition rate and thus enables rapid and low-noise infrared spectroscopy. We demonstrate this by measuring typical molecular vibrations in the range of 3 µm. Combined with surface-enhanced infrared spectroscopy, where the confined electromagnetic near-fields of resonantly excited metal nanoparticles are employed to enhance molecular vibrations, we realize the spectroscopic detection of a molecular monolayer of octadecanethiol. In comparison to conventional light sources and synchrotron radiation, our compact table-top OPO system features a significantly improved performance making it highly suitable for rapid analysis of minute amounts of molecular species in life science and medicine laboratories.