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.
Highlights
Infrared (IR) spectroscopy allows for unambiguous identification and chemical characterization of molecular species based on their infrared vibrational bands
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
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
Summary
Infrared (IR) spectroscopy allows for unambiguous identification and chemical characterization of molecular species based on their infrared vibrational bands. Excited in the infrared, such nanoantennas confine infrared radiation on the nanometer scale and provide locally high electromagnetic field intensities Following this approach of surface-enhanced infrared absorption (SEIRA), vibrational signals of molecules located in the plasmonic hotspots can be enhanced up to five orders of magnitude in comparison to conventional infrared spectroscopy [6,7,8]. Drifts in the intensity or wavelength will directly distort the baseline making it impossible to normalize spectra with reference measurements, while sub-millisecond fluctuations will perturb the acquisition of a single spectrum and will transform into random noise in the spectrum These disadvantages are quite challenging and have so far hampered the use of coherent sources for FTIR spectroscopy, despite their great advantages in terms of globar microscope aperture. We further give a quantitative measure for the fiber-feedback OPO noise performance on different relevant time scales
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