Chemical imaging using infrared photothermal microspectroscopy

There is a growing need for new characterization techniques that can provide information about the chemical composition of surfaces and bulk materials with spatial resolution in the range of 1-10 microns. While FTIR microspectroscopy addresses this problem, the practical resolution limit is still only about 20 microns. Other well-established techniques at the nanometer are impractical at the micro-scale. Raman micro-spectroscopy provides adequate spatial resolution (~1 micron), but may not always be useful due to its low throughput and for samples with strong fluorescence. We are developing a non-contact and non-destructive technique that provides similar information as IR or Raman spectroscopy. It involves photo-thermal heating of the sample with a tunable quantum cascade laser (or other suitable infrared laser) and measuring the resulting increase in thermal emission by either an infrared detector or a laser probe in the visible spectral range. The latter case allows for further increase of the spatial resolution from ~10 microns to ~1 micron, at the right experimental conditions. Since the thermal emission signal from the surface is directly proportional to the absorption coefficient, by tuning the laser wavelength we directly measure the IR spectrum of the sample. By raster-scanning over the surface of the sample we can obtain chemical composition maps. We demonstrate this technique by imaging the surface of several different materials. We analyze the spatial resolution of our photo-thermal imaging system as well as discuss the conditions under which the spatial resolution can be further increased from the infrared far-field diffraction limit.