Photothermal absorption microprobe: Infrared spectroscopy of a single 1 μm organic particle

This letter describes a technique, based on photothermal absorption, for performing infrared spectroscopy with high sensitivity to a single micrometer-sized particle on a surface. The heat absorbed as a result of irradiation with a tunable infrared laser is monitored by observing the induced thermal expansion with a phase-sensitive microscope (an interferometer with a beam focused to a few micrometer spot diameter). The technique has a high specificity to a particle over background from substrate materials because of the poor heat conductance and small thermal mass of the particle. Spectra have been obtained from particles as small as 1 μm in diameter (which corresponds to 0.5 pg of material) with the use of only a few milliwatts of power from a tunable CO2 laser.