Subwavelength infrared spectromicroscopy using an AFM as a local absorption sensor

We describe a new method of infrared micro-spectroscopy. It aims at performing “chemical mapping” of various objects with sub-wavelength lateral resolution, by using the infrared oscillatory signature characterizing different molecular species. Here, we use an atomic force microscope tip, probing the local transient deformation induced by an infrared-pulsed laser tuned at sample absorbing wavelength of the sample. The tip oscillates at resonant frequencies, which amplitude can be correlated with local absorption. We show that this new tool opens the way of measuring and identifying spectroscopic contrasts not accessible by far-field or near-field optical methods and with a sub-wavelength lateral resolution that is not limited by the heat flow through the sample. We exemplify the accuracy of the method by mapping Escherichia coli bacteria at different wavelengths.