LCP/CLIO, ORSAY CEDEX
Performing "chemical mapping" of various objects with sub-wavelength lateral resolution, by using the infrared vibrationnal signature characterizing different molecular species, is an old dream. We have recently demonstrated such a method, called AFMIR [1]. We use the photo-thermal expansion effect, detected by an atomic force microscope tip, probing the local transient deformation induced by an infrared pulsed laser tuned at a sample absorbing wavelength. This method records directly the local infrared absorption without interference due to the medium real index of refraction (i.e. sample topography and inhomogeneities) contrary to near-field optical methods. The pulsed character of the FEL, together with the small size of the AFM tip, allows a spatial resolution better than 100 nm. Local spectroscopy and imaging can be performed. We show different examples. In particular, we have imaged single viruses imbedded into a cell [2] and living cells (in water). In this case, discrimination of the cell signal against water broadband absorption is made by selecting proper modes after Fourier analysis of the AFM cantilever modes of vibration [3].
[1] A. Dazzi et al, Optics Letters, 30 (18), 2388 (2005)
[2] A. Dazzi et al, Ultramicroscopy, 108/7, 635 (2008)
[3] C. Mayet et al, Optics Letters, 33(14), 1611 (2008)
