Near-field probing of vibrational absorption for chemical microscopy

Identification of chemical compounds by vibrational spectroscopy at infrare d wavelengths requires macroscopic samples: the spatial resolution is diffr action-limited to a scale of about half the wavelength, or about five micro metres, The scanning near-field optical microscope(1,2), however, can revea l sub-wavelength detail because it uses near-field probing rather than beam focusing. Here we demonstrate the use of the aperture-less approach to sca nning near-field optical microscopy(3-6) to obtain contrast in vibrational absorption on a scale of about 100 nanometres, about one-hundredth of a wav elength. We record infrared scattering from the tip of an atomic force micr oscope scanned over a composite polymer film. At the boundary between diffe rent polymers we observe contrast changes owing to changes in vibrational a bsorption. The contrast is strongly enhanced in the near field of the probe tip, which we interpret as evidence of surface-enhanced infrared absorptio n(7). When extended to multi-wavelength operation, this approach should ena ble imaging of chemical composition at nanometre resolution.