Near-field IR imaging Diffraction usually limits spatial resolution to half the wavelength of the incident light. Therefore, spatial resolution with IR illumination tends to be around 5 um. However, using an apertureless approach to scanning near-field optical microscopy, B. Knoll and F. Keilmann of the Max-Planck-Institut fur Biochemie (Germany) are able to generate contrast on a scale of 100 nm with IR light. In the apertureless approach, the usual aperture is replaced with a particle or a tip that scatters light. Although the entire tip structure produces background scattering, the scattering from the tip apex depends upon the presence of a nearby sample. Modulating the tip-tosample separation allows the tip's nearfield response to be filtered from the background signal. Knoll and Keilman use commercial sili-
con cantilevered tips, which they coat with a 100-nm-thick layer of gold. The cantilever is dithered perpendicular to the sample surface at 40 kHz. The IR beam from a tun-
able C0 2 laser is focused onto the tip, and forward-scattered radiation is collected by a HgCdTe detector. The method was used to image a sample of polystyrene (PS) and poly (methyl methacrylate) (PMMA), which are immiscible. The sample was prepared by drying drops of the polymers on cleaved NaCl, which was dissolved in water to form a nearly flat polymer surface with sharp boundaries. The IR wavelength affects the image—as the wavelength is tuned to a vibrational resonance of one of the materials, the absorbing material appears darker in the image. The contrast is 5% at 9.68 um and 4% at 10.17 um. The authors attribute the strong contrast enhancement in the IR images (rightt and simultaneously recorded near field of the probe tip to surfaceAFM topography (left) )o PS embedded in enhanced IR absorption They suggest that PMMA. The upper right tmage ii recorded at this technique will allow IR imaging with 9.68 pm, ,nd the lower right image is recorded
