A highly localized light source shows promise for use in scanning near-field optical microscopy. Developed by scientists at the University of Melbourne in Australia and at Virginia Polytechnic Institute in Blacksburg, it combines aspects of two types of conventional probes.William A. Ducker, a federation fellow at the University of Melbourne, and his colleague Valentin Lulevich of Virginia Tech discovered that commercially available silicon nitride atomic force microscope (AFM) probes can photoluminesce. When they illuminate the probe tip with 532-nm light from a thermoelectrically cooled diode laser from World Star Tech of Toronto, the tip absorbs some of the incident light and emits at about 650 nm.The mechanism of this photoluminescence is not yet understood, although the researchers suspect that small regions of crystalline silicon within the silicon nitride chip may be interacting with the absorbed light.To characterize the performance of the photoemission as a scanning near-field optical microscopy source, they used an Asylum Research microscope to position an AFM probe over a Zeiss Axiovert optical microscope. Although the photoemission occurred even without a sample present, it was strongly enhanced by proximity to a sample: The strength of the photoemission grew exponentially as the probe was brought nearer to the sample.Because the light is so localized, the lateral resolution of the optical transmission image can be as good as 20 to 30 nm. Another feature of the system is that it can simultaneously generate optical transmission microscopy and AFM images.Ducker noted that the technique combines aspects of both aperture and apertureless scanning near-field optical microscopy, with the emission strongly dependent on distance from the sample but the absorption proportional to the optical density of the sample. An advantage of the scientists’ new approach, he said, is that the signal can be interpreted in the same way as in conventional light microscopy.Although initial images can be analyzed in a straightforward fashion, the researchers noted that there is a near-field probe interaction that is not yet fully understood. Still, the technique offers the possibility of varying the emission wavelength, of performing spectroscopy on adsorbed material and of quantifying the amount of adsorption based on optical absorption. Applied Physics Letters, Nov. 21, 2005, 214107.