Near-Field Scanning Optical Microscopy Studies of Materials and Devices

Near-field scanning optical microscopy (NSOM) provides a means to study optical and optoelectronic properties of materials at the nanometer scale. The key to achieving resolution higher than the diffraction limit is to place a subwavelength-sized light source—e.g., an aperture—within the near-field zone of the sample. In this case, the area of the sample illuminated is determined by the aperture size and not by the wavelength (see Figure 1). An image can then be formed by moving the sample and light source with respect to each other. While the principle of near-field optics is straightforward, its realization at visible-light wavelengths was not achieved until the invention of scanning-probe techniques in the 1980s. Since Betzig et al. demonstrated in 1991 that bright subwavelength apertures can be made by tapering and metal-coating single-mode optical fibers, research activities involving NSOM have increased tremendously. The later incorporation of shear-force feedback to regulate tip-sample separation adds another strength to NSOM. Using this distance regulation, a topographic image similar to that obtained by a conventional scanning force microscope is acquired simultaneously with the optical image. This provides a way to correlate structural and physical properties at the same sample positions and greatly simplifies interpretation of the NSOM data.