| A dual-wavelength Scanning Near-Field Optical Microscope was developed in order to investigate near-field contrast mechanisms as well as biological samples in air. Using a helium-cadmium laser, light of wavelengths 442 and 325 nanometers is coupled into a single mode optical fiber. The end of the probe is tapered to a sub-wavelength aperture, typically 50 nanometers, and positioned in the near-field of the sample. Light from the aperture is transmitted through the sample and detected in a confocal arrangement by two photomultiplier tubes. The microscope has a lateral topographic resolution of 10 nanometers, a vertical resolution of 0.1 nanometer and an optical resolution of 30 nanometers. Two alternate methods of producing the fiber probes, heating and pulling, or acid etching, are compared and the metal coating layer defining the aperture is discussed. So-called “shear-force” interactions between the tip and sample are used as the feedback mechanism during raster scanning of the sample. An optical and topographic sample standard was developed to calibrate the microscope and extract the ultimate resolution of the instrument. The novel use of two wavelengths enables the authentication of true near-field images, as predicted by various models, as well as the identification of scanning artifacts and the deconvolution of often highly complicated relationships between the topographical and optical images. Most importantly, the use of two wavelengths provides information on the chemical composition of the sample. Areas of a polystyrene film are detected by a significant change in the relative transmission of the two wavelengths with a resolution of 30 nanometers. As a biological application, a preliminary investigation of the composition of Black Spruce wood cell fibers was performed. Comparisons of the two optical channels reveal the expected lignin distributions in the cell wall. |
