Optical contrast mechanisms and shear force interactions in near-field scanning optical microscopy.

Abstract

This dissertation investigates mechanisms that influence image formation in near-field scanning optical microscopy (NSOM) performed with tapered fiber aperture probes. Both the generation of optical contrast for transmission mode NSOM and the force interaction between the probe and sample that is the basis for topographic imaging by shear force microscopy (SFM) are studied. A brief introduction and review of the field of NSOM are given. The lack of understanding in the previous work of the optical and force interactions between the probe and sample is cited as the motivation for the present investigation. A theoretical model is developed that describes the linear scattering of the probe's source field by the complex transmittance of the sample. The imaging of subwavelength features is shown to arise from the spatial mixing of the evanescent waves of the probe's source field with the high spatial frequencies of the object. Calculations of the optical transfer function are presented. The shear force servo that regulates the probe-to-sample separation and facilitates the acquisition of SFM imagery is extensively analyzed. The optical detection scheme that measures the dither vibration of the probe is characterized in order to optimize the servo performance. The shear force interaction is then analyzed by modeling the probe as a simple harmonic oscillator. Measurements of the probe's resonant response while interacting with the sample reveal that the shear force is mainly frictional. The magnitude of the force is derived, and limitations on its measurement are established through analysis of the minimum detectable displacement of the probe. The servo performance is shown to be shot noise limited, as opposed to being limited by the thermal vibration noise of the probe. Experimental SFM and NSOM images of various grating structures and optical data storage materials are presented. The optical contrast mechanisms displayed in the images are identified. Linear scattering generally dominates the contrast, but some images exhibit unique near-field effects due to probe-sample interactions that lead to nonlinear imaging behavior. The origin of these interactions is the boundary conditions imposed on the probe's aperture by the sample's composition and structure.