Investigation of optical near field using near field scanning optical microscopy
Conventional optical imaging techniques have a fundamental resolution limit due to the diffraction limit of light. The advances of science and technology on the nanoscale demand a new tool for characterization. The Near Field Scanning Optical Microscopy (NSOM) has been developed to tackle certain aspects of this problem. The work presented here applies different types of NSOM to explore the near field distribution of metal plasmonic nanostructures. Ridge apertures with shapes like bowtie and C can be used to focus light into sub-diffraction limit spot with enhancement. This localization of the field near the exit of a bowtie aperture is investigated with a home built aperture NSOM. The experiment results confirm the confinement of focused light spot and the fast decay nature of optical near fields. To further increase the transmission efficiency, a concentric grating structure is added to the bowtie aperture. Near field examination of the transmitted energy with NSOM shows a factor of 15 augment in the transmission. To achieve a better resolution, an alternative method is also investigated. The scattering (apertureless) NSOM is demonstrated to have optical resolution only limited by the radius of its tip apex. The physics behind scattering NSOM is explained and the difficulty of its implementation investigated. Among the different proposed methods, the pseudo-heterodyne interferometric method is chosen and applied in this work. A scattering NSOM has been built based on a commercial AFM. With the home-built s-NSOM setup, the optical responses of several different plasmonic structures have been studied. The examination of a sample with a circular aperture array shows that the interferometric pseudo-heterodyne method is indeed effective in suppressing the background noise. The s-NSOM has also been used to investigate the formation mechanism of interferometric patterns observed during the measurement of a single nanoslit. In the end, a revisit of the 3-D optical field distribution of the bowtie aperture has been conducted, with a focus on the Ez field. For s-NSOM measurements, both the amplitude and phase information are obtained, enabling vector analysis of the optical fields.
Xu, Purdue University.
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