Near-field scanning optical microscopy studies of photonic structures and materials

Near-field scanning optical microscopy (NSOM) was used to study a patterned membrane of SiN, a material commonly used in the construction of micro-eletromechanical and optical devices as well as several photonic structures. NSOM is a technique that provides information about optical properties of materials with sub-diffraction limit resolution. Using a transmission mode polarization modulation NSOM, the stress in an unsupported SiN membrane was studied by examining localized birefringent and dichroic variations in the sample. A technique was developed to remove contributions to the measured retardance signal that are caused by dichroism in the sample. The corrected images can then be used to obtain quantitative information about the anisotropic stress variations associated with patterning the membrane. Several photonic structures are studied using an NSOM in collection mode, where the evanescent tail of light confined to a waveguiding material or structure is directly probed by the NSOM, allowing direct measurement of the distribution of guided light inside the material. The intensity distribution inside a two-dimensional photonic crystal structure is mapped and used to determine the dispersion relations of the crystal. The capability of the collection mode NSOM was expanded to measure both optical intensity and phase by incorporating the NSOM into a Mach-Zender interferometer. This was used to study the propagation of light in waveguides patterned on LiNbO₃ and periodically poled LiNbO ₃ (PPLN) to aid in characterization of the waveguides and determine sources of loss. Finally multimode interference imaging phenomena were studied in a 2 x 2 multimode interference coupler, and a wide PPLN waveguide. The results were compared to simulations based on physical properties of the devices.