Polarization Mode Coupling due to Metal-Layer Modifications in Apertureless Near-Field Scanning Optical Microscopy Probes

Apertureless near-field scanning optical microscopy (NSOM) probes are known to produce a highly spatially localized emitted optical near field with an axisymmetric, radially polarized input. Using rigorous three-dimensional electromagnetic modeling tools, we investigate the conversion of a linearly polarized input to this mode in such a probe due to rectangular slit defects introduced in the metal coating layer of the probe. Two types of defect configurations are studied: unilateral slits—a rectangular slit carved radially into one side of the probe—and bilateral slits—consisting of a rectangular slit that transects a chord of the probe. The latter are found to be more effective for this mode conversion process. In addition, the role of the slit width, depth, and offset are studied, and an optimal slit configuration is determined. These results indicate an approach for fabricating aperture-less NSOM probes having efficient polarization conversion properties, facilitating the development of improved-performance NSOM systems.