High resolution SNOM probes

Abstract

Development of nanotechnologies demands optical characterization and measurement techniques that yield information with resolutions well below the diffraction limit. This requires an increase of the resolution of scanning near-field optical microscopes (SNOMs) from 50-70 nm commercially available nowadays in the visible range, to beneficial 30 nm, where λ is the wavelength of light in free space. High resolution SNOM probes would be crucial in measurements of point spread functions of superlenses based on negative refraction and characterization of plasmonic circuitry. The resolution of SNOMs is ▵r = d + 2a, where d is the diameter of a radiating aperture of a tapered-fiber metal-coated probe and a is a skin depth, that is the distance the electromagnetic field penetrates the metal coating. The size of the radiated field does not exceed the diameter ▵r when the aperture-sample distance h is kept constant by the shear-force tuning fork method. One of the resolution parameters, the skin depth a, depends on the metal that coats the dielectric probe and the shape of the metal rim. For Ag and Al, the values of a are on the level of 10nm, when measured on a flat metal surface illuminated with a plane wave. Thus, the other resolution parameter which we intend to decrease is a probe diameter d. The probe should radiate enough energy to be detected in a reasonable scanning measurement time. Recently, we proved that probe emission depends on the charge density induced on the probe rim. To increase this density we propose enhancement of the photon-plasmon coupling on the interface between the dielectric core and the metal coating. To this end we corrugate the interface. In this paper we analyze the role of parameters of the corrugations and report on attempts to fabricate them.