Superfocusing on a dielectric-metal-dielectric apertureless scanning near-field optical microscope probe

Scanning near-field optical microscopy (SNOM) is an important tool for measurements of nanostructures as well as chemical and biosensing applications. A key parameter defining its usefulness is its resolution which depends of the geometrical size of SNOM probes and signal throughput in aperture tips and signal enhancement in apertureless ones. We analyse nanofocusing properties of an apertureless probe in the form of a dielectric tapered fiber with metal apertureless coating and a novel dielectric nanocladding. The new outer nanocladding gives an ability to tune resonant enhancement wavelengths within a wide spectral range by choice of cladding permittivity. The investigation is performed using body-of-revolution finite-difference time-domain simulations. A silica core of diameter decreasing from 2 mum to 5 nm at the apex is covered with a 40 nm thick Ag layer and has a 5 nm dielectric cladding. Internal illumination with a radially polarized Laguerre-Gauss beam guided in fiber is used. We find that with an increase of the refractive index of nanocladdings the maximum enhancement occurs for increasingly longer wavelengths.