Special aspects of subwavelength focal spot measurement using near-field optical microscope

In this paper we numerically and experimentally investigated the influence of the hollow pyramidal shape NSOM cantilever to the measured characteristics of subwavelength focal spot. Using linearly polarized Gaussian beam with wavelength $633$ nm and Fresnel zone plate with focal length $532$ nm it was shown that hollow cantilever with angle $70\circ$ and nanoaperture $100$ nm detects preferably the transverse component of the electric field. The focal length was equal to $0,36\lambda$, the smallest focal spot diameter was $(0,40\pm0,02)\lambda$, the depth of focus was $0,59\lambda$ and the diffractive efficiency was $12\%$.