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° and nanoaperture 100 nm detects preferably the transverse component of the electric field. The focal length was equal to 0,36λ, the smallest focal spot diameter was (0,40±0,02)λ, the depth of focus was 0,59λ and the diffractive efficiency was 12%.