Direct observation of size-dependent features of the optical near field on a subwavelength spherical surface

We investigate the characteristics of the optical near field of a subwavelength dielectric sphere by using a nanometric fiber tip and a sensitive technique of wavelength conversion. The decay length of the near field induced around the single sphere is directly measured to be 0.3 of its radius, obtained by precise positioning of the probe tip. A numerical calculation of the optical near field based on the Mie scattering theory is in good agreement with the experimental results. The enhancement of fluorescence collection efficiency in the near-field region is also observed.