Power spectral analysis for evaluating optical near-field images of 20 nm gold particles

We have demonstrated that the power spectral analysis could be employed as a potential tool for evaluation of near-field optical images of colloidal gold particles of size 20 nm fixed on a glass substrate. It has been found that the power spectral density of the near-field image resembles the Airy pattern due to a circular dot with the first minimum occurring at the spatial frequency 5.6 × 10⁷ m^-1. This corresponds to an estimated particle diameter of 22 nm. From a comparison with the theoretical power spectral density of randomly distributed circular dots, the transfer function of the near-field optical microscope has been estimated. It has been found that the resolution estimated using -3 dB cutoff value obtained from the transfer function was 0.8 nm which is much higher than that determined by aperture size. The high resolving ability of the present microscopic system is believed to be due to the confinement of very high spatial frequency evanescent fields around the edges of the metal-glass fiber boundary at the apex of the apertured probe.