Finite-difference time-domain analysis of refractive index grating on planar light waveguide circuit with optically trapped gold particles

The super-resolution capability of scanning near-field optical microscopy (SNOM) with a gold particle is studied by the two-dimensional finite-difference time-domain (2D FDTD) method. We obtain SNOM signals by integrating the far field within the numerical aperture of an objective lens for a refractive index grating by scanning optically trapped gold particles with different diameters illuminated by focused laser light at the wavelength of 515 nm. The signal is strong at a high refractive index of the grating and exhibits similar behavior to that obtained in the experiment with the grating fabricated on a planar light waveguide circuit with a period of 1060 nm. Furthermore, the signal modulation increases as the gold particle diameter decreases and reaches 0.82 at a diameter of 50 nm.