Design and optimization of tapered structure of near‐field fibre probe based on finite‐difference time‐domain simulation
The finite‐difference time‐domain method was employed to simulate light propagation in tapered near‐field fibre probes with small metal aperture. By conducting large‐volume simulations, including tapered metal‐cladding waveguide and connected optical fibre waveguide, we illustrated the coupling between these guiding modes as well as the electric field distribution in the vicinity of the aperture. The high collection efficiency of a double‐tapered probe was reproduced and was ascribed to the shortening of the cut‐off region and the efficient coupling to the guiding mode of the optical fibre. The dependence of the efficiency on the tapered structure parameters was also examined.
Analysis of image formation in a near-field scanning optical microscope: effects of multiple scattering
We analyze how the interaction between the probe and the field near the sample surface affects the characteristics of the images produced by a near-field scanning optical microscope. The numerical calculation is carried out using the finite-difference time-domain method, and a three-dimensional model that includes a probe of arbitrary shape is considered. Assuming an aperture-type probe (collection-mode near-field microscope), we compare the images produced from the same surface under both the constant-height-scanning and constant-distance-scanning modes. Our results indicate that the probe-sample interaction is critical in defining the characteristics of the generated images. We also investigated image characteristics as function of the polarization state of the illuminating light. When the effects of multiple scattering by the probe tip are taken into account, the calculated intensity image approximates the geometric profile of the sample surface. The dependences of the detected image intensity with the scanning mode utilized, the polarization state of the illuminating light and the sample size are also investigated.
A new method of near field analysis
The problem of the accurate determination of the total electromagnetic field produced by a current distribution on a circular disc type aperture is examined. For comparison of the near and Fresnel region solutions, an exact Maxwellian integral is used as the starting point. Aperture and space coordinates are separated by the use of Bessel function addition theorems, allowing the field to be expressed as a sum of terms containing radial and angular integrals. The radial integrals involve only the disc (aperture) distribution and coordinates while the angular integrals involve only the elevation angle. The distance from origin appears implicitly in the series coefficients. These integrals are suitable for high speed computations and have been evaluated for a number of cases. The results are compared with the small angle and general Fresnel formula values for various observation angles, distances from the aperture, and aperture distributions.