3-Dimensional Finite Element Time Domain Analysis of an Asymmetric Near-Field Optical Probe

Considerable effort has been invested into numerical models of scanning near-field optical microscopy during the last years. The finite difference time domain method, using an orthogonal discretization scheme, has often been used for full-wave three-dimensional studies. Because optical near-field configurations are often characterized by curvilinear shapes, locally refined, tetrahedral grids are better suited to describe the geometry. Where fine geometrical details must be resolved or the field solution is expected to vary rapidly, the elements are made smaller while in the other regions a coarser mesh can be used, thereby reducing the size of the problem and promoting computational efficiency. In this study, we use a finite element approach that solves the electric field vector wave (curl–curl) equation in the time domain (FETD) to investigate a novel, scanning near-field optical probe concept with asymmetric cladding. A specific advantage of the finite element method is its inherent capability to discretize the curl–curl equation in a non-uniform way. The finite element method is therefore particularly suited to approximate the geometry of an optical near-field configuration. We model a simplified setup, introduce specific approximations and discuss the method's capabilities and its potential for modeling more complex configurations.