Modeling of Nanophotonic Resonators with the Finite-Difference Frequency-Domain Method

Finite-difference frequency-domain method with perfectly matched layers and free-space squeezing is applied to model open photonic resonators of arbitrary morphology in three dimensions. Treating each spatial dimension independently, nonuniform mesh of continuously varying density can be built easily to better resolve mode features. We explore the convergence of the eigenmode wavelength $lambda $ and quality factor $Q$ of an open dielectric sphere and of a very-high- $Q$ photonic crystal cavity calculated with different mesh density distributions. On a grid having, for example, 10 nodes per lattice constant in the region of high field intensity, we are able to find the eigenwavelength $lambda $ with a half-percent precision and the $Q$-factor with an order-of-magnitude accuracy. We also suggest the $lambda /n$ rule (where $n$ is the cavity refractive index) for the optimal cavity-to-PML distance.