The whispering gallery modes (WGMs) of optical resonators have prompted
intensive research efforts due to their usefulness in the field of biological
sensing, and their employment in nonlinear optics. While much information is
available in the literature on numerical modeling of WGMs in microspheres, it
remains a challenging task to be able to predict the emitted spectra of
spherical microresonators. Here, we establish a customizable Finite- Difference
Time-Domain (FDTD)-based approach to investigate the WGM spectrum of
microspheres. The simulations are carried out in the vicinity of a dipole
source rather than a typical plane-wave beam excitation, thus providing an
effective analogue of the fluorescent dye or nanoparticle coatings used in
experiment. The analysis of a single dipole source at different positions on
the surface or inside a microsphere, serves to assess the relative efficiency
of nearby radiating TE and TM modes, characterizing the profile of the
spectrum. By varying the number, positions and alignments of the dipole
sources, different excitation scenarios can be compared to analytic models, and
to experimental results. The energy flux is collected via a nearby disk-shaped
region. The resultant spectral profile shows a dependence on the configuration
of the dipole sources. The power outcoupling can then be optimized for specific
modes and wavelength regions. The development of such a computational tool can
aid the preparation of optical sensors prior to fabrication, by preselecting
desired the optical properties of the resonator.Comment: Approved version for SPIE Photonics West, LASE, Laser Resonators,
Microresonators and Beam Control XV
