We report our proof-of-principle experiment and modeling of hexagonal micro-pillar cavities. A commercial hexagonal silica fiber (125μm plane-to-plane) was side-coupled perpendicularly with a Gaussian beam, thus the fiber acted as a μ-pillar cavity. We observed multimode resonances with typical Q ≈ 2,500 in the tangential directions that are ≈ 120° to the input-coupling cavity sidewall. The observed free spectral range (FSR) ≈ 4.5 nm is consistent with a six-bounce cavity round-trip length. By using wavefront-matching concept, the observed multimode resonances can be attributed to open-loop trajectories. The multiple wavefront-matched open-loop trajectories of the same ray incident angle can result in resonance linewidth broadening. We employed a k-space representation to calculate the hexagonal cavity normal mode locations.
Hexagonal micropillar (μ-pillar) cavities have been studied using 2-D finite-difference time-domain (FDTD) method. Singlemode resonances have been observed from a 2-μm sized hexagonal μ-pillar cavity that is selectively input-coupled. The simulated resonance wavelengths are consistent with the wavefront-matching condition based on ray-optics. The resonant field distribution has an integer number of field maxima along the cavity rim similar to whispering-gallery modes. Laterally waveguide-coupled hexagonal μ-pillar cavity channel add/drop filters with parallel and non-parallel waveguides have also been designed and simulated. Preliminary simulation of a 20-μm sized hexagonal μ-pillar cavity laterally coupled with 0.4-μm wide parallel waveguides demonstrated an extinction ratio of ≈ 8 dB, a signal/background ratio of ≈ 11 dB and a finesse of ≈ 5.