Hexagonal micro-pillar cavities: multimode resonances and open-loop resonance linewidth broadening Ning Ma, Frankie Kin Lam Tung, Shing Fai Lui, Andrew W. Poon Dept. of Electrical and Electronic Engineering,Hong Kong University of Science and Technology Clear Water Bay, Hong Kong, China ABSTRACT We report our proof-of-principle experiment and modeling of hexagonal micro-pillar (µ-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. Keywords: micro-pillar cavities, optical resonances, hexagonal, linewidth broadening, WDM add/drop filters 1. INTRODUCTION Optical µ-pillar cavities have attracted recent interest for applications in integrated photonics due to their compact size (10 - 100 µm lateral dimensions and ≈ µm height) and high-Q resonances. Light can be partially confined by nearly total internal reflection (TIR) at the µ-pillar resonator sidewall. Optical resonances can be excited only when the cavity round-trip wavefronts are wavefront-matched. The µ-pillar cavity can be laterally or vertically coupled with waveguides, and circular µ-pillar wavelength-division multiplexing (WDM) channel add/drop filters have been demonstrated [1, 2, 3]. However, the main drawback of the laterally coupled circular ring and disk micro-cavities is the short interaction length between the curved cavity sidewall and the straight waveguide sidewall. Such short interaction length imposes a sub-micrometer air-gap spacing for evanescent coupling. In order to improve the lateral coupling length, channel add/drop filters based on race-track shaped resonators [4] and polygonal µ-pillar resonators have been recently proposed [5]. The key advantages of the polygonal µ-pillar cavities are two-fold: (1) the entire flat cavity sidewall allows a longer lateral coupling length, and (2) the same cavity modes can be input and output-coupled along the sidewall. Therefore, a wide air-gap spacing can be tolerated for evanescent coupling between the cavity and the laterally coupled straight waveguides. Recently, multimode resonances in square-shaped µ-pillar cavities were experimentally observed by Gaussian beam coupling [6]. Hexagonal microlasers have been reported [7].