Measurement of multimode resonances in hexagonal micro-pillar optical cavities

Optical micro-pillar (*m-pillar) resonators have attracted recent interests for potential applications in integrated photonics due to their compact size (of 10-100 *mm lateral dimensions and of #~*mm height) and high-Q resonances. Ring and disk *m-pillar wavelength-division multiplexing (WDM) channel add/drop filters have been demonstrated. Very large-scale integrated (VLSI) photonic chips using *m-pillar cavities are feasible. High-Q optical resonances can be confined by nearly total internal reflection (TIR) at the *m-pillar resonator sidewall. The *m-pillar cavity can be evanescently side-coupled or vertically coupled to input and output waveguides. The main drawback of the conventional side-coupled circular ring and disk cavities is the short interaction length between the curved cavity sidewall and the straight waveguide sidewall. Such short interaction length imposes a sub-micrometer gap distance for evanescent coupling. Polygonal *m-pillar resonators provide an alternative means of increasing the evanescent side-coupled length for potential WDM add/drop filter applications. The advantages of the polygonal cavities are twofold: The entire flat polygonal sidewall allows a longer interaction length, and therefore a wider gap distance for evanescent coupling between the cavity and the straight waveguides; and the optical path length is identical for rays having the same incident angle along the sidewall, and hence the same cavity mode can be coupled anywhere along the flat sidewall. Fabrication of polygonal *m-pillar cavities can be readily achieved with established microelectronic fabrication processes. Recently, multimode resonances in square-shaped *m-pillar cavities were demonstrated, and hexagonal microlasers have been reported. In this paper, we report our recent measurement of multimode resonances in the elastic-scattering spectrum of hexagonal *m-pillar optical cavities, using Gaussian beam coupled along the cavity sidewall. The observed free spectral range (FSR) is consistent with the six-bounce closed-loop path length. By using the wavefront-matching concept, the observed multimode resonances can be attributed to round-trip trajectories that need not be closed after each round trip.