We analyzed regular polygonal ring resonators based on multi-mode waveguide using finite-difference time-domain simulation. It consists of the regular polygonal ring waveguide, total internal reflection mirror, and MMI coupler. In general, multi-mode waveguide-based resonator is difficult to use as sensors because of poor output characteristics. By using the low reflectance of the higher-order mode compared to the fundamental mode in the TIR mirror, we designed a regular polygonal ring resonator that can be used as sensors even when a multi-mode waveguide is used instead of a single-mode waveguide. In fabrication, the multi-mode waveguide has a wider line width than the single-mode waveguide, which reduces the process cost and enables mass production. The width and height of the multi-mode waveguide are designed to be 2.5 μm and 2 μm, respectively using SU-8 polymer. The regular hexagon ring resonator shows the highest Q-factor of 1.03×10<sup>4</sup> among the various regular polygonal ring resonators.
In this paper, we designed hexagonal ring resonator using localized surface plasmon resonance (LSPR) phenomenon to enhance the sensitivity which is a significant factor in bio-chemical sensors. We used a hexagonal ring resonator structure to eliminate the bending loss which is one of the prime factors affects sensitivity. The sensing area of the hexagonal ring resonator with LSPR is deposited metal nanoparticle on cladding which makes difference with general sensing region of the hexagonal ring resonator. In this sensing region, the wavelength of light should be longer than the size of the nanoparticle because the metal nanoparticle reacts the light in specific condition. The sensitivity of the resonator can be improved with using this phenomenon. We used finite difference time domain (FDTD) methods for theoretical analysis. Also, we optimized the structure to reduce LSPR loss and enhance the sensitivity by adjusting type, size, thickness of the metal nanoparticle. As a simulation result, we verified that sensitivity of hexagonal ring resonator with LSPR can be 2.5 times higher than without LSPR.