In this paper, we propose a novel sensor structure based on the rectangular ring resonator with the photonic crystal
microcavity (PCM), and optimize the structure using finite-difference time-domain (FDTD) method. This sensor
consists of the rectangular resonator with total internal reflection mirror and the PCM, which can be placed at the nearby
optical waveguide of the rectangular ring resonator. The PCM is composed of a defect cavity with different holes on
the center of it. The Q-factor of the PCM can be significantly enhanced when the PCM has the resonance wavelength.
The PCM can be evanescently coupled to a side waveguide arm of the rectangular ring resonator. The sensitivity of the
ring resonator in the presence of gas or biomolecules composition was calculated using the FDTD method. When the
injected gas or biomolecules pass through the PCM, the variation of effective index due to their concentration affects the
resonance condition of the rectangular ring resonator. We have investigated how the shift of the resonance peak in the
resonance wavelengths depends on the gas or biomolecules concentration. We also have optimized the sensor structure
for the waveguide width and length, the hole radius, and the number of hole on the PCM. The optimum lattice
constants, hole radius, and cavity length are 370, 100, and 580 nm, respectively. The rectangular ring resonator sensor
with microcavity significantly enhances the quality factor and the sensitivity compared to the directional coupler sensor
with PCM. The change of normalized output power in rectangular ring resonator with PCM is approximately twice
larger than the change in directional coupler with PCM.