Excitation of Confined Modes in Silicon Slotted Waveguides and Microring Resonators for Sensing Purposes

An important class of biochemical sensors are based on slotted waveguides having an increased sensitivity due to the confinement of the optical field in the slot region. The most sensitive devices of such a type contain microrings, and in these devices, the resonant frequency shift due to presence of the analyte in the microring slot is converted to the sensing signal. However, to the best of our knowledge, in none of the publications dealing with slotted microring sensors, there is no numerical or experimental analysis if the light is actually confined in the ring slot region. Here, we investigate numerically the conditions for the excitation of the modes confined in the slot of the ring. The 3-D finite element method (FEM) is used to calculate the optical power transmission coefficients of all-pass configuration (slotted ring +bus waveguide) in wide spectral range, while the eigenfrequency spectrum of slotted rings and its dependence on the top cladding refractive index is analyzed by means of 2-D axisymmetric FEM approach. It is shown that for the sensors based on slotted microrings sensitivity attains much higher value, up to four times, when the mode is confined in the slot of the ring is excited. This can be important for the design of different biochemical sensors.