Optimized design for 2 × 106 ultra-high Q silicon photonic crystal cavities

We propose an optimized design for the measurement in transmission of photonic crystal width-modulated line-defect cavities. By controlling the number of holes and rows that separate the cavity from the coupling waveguides, the measured quality factor of the cavity can be tuned to be close to the unloaded one. In the case of a weakly coupled cavity, we measure an ultra-high quality factor that reaches a value of 2 × 106. This value is not obtained for the largest spacing but for an intermediate one. This counter-intuitive result is supported by 3D-FDTD modeling.