Low-threshold lasing with a stationary inflection point in a three-coupled-waveguide structure

The frozen mode regime is a unique slow-light scenario in periodic structures, where the flat bands (zero group velocity) are associated with the formation of high-order stationary points (also known as exceptional points). The formation of exceptional points is accompanied by enhancement of various optical properties such as gain, Q factor, and absorption, which are key properties for the realization of a wide variety of devices such as switches, modulators, and lasers. Here we present and study an integrated optical periodic structure consisting of three waveguides coupled via microcavities and a directional coupler. We study this design theoretically, demonstrating that a proper choice of parameters yields a third-order stationary inflection point (SIP). We also show that the structure can be designed to exhibit two almost overlapping SIPs at the center of the Brillouin zone. We study the transmission and reflection of light propagating through realistic devices composed of a finite number of unit cells and investigate their spectral properties in the vicinity of the stationary points. Finally, we analyze the lasing frequencies and threshold level of a finite structure (as a function of the number of unit cells) and show that it outperforms conventional lasers utilizing regular band-edge lasing (such as distributed-feedback lasers).