Topology Optimal Design of THz Standard NRD and SWG-NRD Guides Connecting Coupler

Sub-wavelength grating NRD guide (SWG-NRD guide) with single-mode transmission of LSM01 mode is drawing significant attention for THz-wave integrated circuit applications. In order to suppress the reflection loss between standard NRD and SWG-NRD, matching circuits are essential and previously taper couplers have been introduced though a considerable length of taper waveguide is needed. Therefore, it is necessary to design these couplers optimally with a reduced coupling length. In this paper, we present topology optimal design of standard NRD and SWG-NRD guides connecting coupler utilizing function expansion method and covariance matrix adaptation evolution strategy (CMA-ES). The reflection loss is greatly suppressed by the optimized device with a reduced coupling length, and 100 % of single-mode transmission is achieved.