Controlling the launching efficiency and the directionality of surface plasmon polaritons is a major goal for the development of plasmonic devices. This study presents the use of the damped least-squares method to optimize the geometry parameters of a surface plasmon polariton unidirectional coupler. A damped least-squares algorithm has been constructed and the performance of a surface plasmon polariton unidirectional coupler has been obtained with COMSOL LiveLink for MATLAB. We have optimized a quasi-periodic surface plasmon polariton unidirectional grating coupler, which consists of five groups of sub-wavelength groove-doublets with an extinction ratio of nearly 71 dB. Combination of the COMSOL and the damped least-squares method in a MATLAB environment is a practicable method for the design of a surface plasmon polariton unidirectional coupler. Moreover, the general design principles behind this study can readily be extended to other numeric algorithms and other plasmonic devices.
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