Design of a terahertz even splitter and its tolerance analysis

A novel method is presented in this paper to realize terahertz even beam splitting by using a subwavelength binary simple periodic rectangular structure, for making comprehensive application of both the RCWA (Rigorous Couple-Wave Analysis) and the GA (Genetic Algorithm). By applying RCWA, the efficiency of each diffraction order can be numerically solved by using the structure parameters. To design an even beam splitter with a subwavelength structure is to find the optimal duty cycle f, period d, the grating depth h1 and the substrate thickness h2 to approach the minimum zero-order diffraction efficiency, the maximum sum of each non-zero-order diffraction efficiency, and the uniform distribution. Considering the three goals above, an evaluation function is established. GA is applied to optimize the evaluation function F, and then the optimal parameters of the splitter are obtained. When its period, groove depth, substrate thickness and duty ratio respectively equal to 269.7 μm, 175.2 μm, 18.1 μm and 0.409, the normal-incident TE-polarized terahertz plane wave with its frequency equal to 2.52 THz is divided evenly into the diffraction orders±1 and±2. It has a total efficiency up to 92.23% with a preferable result of reducing zero-order diffraction efficiency to 0.192% and an error of uniformity down to 6.51×10- 6, indicating an excellent performance of diffraction efficiency, uniformity and zero-order suppression as a terahertz even splitter. These results break the limitation of realizing even beam splitting in which the traditional scalar theory applies. In addition, this paper exposes the law of influence of the structure parameters, including ridge width, groove width, groove depth, duty ratio and substrate thickness, on the diffraction efficiency and its uniformity. It is found that only a small deviation of the structure parameters, corresponding to a deviation of ridge width a, groove width b, groove depth h1, and substrate thickness h2, less than 1 μm from the optimum design values, the element maintains good beam splitting performance. P0 is permitted to suppress to less than 2%, the error of uniformity U is better than 19.60%, and the diffraction efficiency maintains higher than 79.10%. With a substantial deviation from the design values of the structure parameters, the performance of the splitter will severely degrade and need to be redesigned.