Design results: (a) for Grating 1 and (b) for Grating 2. The color map and contour lines depict η calculated by using the formulas in Section 3.1. The blue rectangular boxes delineate the boundaries of Ω(ϕ^*), inscribed within the contour lines of η = 97% (η^*). The upper-right white triangular area in (b) belongs to the domain where the lower base width of the trapezoid is greater than f_maxd = 0.9d.