Design of two-way beam splitters. (a) Optimum values of the fill factor f (red) and the relief depth h/λ₀ (blue) as a function of the exit angle of the first diffracted order in TE polarization. (b) The corresponding first-order diffraction efficiency η_+1,0 = η_−1,0 (black), the efficiency η_0,0 of the zeroth transmitted order (red), and that of the zeroth reflected order (blue), which is 1 − 2η_+1,0 − η_0,0 due to energy conservation. The inset shows the grating structure and direction of illumination.