(a) Variation of the transmissivity of positions 5 and 6 over the coupling length L when t = 20 nm for the wavelengths of 1 μ m , 1.55 μ m , and 2 μ m ; Distribution of the absolute value of the electric field ( E ) at each port under the coupling length of (b) 100 nm, (c) 750 nm, and (d) 1.5 μ m for λ = 1.55 μ m .