Numerically simulated SFG efficiency from Eqs. (1) versus the coupling Ω ˜ and the phase mismatch Δ k . Frames (a) and (b) are for a single crystal ( N = 1 ) , while frames (c) and (d) are for a composite crystal of N = 4 domains and frames (e) and (f) are for a composite crystal of N = 6 domains, with the thicknesses listed in Table 1. The left columns (a), (c), and (e) are for the undepleted-pump regime ( | A 1 ( z i ) | = 10 | A 2 ( z i ) | ) , while the right columns (b), (d), and (f) are for the depleted-pump regime ( | A 1 ( z i ) | = | A 2 ( z i ) | ) . The inner curves mark the 90% efficiency level, and the outer curves are for the 50% level. The reciprocal of the total crystal length L is used as a unit for Ω ˜ and Δ k .