Figure 6: Numerical simulation of second harmonic diffraction.

(a) Finite difference time domain simulation of a nonlinear grating modelled after the oblique-angle-modulated DAS multilayer structure illustrated in Fig. 4a. The sinusoidal grating has a period Λ=280 nm and a modulation depth h=50 nm with a 15 nm thick nonlinear region defined on the right-hand grating facets indicated by dashed black lines. The top and bottom false-colour images show the relative optical field intensity (|E|², displayed with a logarithmic colour scale) at the fundamental frequency for left and right incidence, respectively, using the same θ_i=32.5° as the experiments in Fig. 5. (b) Similar images showing the optical field intensity distribution produced at the second harmonic frequency. The asymmetry in second harmonic generation is a straightforward consequence of the larger fundamental field intensity in a that occurs in the nonlinear layer for right versus left incidence. (c) Ratios of the SHG and linear T₋₁ diffraction intensities for right versus left incidence predicted from the model in comparison to the experimental results obtained from Fig. 5b.