Polarization change in the first order transmitted plane wave for an incoming plane wave at the Bragg wavelength and incoming wavevector k, with kX tuned to the Bragg angle, but with a varying ky. The lines for [0, 1] → [1,0] and [1,0] → [0,1] overlap. The lower transmission of the [0,1] → [0,1] mode can also be seen in Fig. 4, and the thicker lines at higher |k_y| are unresolved Fabry-Perot fringes.