Fig. 4. Polarization selection rule for NOA.

a The spectrum of electromagnon (${\epsilon }_{xx}$) obtained for (001) surface at 0 T in the screw spin phase. Experimental configuration (inset to (b)) is the same as that in Figs. 1, 2, and 3. b The rotation angle θ and ellipticity η at 0 T. The GB (${\epsilon }_{xy}^{\mathrm{S}}$) is forbidden (see Fig. 2f) and the NOA (${\epsilon }_{xy}^{\mathrm{A}}$) solely induces the optical rotation. c The spectrum for ${\epsilon }_{yy}$ (y||[$1\overline{1}0$]) was obtained by using different samples with (110) surface at 0 T. The light polarization plane is perpendicular to P||[110] (inset to (d)). The observed spectrum is similar to ${\epsilon }_{yy}$ obtained by using (001) surface (Fig. 2a). d The rotation angle θ and ellipticity η for (110) surface.