Fig. 3. Correlation of optical activities with spin orders.

Color-coded spectra of a electromagnon (Im[${\epsilon }_{xx}$]), b GB (Im[${\epsilon }_{xy}^{\mathrm{S}}$]), and c NOA (Im[${\epsilon }_{xy}^{\mathrm{A}}$]) in the magnetic field (5 T, H_dc||[110]). The open circles in a indicate the data acquisition temperatures. The GB and NOA emerge in the screw spin phase, manifesting that the ferroelectric and chiral screw spin order gives rise to the GB and NOA. (See Supplementary Note 3 for details of spectra).