Fig. 4

Broadband tunable symmetry-broken HPs in calcite. a, b Experimental near-field images of symmetry-broken HPs with and without background, respectively, at three different frequencies. c Absolute value of the Fourier transforms of the images shown in a; d The filtered experimental data extracted from (b). White dashed lines denote asymptotes of the experimental IFC. d Experimental open angle θ (orange rhombus) and theoretical polarization angle φ (green circle and line) of HPs plotted against frequency. Note that we used different disk antennas but with the same size at ω = 1460 cm⁻¹ compared to the case at ω = 1450 cm⁻¹ or 1440 cm⁻¹, all of them exhibited obvious symmetry-broken HP phenomena, which proved the robustness of the experimental results