Reciprocal Asymmetric Transmission in Directionally Metallized Silica Nanogratings With Broken Lateral Mirror...

We study here lateral asymmetric transmission (AT), a phenomenon in which the transmission under illumination from the same half-space depends on the sign of the in-plane wavevector, $+k_x$ versus $-k_x$ . Such lateral AT can emerge in reciprocal systems without polarization conversion when broken lateral mirror symmetry is combined with multiport diffraction. We experimentally and numerically investigate this effect in directionally Ag-coated ${\text{SiO}}_2$ nanogratings, whose one-sided metal coating breaks the lateral mirror symmetry. Rigorous coupled-wave analysis shows that channel opening establishes the multiport scattering basis, whereas structure-dependent resonant responses modulate the redistribution of power among transmission, reflection, and absorption channels. Field and normalized local-loss distributions corroborate stronger metallic dissipation under $+k_x$ excitation and greater transmission under $-k_x$ excitation. Angle-resolved measurements and morphology-informed simulations show the same sign and principal angular-spectral evolution of the lateral transmission contrast. The design principle is further examined in directionally Al-coated PMMA- ${\text{TiO}}_2$ gratings on a ${\text{SiO}}_2$ substrate. These results identify broken lateral mirror symmetry as the structural prerequisite and higher-order diffraction-channel opening as the multiport basis for reciprocal lateral AT, with structure-dependent resonant coupling modulating the attainable contrast.