Asymmetric optical transmission from broken interface-orientation symmetry in self-shadowed grating metallizations

Abstract

Reciprocal optical structures can transmit finite-angle illumination asymmetrically when the two illumination directions couple differently to the available diffraction channels. We realize this in a single overlay-free step: oblique metallization of a conventional diffraction grating, where ballistic self-shadowing grows a one-sided, louver-like silver profile. Electron microscopy and Ag mapping show the one-sided silver redistribution on oblique-coated films and its absence on a normal-incidence control. Full-wave Maxwell calculations on the ballistic-growth geometry predict broadband substrate-favored transmission, carried by the transverse-magnetic (TM) polarization in the diffraction regime (contrast up to 2.1) and polarization-independent and spectrally flat (1.5-1.9) deep in the geometric regime; the rejected power is redirected predominantly into specular reflection, and the deposition angle selects the magnitude and sign of the effect (optimum near $45^\circ$). Calibrated three-color sample-reversal measurements yield dark-corrected substrate-favored ratios that persist in both output-analyzer projections on a $45^\circ$-coated 1-$μ$m-period film (up to 1.8) and lie within the computed geometric-regime band on a 30-$μ$m-period film (1.5-1.8, with reference-normalized absolute transmittances).