Giant Goos–Hänchen Shift with Stable High-Efficiency Emission Enabled by Topological High-<i>Q</i> Unidirectional Guided Resonances

Achieving an enhanced Goos–Hänchen (GH) shift while maintaining consistently high-efficiency transmission or reflection is of great importance for photonic applications such as information storage and ultrasensitive sensing. While enhanced GH shifts have been realized via resonance mechanisms or bound states in the continuum, these approaches inherently introduce sharp spectral modulations─characterized by angularly sensitive reflection or transmission peaks. This fundamentally constrain their applicability in pure-phase optical devices where uniform intensity is essential. Here, leveraging topologically protected high quality factor (high-Q) unidirectional guided resonance (UGR) in metagratings with either C2z symmetry or space-inversion symmetry, we theoretically demonstrate giant GH shifts reaching values of ∼ 103 wavelengths with constant high-efficiency reflection or transmission. Furthermore, we reveal that the topological nature of UGR ensures exceptional robustness of the enhanced GH shifts against varying geometrical parameters, with their magnitudes directly proportional to the quality factor of the UGR. Finally, by exploiting the high sensitivity of UGR-induced GH shifts to external environments, we design a highly sensitive refractive index sensor with a sensitivity exceeding 1.2 × 106 wavelengths per refractive index unit.