Observation of Goos-Hänchen Shift under Normal Incidence in Slanted TiO2 Nanogratings

Goos-Hänchen (GH) shifts at natural optical interfaces are typically small. Recent studies have shown that strong optical resonances of metasurfaces and nanogratings can dramatically enhance the GH shifts. However, GH shifts are typically observed when light is obliquely incident while those under normal incidence have rarely been explored. Here, we demonstrate, both theoretically and experimentally, that GH shifts can also be realized under normal incidence by employing slanted TiO2 nanogratings. The slight slant of the nanograting introduces symmetry breaking, enabling quasi bound states in the continuum (Q-BICs) with extremely high Q factors. Because the transmission phase φ(k) becomes asymmetric with the in-plane wavevector k, its derivative dφ(k)/dk acquires a large magnitude at k = 0. This steep phase variation results in the observable normal-incident GH shifts. Experimentally, we use reactive ion beam etching for high-quality TiO2 slanted nanogratings. The presence of Q-BICs resonances was revealed by reflection spectra measurements, and the corresponding GH shifts were measured using light-field detection scheme. Our results reveal normal‑incidence GH shifts, thereby opening a new route for compact, on‑chip beam‑steering devices that operate without the need for angular alignment.