Spectrally tunable singular phase at normal incidence in nanophotonic cavities

The concept of singular phases at the point of darkness has been extensively studied using various optical systems, such as metamaterials and thin-film cavities, mainly for refractive index sensing. However, these systems typically work at an oblique angle of incidence. Achieving a singular phase at normal incidence is vital for practical sensing and flat-optics applications, but it remains challenging. Here, we demonstrate the singular phase at normal incidence using a grating-coupled thin-film cavity that functions at optical wavelengths. By optimizing the structural parameters of the grating and thin-film cavity, we achieve zero reflection with an abrupt phase shift at the polarization-dependent resonance of the grating. In addition, we introduce a spectrally tunable singular phase at normal incidence by integrating a phase change material (PCM) into the cavity. Using a lossless PCM, such as Sb2S3, we achieve a phase singularity with continuous spectral tunability up to 203 nm by altering its phase from amorphous to crystalline, a key feature for phase engineering in flat optics.