All-Dielectric Metasurface with a Two-Dimensional Locally Flat Photonic Band

Photonic flatbands offer promising light-matter interaction due to their unique slow-light nature. In recent years, flatbands have also attracted significant interest in optical engineering because of their angle-insensitive resonant characteristics. However, to date, no studies have reported the dispersionless behavior of flatbands under arbitrary two-dimensional incident angles and polarizations. Here, we present a two-dimensional photonic flatband created using a silicon metasurface with a Lieb lattice-inspired structure that demonstrates a locally flat photonic band for both transverse electric (S-) and transverse magnetic (P-) polarized light. Employing Fourier imaging, we analyze the energy-momentum relation of the flatband metasurface under arbitrary two-dimensional incident angles, demonstrating flatbands with dispersion (change in resonance) less than the resonance line width up to a numerical aperture of 0.22 (polar angle θ = ± 12.7°) for all polarizations and arbitrary azimuthal angles (ϕ). The maximum flatband extent goes up to 0.6 (θ = ± 36.8°) for p-polarization at ϕ = 0° in the experiment. This geometry can be adapted for various applications in local field enhancement, reconfigurable metasurfaces, enhanced photodetection, and augmented reality displays.