Abstract
High-quality (
Q
) factor resonances in nonlocal metasurfaces are highly desired for enhancing light-matter interactions. However, high-
Q
resonances typically suffer from significant wavelength shifts or
Q
-factor degradation with varying incident angles. Current strategies to improve angular robustness have been largely confined to one-dimensional cases and work in reflection mode. In this work, we propose a high-
Q
transmissive metasurface that addresses this limitation by engineering quasi-guided modes to suppress angular dispersion and
Q
factor degradation in the two-dimensional Brillouin zone. This is achieved through two-dimensional Brillouin zone folding, which replicates geometrically tuned flat bands from the boundary to the center, coupled with asymmetry-engineered far-field excitation. Our approach eliminates the inherent trade-off between maintaining a high
Q
-factor and ensuring angular stability of nonlocal resonances, demonstrating exceptional robustness: the
Q
-factor remains above 10
3
with stable resonant wavelengths and circular-polarization independence over a wide angular range of ±18° in both the x-z and y-z planes. Moreover, our design extends angle-robust high-
Q
flat-band spectral filtering from reflection to transmission, enabling ultra-narrowband filtering in transmission. This work paves the way for advancements in nonlinear photonics, spectroscopic sensing, and quantum sensing.