Ultrafast All‐Optical Switching with Engineered Wavefronts via Chiral Nonlocal Phase Gradient Metasurfaces

The rapid‐growing demands in high‐speed telecommunication and optical computing necessitate the advances in all‐optical switching based on high quality (Q) factor photonic nanostructures. However, current all‐optical switching approaches cannot simultaneously shape the output wavefront, because combining pixelated phase control with high‐Q nanostructures remains extremely challenging. Here, we demonstrate picosecond all‐optical switching of the unpolarized incident light with engineered output wavefronts via nonlocal geometric‐phase gradient metasurfaces supporting high‐Q chiral quasi‐bound states in the continuum (quasi‐BICs). The nonlocal geometric‐phase gradient metasurfaces enable pixelated wavefront shaping of the transmitted light in both real and momentum spaces, while the high‐Q quasi‐BIC resonances underpin the resonantly enhanced responses critical for ultrafast optical modulation. Notably, leveraging the chiral‐selectivity of the chiral quasi‐BICs, the metasurfaces are functionally integrated with circular polarizers, which thereby overcome the inherent constraint of nonlocal geometric‐phase gradient metasurfaces on the incident polarization. Our work provides an ultrafast and versatile platform for multi‐dimensional optical field manipulation.