A super-compact Galilean beam expander enabled by a sandwich-type double-metasurface

We present a super-compact Galilean beam expander based on a sandwich-type double- metasurface, enabling efficient expansion and collimation of a highly divergent VCSEL beam (14° within a 300 μm-thick device. The expander, comprising two amorphous-silicon metasurfaces on a 300 μm glass substrate, expands the beam from 18 μm to 87 μm and achieves a 75 μm waist at 12 mm, maintaining a narrow 0.62° divergence over 12–30 mm. With a footprint of just 500 μm × 500 μm and >70% focusing efficiency, this metasurface expander offers a compact, integrable solution for next-generation miniaturized photonic systems.