Stacking c-Si metasurfaces by combing floating-off and soft transfer technique

Conventional metasurfaces fabricated on rigid substrates (e.g., silicon, quartz) face significant limitations in flexible optics, conformal devices, and dynamically tunable systems due to their bulky form factors, poor mechanical flexibility, and limited functional integration. To address these challenges, this study introduces an innovative fabrication strategy involving the transfer and vertically stacked integration of monocrystalline silicon metasurfaces on flexible polydimethylsiloxane (PDMS) substrates, overcoming the performance constraints of traditional rigid platforms. First, a "flipping" technique for monocrystalline silicon metasurface membranes is developed based on optimized wet transfer processes. A custom-built 3D electronically controlled transfer platform, integrated with high-resolution microscopic imaging, achieves subwavelength-level alignment (±5 nm positional accuracy, angular misalignment below 10°) between bilayer metasurfaces using predefined nanoscale fiducial markers. Second, polymethyl methacrylate (PMMA) films with precisely controlled thicknesses are employed as tunable interlayer spacers, enabling tailored optical coupling via interlayer resonance and van der Waals interfacial bonding. This approach breaks the dimensional limitations of conventional singlelayer metasurfaces, expanding light-field modulation capabilities in polarization, phase, and spectral domains. By synergizing flexible transfer, high-precision stacking, and dynamically adjustable interlayer spacing, this technology resolves the functional rigidity and spatial-dimensional constraints inherent to rigid-substrate devices. It establishes a versatile fabrication platform for advancing reconfigurable metalenses, adaptive spectral filters, and bio-integrated optical sensors, offering new paradigms for next-generation photonic systems.