Dual-focal metalenses based on complete decoupling of amplitude, phase, and polarization

The simultaneous control of amplitude and phase via metasurfaces affords us an unprecedented degree of freedom in manipulating electromagnetic waves. However, currently most designs suffer from low efficiency, which raises certain concerns for real-world applications. Moreover, complete amplitude, phase, and polarization modulation is particularly challenging. This typically requires a combination of attenuators, optically thick wave plates, and large dielectric lenses. Here, we propose an alternative scheme by introducing vertical-mode cross-coupling for polarization control and high efficiency, while involving spatially-varied orientations and structures for independent amplitude and phase modulation. The vertical-mode cross-coupling is synthesized by stacking triple-layer twisted split-ring resonators (SRRs) operated in a transmissive scheme. Such tight cross-coupling and chirality-assisted coherent multiple resonances facilitate high cross-polarization conversion efficiency (∼100%) and a broadband transmission window with full phase coverage. As a proof of concept, two dual-focal metalenses that are challenging to be actualized through conventional metasurfaces were designed and studied numerically and experimentally, with a total thickness of λ0/12 at microwave frequencies. Desirable dual-focusing behavior with axial and lateral alignment of two foci were demonstrated. Our findings, not confined to microwave operation, open up an alternative way to the fine control of light, and can stimulate novel and high-performance versatile photonic metadevices.