Dual-Focal Metalenses Based on Complete Decoupling of Amplitude, Phase and Polarization

The simultaneous control of amplitude and phase via metasurfaces affords us unprecedented degree of freedom in manipulating electromagnetic waves, but currently most designs suffer from low efficiency which raises certain concerns for real-world applications. Moreover, the complete amplitude, phase and polarization modulation is particularly challenging, which 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 by 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 transmissive scheme. Such tight cross-coupling and chirality-assisted coherent multiple resonances facilitates high cross-polarization conversion efficiency (~100%) and broadband transmission window with full phase cover. As a proof of concept, two dual-focal metalenses that are challenging to be actualized through conventional metasurfaces are designed, numerically and experimentally studied with a total thickness of λo/12 at microwave frequencies. Desirable dual focusing behavior with axial and lateral alignment of two foci are demonstrated. Our findings, not confined to microwave operation, opens up an alternative way in fine controlling light and can stimulate novel and high-performance versatile photonic metadevices.