Unidirectional Far-Field Radiation of Surface Waves by On-Chip Achromatic Metasurfaces.

Converting surface waves to unidirectional far-field beams is crucial for information readout in on-chip photonic applications, but conventional grating-like couplers suffer from low-efficiency issue due to multi-mode generation. Although recently proposed metasurfaces have addressed this challenge, meta-couplers realized so far exhibit inevitable frequency dispersions, restricting their practical applications. Here, we propose a scheme to design on-chip achromatic metasurfaces that can decouple SWs within a continuous frequency band to far-field beams traveling along the same direction. Our metadevices, composed of composite meta-atoms exhibiting pre-designed reflection phases with tailored frequency dispersion dictated by both resonant and Pancharatnam-Berry phase mechanisms, can compensate the desired frequency-dependent wavevectors for SWs at different frequencies, thereby generating unidirectional far-field beams. As a proof of concept, we design and fabricate two on-chip achromatic metasurfaces working in the microwave regime and experimentally demonstrate that they can achieve in-plane and out-of-plane unidirectional far-field decoupling of SWs, exhibiting relative frequency bandwidths of 22.22% and 37.04%, respectively. Both microwave experiments and full-wave simulations successfully confirm our theoretical predictions. These findings pave the way for on-chip photonic manipulations and related applications, including wireless communication, signal processing, high-sensitivity sensing, and so on.