Ultra-thin double-layer plasmonic metalens with ultra-broadband bandwidth and high numerical aperture

Broadband or ultra-broadband metalenses with high-performance focusing are essential for various applications across from the optical band to lower microwave spectrums. To design an ultra-broadband metalens with both a high numerical aperture (NA) and an ultra-thin device profile is still a great challenge in the field. In this paper, an ultra-broadband (within the entire Ku band: 12-18 GHz) bi-layer plasmonic microwave metalens is designed and presented both numerically and experimentally. Simulated and experimental results indicate that the dispersion characteristic of ultra-broadband metalens is regularly negative, which shows a linearly increased focal length along with the increased operation frequency. Besides, the presented ultra-broadband microwave metalens has an average transmission efficiency of 35%, focusing efficiency of 20% and a high NA of 0.7 on average in the simulations. The device thickness of the presented ultra-broadband metalens is only 0.125λ₀ (λ₀ is the central wavelength of 15 GHz), which shows an ultra-compact design advantage compared to previous works. The sample is also fabricated, and a much higher focusing efficiency as well as a smaller NA are obtained in the measurements. The presented diffractive characteristic is different from previously reported refractive-type broadband microwave metalenses based on dielectric metasurfaces. The presented microwave metalens can find many potential applications, such as metalens antennas, high-resolution imaging, and broadband meta-lens retro-reflectors in the Ku band.