Dispersion characteristics analysis of a four-layer high-numerical-aperture focusing metalens

High-numerical-aperture (NA) metalenses are an important ultra-thin flat meta-device, which can produce a diffraction-limited spot size comparable to half-wavelength at far field. High NA metalens have been successfully demonstrated by using all-dielectric metasurface design in the past few years. However, high NA metallic metalenses in the low spectrums such as Terahertz and microwave are rarely seen. In this paper, a high NA metalens consist of four-layer high transmission double complementary split-ring-resonator gradient arrays are designed and presented. The numerical simulations show that the full-width-at-half-maximum (FWHM) of focusing spot has only 0.57 lambda at 9GHz thus leading to a high NA metalens of 0.88. Besides, the dispersion characteristic of the presented high NA is studied and it is found that the focal length decrease first along with operation frequency and then increase from the considered frequency range from 8.5GHz to 9.5GHz. The dispersion characteristic of presented high NA metalens is distinct to all of previous demonstrated broadband microwave metalens, thus providing versatile dispersion-control for further broadband metalens applications. The dispersion analysis of high NA metalens here pave the way for further applications such as a high-gain lens antennas where the bare patch deposits on the different focal length with varying frequency.