A polarization-insensitive broadband terahertz (THz) metamaterial absorber is presented in this paper. The metamaterial absorber consists of several circular and rhombus metallic patches as a unit cell and a metallic ground plane separated by a polyimide dielectric layer. By increasing the number of circular and rhombus patches with different sizes, the operating bandwidth of the absorber is broadened because of the resonances of each single patch getting close enough and merging together in the frequency range. The simulated results show that the absorption bandwidth covers a broadband frequency range of 0.22–0.33 THz with the highest absorptivity of 99.93% at 0.3035 THz. The full width at half maximum is as wide as 0.155 THz, and the bandwidth of over 80% absorptivity is 0.1315 THz. This study promises applications in THz detection and communication systems, and the operation bandwidth is very convenient and compatible for electronic THz sources.

The numerical aperture (NA) of a lens determines its focusing resolution capability and the maximum light collection or emission angle. In this Letter, an ultrathin high NA metalens operating in the microwave band is designed and demonstrated both numerically and experimentally. The proposed element is constructed by a multi-layer complementary split ring resonator, which can cover full 2 π phase shift simultaneously with high transmission magnitude by varying its radius gradually. The numerical and experimental results reveal that the designed ultrathin (thickness is only ∼ 0.23 λ ) metalens can focus normal incident microwave efficiently to a spot of full width at half-maximum (FWHM) as small as ∼ 0.54 λ with a corresponding high NA exceeding 0.9. Besides, the high NA metalens also possesses a relatively large focusing efficiency with a peak 48% within considered broad frequency range from 7.5 to 10 GHz. The performances of the presented metalens can be comparable or even superior to nowadays high-quality optical metalenses and represent an important step to develop a high-performance metalens in low spectrum. Besides, it can greatly facilitate the development of some novel miniaturized devices like a high-gain low profile scanning antenna, an ultra-compact retroreflector, and cloaks.

Broadband metalenses consisting of sub-wavelength phase gradient elements are indispensable in modern science and technology. So far, several broadband optical metalenses are demonstrated but mostly with either small NA or relatively low focusing efficiency. Herein, an ultra-thin broadband microwave metalens (frequency range from 8.0 to 10.5 GHz) with a relatively high-efficiency above 40% and a large NA of more than 0.6 is presented. The metalens is also fabricated and the measurement results agree with the simulations very well. The performances of the presented broadband metalens can surpass nowadays microwave metalenses largely and open up new vistas for low-profile, low-cost and light-weight microwave components.

Controlling the dispersion characteristic of metasurfaces (or metalenses) along a broad bandwidth is of great importance to develop high-performance broadband metadevices. Different from traditional lenses that rely on the material refractive index along the light trajectory, metasurfaces or metalenses provide a new regime of dispersion control via a sub-wavelength metastructure, which is known as negative chromatic dispersion. However, broadband metalenses design with high-performance focusing especially with a reduced device dimension is a significant challenge in society. Here, we design, fabricate, and demonstrate a broadband high-performance diffractive-type plasmonic metalens based on a circular split-ring resonator metasurface with a relative working bandwidth of 28.6%. The metalens thickness is only 0.09λ₀ ( λ₀ is at the central wavelength), which is much thinner than previous broadband all-dielectric metalenses. The full-wave simulation results show that both high transmissive efficiency above 80% (the maximum is even above 90%) and high average focusing efficiency above 45% (the maximum is 56%) are achieved within the entire working bandwidth of 9–12 GHz. Moreover, an average high numerical aperture of 0.7 ( NA=0.7 ) of high-efficiency microwave metalens is obtained in the simulations. The broadband high-performance metalens is also fabricated and experimental measurements verify its much higher average focusing efficiency of 55% (the maximum is above 65% within the broad bandwidth) and a moderate high NA of 0.6. The proposed plasmonic metalens can facilitate the development of wavelength-dependent broadband diffractive devices and is also meaningful to further studies on arbitrary dispersion control in diffractive optics based on plasmonic metasurfaces.