Planar achromatic metalenses with a thickness of the order of wavelength have attracted much attention for their potential applications in ultra-compact optical devices. However, realizing single-layer achromatic metalenses across a wide bandwidth requires that the corresponding meta-atoms have complex cross-sections for correct phase profile and dispersion compensation. Herein, we introduce an effective Abbe number and use lens maker equations to design a dual-layer achromatic metalens in which we compensate the dispersion by using a plano-convex liked metalens combined with a plano-concave liked metalens. The stacked metalens are designed based on simple high refractive index dielectric cylindrical meta-atoms with different radii, which simplify the design and fabrication processes. We demonstrate that a dual-layer achromatic metalens has a small focal length difference across the visible wavelength range and an average focusing efficiency above 50%, which proves that the design method is promising for many potential applications in multi-functional flat optical devices.

Space-time-modulated metasurfaces can manipulate electromagnetic waves in space and frequency domains simultaneously. In this article, an analytical design of space-time-modulated metasurfaces with modulation elements composed of two paths, In-phase (I) and Quadrature (Q), is proposed. The model is derived analytically, the space-/frequency-domain manipulations are achieved by designing the dimension and time sequence of I and Q paths. In the specular reflection direction, an objective frequency shift of the reflected first-order harmonic can be obtained. While, in other directions, the opposite first-order harmonic can be easily controlled by changing the dimension of I/Q paths, and the objective first-order harmonic remains unchanged. Furthermore, with a small dimension of I/Q paths, the first-order harmonic can be used for beam scanning by predesigning the start time of the modulation element. To realize the space-time-modulated metasurface with the required periodically time-varying responses, 2 bit unit cells loaded with dynamically switchable pin diodes are used as I/Q modulation. Both the analytical and numerical results demonstrate that space- and frequency-domain manipulations of the reflected fields by the first-order harmonics can be simultaneously obtained. The proposed designs have potential applications in wireless communications, radar camouflaging, and cloaking.

A three-dimensional (3-D)-printed dual-fed dual-frequency dielectric antenna is investigated for vehicular communications. It combines a dielectric resonator (DR) antenna (DRA) and a dielectric lens antenna (DLA) for the low and high frequency bands, respectively. Both of the antenna parts share the same dielectric body that has three different effective dielectric constants. The DRA is excited by a vertical conducting adhesive strip on its sidewall, whereas the DLA is fed by a separate embedded slot-coupled cylindrical DRA. To verify the idea, a dual-frequency dielectric antenna operating at S - and X -bands is designed and fabricated with a 3-D printer. The S -parameters, radiation patterns, antenna gains, and antenna efficiencies of the two antenna parts are measured, and reasonable agreement between the measured and simulated results is found. The prototype has wide measured S - and X -band 10-dB impedance bandwidths of 40.2% and 19.5%, respectively. Also, its measured isolation between the two ports is desirably over 40 dB across the two bands. The measured peak antenna gains are given by 5.8 and 12.0 dBi in S - and X -bands, which are suitable for short-distance vehicle-to-basestation and long-distance vehicle-to-satellite communications, respectively.

A new decoupling method is proposed in this article, which simultaneously realizes port and radiation pattern decoupling, namely, high isolation and undistorted radiation patterns, respectively. The proposed decoupling method shows its effectiveness in $1\times 2$ dielectric resonator (DR) antennas (DRAs) even with zero spacing. Conformal strips are attached to the sidewall of the DRs and parallel to the E-plane of the DRAs. Coupled from the resonant DR, the conformal strips can generate an induced current, thus forming an induced field inside the DR. It can be noted that, inside the two DRs, the DR resonant field and the strip induced field are antiphase and in-phase, respectively. Tuning the conformal strips can change the amplitude ratio and phase difference between the resonant and induced fields, which can be used to cancel the fields inside the DR based on the superposition principle. When one DR is excited, the field inside the other DR can be very weak. The DRs can then work at their fundamental TE $_{\delta 11}$ modes independently. Therefore, their radiation patterns are not distorted together with high port isolation. A new indicator, energy storage proportion (ESP), is proposed to evaluate the decoupling effect. It can be found that, when the ESP gets close to 1, the radiation patterns will be restored together with high isolation. To verify the idea, $1\times 2$ DRAs are designed, fabricated, and measured. A reasonable agreement can be observed between the measured and simulated results for both H- and E-plane decoupled cases. All the measured overlapping impedance bandwidths are larger than 6%, the envelope correlation coefficients are lower than 0.059, and the isolations are higher than 20 dB. It should be highlighted that all the radiation patterns in this article have been restored, useful for practical applications using multiple-input multiple-output techniques.

A pattern- and polarization-reconfigurable dielectric resonator antenna (DRA) is presented. It deploys a conventional cylindrical dielectric resonator (DR) with a new switchable feeding network. The feeding network has four slots that are etched in the ground. By tuning the phase distributions in these slots, three DR modes (the TM _01δ and two degenerated HEM ₁₁₃ modes) are excited. The former has an omnidirectional radiation pattern, whereas the latter two have boresight patterns with orthogonal polarizations ( x - and y -polarization). By tuning the height-to-radius ratio, these modes can be designed to operate at the same frequency, thus enabling both the pattern and polarization reconfigurability. A prototype of the reconfigurable DRA working in 2.4 GHz is fabricated and measured to verify ideas. Reasonable agreement between measurements and simulations is obtained. Our design provides both pattern and polarization reconfigurability, with good efficiency in the band of interest.