Metalenses are essential components in terahertz imaging systems. However, without careful design, they show limited field of view and their practical applications are hindered. Here, a wide-angle metalens is proposed whose structure is optimized for focusing within the incident angles of ±25°. Simulation and experiment results show that the focusing efficiency, spot size, and modulation transfer function of this lens are not sensitive to the incident angle. More importantly, this wide-angle metalens follows the ideal Gaussian formula for the object-image relation, which ensures a wider field of view and better contrast in the imaging experiment.

Terahertz (THz) lenses have numerous applications in imaging and communication systems. Currently, the common THz lenses are still based on the traditional design of a circular convex lens. In this work, we present a method for the design of a 3D-printed multilevel THz lens, taking advantage of the benefits offered by 3D printing technology, including compact size, lightweight construction, and cost-effectiveness. The approach utilizes an inverse design methodology, employing optimization methods to promise accurate performance. To reduce simulation time, we employ the finite-difference time-domain method in cylindrical coordinates for near-field computation and couple it with the Rayleigh–Sommerfeld diffraction theory to address far-field calculations. This technology holds great potential for various applications in the field of THz imaging, sensing, and communications, offering a novel approach to the design and development of functional devices operating in the THz frequency range.

Chiral metasurfaces integrated with active materials can dynamically control the chirality of electromagnetic waves, making them highly significant in physics, chemistry, and biology. Herein, we theoretically proposed a general and feasible design scheme to develop a chiral metadevice based on a bilayer anisotropic metasurface and a monolayer liquid crystal (LC), which can construct and flexibly manipulate arbitrary terahertz (THz) chirality. When the twist angle between the anisotropic axes of two metasurfaces θ is not 0°, the spatial mirror symmetry of the chiral metadevice is broken, resulting in a strong THz chiral response. In addition, the introduction of anisotropic LCs not only enhances the chiral response of the metadevice but also induces the flipping modulation and frequency tunability of the chirality. More importantly, by optimizing the θ, we can flexibly design the arbitrary chiral response and the operating frequency of chirality, thereby promoting the emergence of various chiral manipulation devices. The experimental results show that the maximum circular dichroism can reach -33dB at 0.94 THz and flip to 28 dB at 0.69 THz by rotating the LC optical axis from the x to y axis, with the maximum operating frequency tunable range of ∼120GHz. We expect this design strategy can create new possibilities for the advancement of active THz chiral devices and their applications, including chiral spectroscopy, molecular recognition, biosensing, and fingerprint detection.

The powerful wavefront manipulation capability of metasurfaces originates from their subwavelength or deep subwavelength elements with designable optical responses, especially phase responses. However, they usually suffer from performance degradation as the spatial phase gradient is large. To solve this issue, we propose an accurate and efficient nonlocal diffraction engineering mechanism to tailor an arbitrary large-gradient wavefront utilizing superwavelength-scale elements. The fast-varying phase profile is cut into segments according to 2π zones rather than subwavelength discretization. Each phase segment is accurately implemented by precisely tailoring the diffraction pattern of the element, where diffraction angles, efficiencies, and phases are controlled simultaneously. As proof of the concept, high numerical aperture cylindrical metalenses are designed using this method and experimentally validated at the terahertz band. The cylindrical metalens is further extended to a full-space metalens, which enables high-quality subwavelength imaging with resolved details of 0.65λ. The proposed mechanism offers an efficient way to capture the fast-varying wavefront using relatively coarse geometries with new physical insights.

Compared with traditional optical elements, metasurfaces have shown unique advantages in multifunctionality encoded in different frequencies, polarization states, and orbital angular momentums. However, the study of metasurfaces with well-controlled functions under different incident angles is still in its infancy. Here we propose a general method to tailor the angular dispersion over the simplest binary dielectric grating in the transmission mode. We demonstrate that the angular response is strongly related to the number of waveguide modes inside the grating, so one can intentionally reduce or enhance the angular dispersion by controlling the number of waveguide modes. Independent phase manipulation over incident angles is experimentally demonstrated by a metalens with angle-dependent focus. The angular dispersion in orthogonal polarization states is further utilized to demonstrate angle-insensitive and angle-multiplexed wave plates. These devices with simple configuration and clear physics offer a general platform to expand the scope of beam manipulation over metasurfaces.

We experimentally investigate the linear polarization conversion for terahertz (THz) waves in liquid crystal (LC) integrated metamaterials, which consist of an LC layer sandwiched by two orthogonally arranged sub-wavelength metal gratings. A Fabry–Perot-like cavity is well constructed by the front and rear gratings, and it shows a strong local resonance mechanism, which greatly enhances the polarization conversion efficiency. Most importantly, the Fabry–Perot-like resonance can be actively tuned by modulating the refractive index of the middle LC layer under the external field. As a result, the integrated metamaterial achieves multi-band tunable linear polarization conversion.

Subtractive imaging is used to suppress the axial sidelobes and improve the axial resolution of 4pi microscopy with a higher-order radially polarized (RP) Laguerre–Gaussian (LG) beam. A solid-shaped point spread function (PSF) and a doughnut-shaped PSF with a dark spot along the optical axis are generated by tightly focusing a higher-order RP-LG beam and a modulated circularly polarized beam, respectively. By subtracting the two images obtained with those two different PSFs, the axial sidelobes of the subtracted PSF are reduced from 37% to about 10% of the main lobe, and the axial resolution is increased from 0.21λ to 0.15λ.