All-dielectric metalens for terahertz wave imaging

Phase and amplitude simultaneously coding metasurface with multi-frequency and multifunctional electromagnetic modulations

The integration of multiple functionalities into a single, planar, ultra-compact metasurface has presented significant opportunities for enhancing capacity and performance within compact 5G/6G communication systems. Recent advances in multifunctional metasurfaces have unveiled comprehensive wavefront manipulations utilizing phase, polarization transmission/reflection, and coding apertures. Despite these developments, there remains a critical need for multifunctional metasurfaces with expanded channel capabilities, including multiple operational frequencies, minimal crosstalk, and high-efficiency computable array factors. This study introduces a multifunctional metasurface that integrates phase- and amplitude simultaneous coding meta-atoms at dual frequencies. By altering the polarization of electromagnetic (EM) waves, it is possible to reshape the wave-fronts of reflected waves at these frequencies. The coding metasurface proficiently manipulates both x and y linearly polarized waves through phase and amplitude coding at dual frequencies, thereby enabling distinct functionalities such as anomalous reflection, reflection imaging, and vortex wave beam generation. Both theoretical analysis and full-wave simulation confirm the anticipated functionalities of the designed devices, paving the way for advancements in integrated communication systems with diverse functionalities.

Super-resolution terahertz imaging based on a meta-waveguide

A terahertz metamaterial waveguide (meta-waveguide) and a meta-waveguide-based lens-free imaging system are presented. The meta-waveguide not only inherits the low-loss transmission performance of a waveguide but also breaks through the diffraction limit under the action of the metamaterial, achieving subwavelength focusing. The focusing distance is far greater than the Rayleigh length, thus enabling far-field scanning imaging. For verification, a metal ring-based meta-waveguide was fabricated by 3D printing and metal cladding technology. Then, a transmission scanning imaging system working at 0.1 THz was built. High quality terahertz images with a resolution of 1/3 of the wavelength were obtained by placing the imaging targets at the focus and performing two-dimensional scanning. The focusing and transmission of terahertz wave in the meta-waveguide were simulated and analyzed.

Micro-dimensional oscillation-based optimization for a dielectric metalens in the mid-infrared

In the past few decades, there has been significant progress made in metasurfaces and integrated and miniaturized optical devices. As one of the most prominent applications of metasurfaces, the metalens is the subject of significant research. In this paper, for achieving better focusing performance of the initial metalens designed by the Pancharatnam-Berry (PB) phase, a concept of micro-dimensional oscillation is proposed to optimize the geometric parameters of nanopillars. A strategy of grouping iteration is proposed to reduce the loss rate and computational effort in a holistic way. Its essence is to divide an extremely large-scale optimization space into many overlapping groups. Meanwhile, an improved genetic-simulated annealing (IGSA) algorithm is presented for the optimal solution of each group. By introducing the adaptive crossover and mutation probabilities in traditional genetic algorithms, the IGSA algorithm has both strong global searching capability and excellent local searching capability. After optimization, the maximum field intensity of the central hot spot can be increased by about 8% compared to the initial metalens. Moreover, the field intensity of the side lobes around the hot spot is almost constant, and the central hot spot increases, which provides a potential for the realization of high imaging contrast.

Shape dependence of all-dielectric terahertz metasurface

All-dielectric metasurfaces have been attracting attention in the terahertz spectral range for low-loss planar optical elements such as lenses, beam splitters, waveplates, vortex plates, and magnetic mirrors. Various shapes of meta-atoms have been used in many studies; however, no systematic comparative study of each shape has been reported. The optical properties of various shapes of metasurfaces are reported in this work using finite difference time domain simulation. The phase of a pillar-type all-dielectric metasurface is mainly determined by the cross-sectional area, rather than its detailed shape. Consequently, in the square lattice geometry, the square shape meta-atom performs best in terms of full phase control at the lowest pillar height with negligible polarization dependence. Furthermore, we compare the transmission, phase, and polarization dependence of the hexagonal and square lattices. Square-shape metasurface successfully realizes subwavelength focusing metalens and vortex plate.

All-dielectric metalens for quasi-optical mode and polarization conversion

Quasi-optical mode conversion technology plays a very important role in the development of high-power terahertz radiation sources. The ability of metamaterials to manipulate wave-front paves a new way in the field of quasi-optical mode conversion. In this paper, the approach for quasi-optical mode conversion by all-dielectric metalens and polarization conversion is proposed and investigated. Three metalens are designed to converter cylindrical waveguide TE₀₁ mode to linear polarized (LP), left-hand circularly polarized (LHCP), and right-hand circularly polarized (RHCP) Gaussian beams at 350 GHz. Electromagnetic simulations show that the Gaussian mode contents of output waves from three metalens are all over 98% with high polarization contents. Furthermore, a metalens is designed for dual circularly polarized (DCP) which could convert cylindrical waveguide TE₀₁ mode to LHCP and RHCP simultaneously. This work unveils the potential application for metalens in terahertz region.

Coaxial double-hole PEDOT: PSS electrodes achieving tunable terahertz zoomable convergence

The local wavefront modulation technique in the terahertz band is an important basis for the development of terahertz modulation technology. Here, an electrically controlled convergent tunable device based on patterned transparent electrode poly (3, 4-ethylenedioxythiophene): poly (styrene sulfonate) (PEDOT: PSS) is realized to locally tune the terahertz wavefront. The device consists of two substrates with circular-hole electrodes and liquid crystal sandwiched between them. The refractive index gradient of liquid crystal in the device can be generated by the coaxial double-hole electrodes, which realize continuous control of significant focusing of the terahertz wave. The test results show that the focal length can be modulated in the range of 3-12 cm with varied external voltage; when it varies from 3 to 8 V, the 1/e² radius of the spot decreases to 1.3 mm, 0.27 times the initial state, and the spot central intensity magnification increases gradually with the change, up to 3.31 times. The acquisition of the large tunable focal length range of the continuous terahertz zoom device shows that the construction of the gradient refractive index is an important method to regulate the terahertz wavefront by optical means, which greatly promotes the research of terahertz imaging devices.

Spherical Aberration-Corrected Metalens for Polarization Multiplexed Imaging

We present a terahertz spherical aberration-corrected metalens that uses the dynamic phase to achieve polarization multiplexed imaging. The designed metalens has polarization-dependent imaging efficiencies and polarization extinction ratios that exceed 50% and 10:1, respectively. Furthermore, opposite gradient phases can be applied to orthogonal polarizations to shift the imaging of the two polarized sources in the longitudinal and transverse directions. Indeed, we find that the metalens has a smaller depth-of-focus than a traditional metalens when imaging point sources with limited objective lengths. These results provide a new approach for achieving multifunctional beam steering, tomographic imaging and chiroptical detection.

Metalens mounted on a resonant tunneling diode for collimated and directed terahertz waves

Refraction in materials is a fundamental phenomenon in optics and is a factor in the manipulation of light, such as wavefront shaping and beam control. However, conventional optical lenses incorporated in numerous optical sources are made of naturally occurring materials, and material properties predetermine the lens performance. For the development of terahertz flat optics, we experimentally demonstrate a gradient-refractive-index (GRIN) collimating metalens made of our original reflectionless metasurface with an extremely high refractive index, above 10 at 0.312 THz. The planar collimating metalens converts wide-angle radiation from a resonant tunneling diode (RTD) to a collimated plane wave and enhances the directivity of a single RTD 4.2 times. We also demonstrate directional angle control of terahertz waves by moving the metalens in parallel with the incoming wave. The metalens can be simply integrated with a variety of terahertz continuous-wave (CW) sources for 6G (beyond 5G) wireless communications and imaging in future advanced applications. Flat optics based on high refractive index metasurfaces rather than naturally occurring materials can offer an accessible platform for optical devices with unprecedented functionalities.

Terahertz focusing metalens of reflectionless meta-atoms with negative refractive indices

Terahertz continuous-wave (CW) sources oscillating around the 1.0 THz band at room temperature have rapidly been developed to bridge the terahertz gap. However, reflectionless metasurfaces suitable for integration with terahertz CW sources as optical components have yet to be developed in the terahertz gap. Here, we propose a terahertz-focusing metalens consisting of reflectionless meta-atoms with a discrete distribution of negative refractive indices from ${-}{1.1}$ to ${-}{2.8}$. The proposed 2D gradient-refractive-index metalens converts an incident terahertz Gaussian beam to a line focus. We also experimentally demonstrate a metasurface of reflectionless meta-atoms with a negative refractive index of ${-}{2.8}$ adopted in the periphery of the metalens. The reflectionless metasurface in the terahertz gap would be a welcome contribution to the rapid growth of terahertz industrial applications with terahertz CW sources. Further, the design approach based on reflectionless meta-atoms with negative refractive indices could be applied to various 2D planar optical components with attractive functionalities such as collimating, arbitrary wavefront shaping, and light vortices.

Focusing and imaging of a polarization-controlled bifocal metalens

Metalenses are a kind of flat optical device, which consist of an array of nanoantennas with subwavelength thickness that manipulates the incoming light wavefront in a precisely tailorable manner. In this work, we proposed a bifocal metalens that can realize switchable multiplane imaging, controlled by changing the polarization state of an incident light. The polarization-dependent metalens was designed and fabricated by arranging polysilicon nanobeam unit elements. We simulated and experimentally characterized the focus performance of the bifocal metalens. Under the light incidence with left-handed circular polarization, the focal length is 250 µm. By changing the polarization state to right-handed circular polarization, the focal length is tuned to 200 µm. Experimental results and numerical simulations are in good agreement. Moreover, when a linear polarization light is used, two focal spots will appear at the same time. Such a bifocal metalens is suitable for multiplane imaging applications.

The Magic of Optics-An Overview of Recent Advanced Terahertz Diffractive Optical Elements

Diffractive optical elements are well known for being not only flat but also lightweight, and are characterised by low attenuation. In different spectral ranges, they provide better efficiency than commonly used refractive lenses. An overview of the recently invented terahertz optical structures based on diffraction design is presented. The basic concepts of structure design together with various functioning of such elements are described. The methods for structure optimization are analysed and the new approach of using neural network is shown. The paper illustrates the variety of structures created by diffractive design and highlights optimization methods. Each structure has a particular complex transmittance that corresponds to the designed phase map. This precise control over the incident radiation phase changes is limited to the design wavelength. However, there are many ways to overcome this inconvenience allowing for broadband functioning.

Terahertz smart dynamic and active functional electromagnetic metasurfaces and their applications

The demand for smart and multi-functional applications in the terahertz (THz) frequency band, such as for communication, imaging, spectroscopy, sensing and THz integrated circuits, motivates the development of novel active, controllable and informational devices for manipulating and controlling THz waves. Metasurfaces are planar artificial structures composed of thousands of unit cells or metallic structures, whose size is either comparable to or smaller than the wavelength of the illuminated wave, which can efficiently interact with the THz wave and exhibit additional degrees of freedom to modulate the THz wave. In the past decades, active metasurfaces have been developed by combining diode arrays, two-dimensional active materials, two-dimensional electron gases, phase transition material films and other such elements, which can overcome the limitations of conventional bulk materials and structures for applications in compact THz multi-functional antennas, diffractive devices, high-speed data transmission and high-resolution imaging. In this paper, we provide a brief overview of the development of dynamic and active functional electromagnetic metasurfaces and their applications in the THz band in recent years. Different kinds of active metasurfaces are cited and introduced. We believe that, in the future, active metasurfaces will be combined with digitalization and coding to yield more intelligent metasurfaces, which can be used to realize smart THz wave beam scanning, automatic target recognition imaging, self-adaptive directional high-speed data transmission network, biological intelligent detection and other such applications. This article is part of the theme issue 'Advanced electromagnetic non-destructive evaluation and smart monitoring'.

Multi-foci metalens for terahertz polarization detection

We propose a reflective terahertz (THz) metalens with four focal points for polarization detection of THz beams. The metalens is composed of Z-shaped resonators with spatially variant orientations, a reflective gold layer, and a dielectric spacer between them. The polarization states of the focal points include left circular polarization, right circular polarization, an incident polarization state, and a polarization state whose major axis is rotated π/4 in comparison with that of the incident polarization. The handedness, ellipticity, and major axis of the polarization state can be determined based on the light intensities of the focal points. The uniqueness of the designed device renders this technique very attractive for applications in compact THz polarization detection and information processing.

Broadband achromatic metalens in terahertz regime

Achromatic focusing is essential for broadband operation, which has recently been realised from visible to infrared wavelengths using a metasurface. Similarly, multi-terahertz functional devices can be encoded in a desired metasurface phase profile. However, metalenses suffer from larger chromatic aberrations because of the intrinsic dispersion of each unit element. Here, we propose an achromatic metalens with C-shaped unit elements working from 0.3 to 0.8 THz with a bandwidth of approximately 91% over the centre frequency. The designed metalens possesses a high working efficiency of more than 68% at the peak and a relatively high numerical aperture of 0.385. We further demonstrate the robustness of our C-shaped metalens, considering lateral shape deformations and deviations in the etching depth. Our metalens design opens an avenue for future applications of terahertz meta-devices in spectroscopy, time-of-flight tomography and hyperspectral imaging systems.

Sub-wavelength tight-focusing of terahertz waves by polarization-independent high-numerical-aperture dielectric metalens

A focusing device is one of the key elements for terahertz applications, including homeland security, medicine, industrial inspection, and other fields. Sub-wavelength tight-focusing of terahertz waves is attractive for microscopy and spectroscopy. Flat optical lenses based on metasurfaces have shown potential in diffraction-limit focusing and advantages of ultrathin thickness and lightweight for large-aperture optics. However previously reported THz metalenses suffered from either polarization-dependency or small numerical aperture (NA), which greatly limits their focusing performance. In this paper, to achieve high-NA and polarization-free operation, we proposed a polarization-independent dielectric metasurface with a sub-wavelength period of 0.4λ. A planar terahertz lens based on such metasurface was designed for a wavelength of λ = 118.8 μm with a focal length of 100λ, a radius of 300λ, and a high NA of 0.95, which was fabricated with a silicon-on-insulator wafer. The experimental results demonstrate a tight focal spot with sub-wavelength full widths at half-maxima of 0.45λ and 0.61λ in the x and y directions, respectively, on the focal plane. In the x direction, the size of 0.45λ is even smaller than the diffraction limit 0.526λ (0.5λ/NA). Such a metalens is favorable for sub-wavelength tight-focusing terahertz waves with different polarizations, due to its polarization independence. The metalens has potential applications in THz imaging, spectroscopy, information processing, and communications, among others.