Edited by: Sreekanth Kandammathe Valiyaveedu, Nanyang Technological University, Singapore
Reviewed by: Jining Li, Tianjin University, China; Yogesh Kumar Srivastava, Nanyang Technological University, Singapore
This article was submitted to Optics and Photonics, a section of the journal Frontiers in Physics
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
Metasurfaces, composed of prearranged artificial unit cells possessing different electromagnetic (EM) responses, provide unprecedented abilities to realize versatile wave manipulations. Especially in the terahertz spectrum, metasurfaces attract broad attention by opening up further possibilities for wave regulating. Terahertz applications in various fields, for instance, spatial light modulation (SLM), radar, imaging, time-domain spectroscopy (TDS), and high-speed communication, have been facilitated and improved. In this article, we first give a simple review on recent advances on terahertz metasurfaces, including discussion of passive metasurfaces with fixed structures and active metasurfaces integrated with tunable components. Then, we briefly review the development of coding metasurfaces and programmable metasurfaces represented by digitized bits. We mainly focus on some powerful functions, functional multiplexing, and real-time controlled applications in terahertz frequencies. Finally, we give an abbreviated overview of developing terahertz multifunctional metasurfaces and programmable metasurfaces.
Terahertz spectrum, where frequency covers 0.1 to 100 THz (wavelength ranges from 3 μm to 3 mm), has conspicuous distinctions with microwave, millimeter-wave, and optical field. Due to the inclusion of characteristic frequencies of most biological and chemical molecules, abundant biological and material information can be detected [
Different from metamaterial that is characterized under effective medium theory with retrieved permittivity and permeability [
More recently, multifunctional metasurface, coding metasurface, and programmable metasurface turn into research hotspots because of demands of device integration and real-time control. The multifunctional feature gives rise to the advantage of compactness, which may contribute to promoting the miniaturization of terahertz devices or systems. Coding metasurface, of which EM responses are characterized by digitalized bits, provides a direct joint between digital sequences and metasurface. Programmable metasurface is an extended form of coding metasurface, which can activate tunable elements by importing digital coding sequence in the real-time manner, thus realizing dynamic switching of functions. In this article, we intend to review metasurfaces reacting in the terahertz spectrum, of which multifunctional and programmable properties are particularly focused and further regarded as a develop prospect. The organization of this review is as follows. In section Passive Metasurfaces for Terahertz Manipulation, we briefly retrospect several passive metasurfaces aimed for terahertz wave regulating and functional versatility. In section Active Metasurfaces for Terahertz Manipulation, we introduce some terahertz metasurfaces combining activated items to realize tunable responses, while dynamically functional multiplexing is included. In section Coding and Programmable Metasurfaces for Terahertz Manipulation, we review the development of coding metasurfaces and subsequent programmable metasurfaces by listing certain remarkable achievements and real-time controlled cases in terahertz category. In conclusion, we summarize the review along with our prospection on further directions of terahertz metasurfaces with multifunctionality and instantaneity.
In contrast to traditional passive terahertz devices, metasurfaces possess superiorities like ultra-thin profile, remarkable lightness, easy fabrication, acceptable loss, and the tremendous potential to manipulate terahertz waves freely. Up to now, passive metasurfaces have been utilized to achieve beam deflection [
Beam deflection is common and easy to achieve by using conventional technics but need thick terahertz/optical elements or complex light paths. Fortunately, metasurface can be adopted instead to regulate incident waves to expected directions. An external wave vector is applied to the transmitted or reflected wave by etching periodically metasurface unit cells, which realizes the beam deflection effect. According to the generalized Snell's law, a desired diffracting angle θ
in which
where θ
Schematic diagrams of generalized Snell's law.
To achieve a complex EM pattern, the phase distribution, amplitude distribution, or polarization conversion will be more variable than that of beam deflection, and are no longer monotonic gradient situations. Through endowing complex EM responses to the metasurface, optical elements such as deflectors, splitters, and waveplates can be replaced. The system complexity can be reduced and proper shaping effects can be maintained. For example, Lorentz beam can be used in a wide range of scenarios due to its non-diffraction characteristic, yet multiple optical elements are required to generate it. In 2017, Guo et al. demonstrated a radially polarized Lorentz beam under circular polarized incidences in the terahertz spectrum utilizing a single-layer metasurface composed of cross-shaped unit cells [
In line with the pursuit of high integration and miniaturization, multifunctional devices are undoubtedly one of the current research hotspots. Designing functional devices by traditional methods may be impractical or undesirable ascribing to limitations imposed by their own mechanisms. Fortunately, the superb degree of freedom of metasurface makes functional multiplexing a reality. Although the significant feature of the passive metasurface is its fixed structure, functional multiplexing can still be achieved. Multifunctional passive metasurfaces can be divided into frequency multiplexing metasurfaces [
We start from the frequency multiplexing situation. Because some metasurface unit cells can function at more than one frequency band, the finally constructed metasurface can realize multiple effects under an overlapping arrangement. As an example, Wang et al. proposed a multi-color metasurface hologram depending on the phase modulating of C-shaped unit cells effective at different working frequencies [
Furthermore, there are many circumstances belonging to polarization multiplexing. To our knowledge, two principles are commonly employed to design polarization multiplexed metasurfaces, one is Pancharatnam–Berry (PB) phase [
Although functional integration has been realized and widely studied using passive metasurfaces, some intractable situations need active items to perform dynamic control. At microwave frequencies, PIN diodes, varactors, and triodes are widely embraced to provide optional responses under different bias signals, while in the terahertz spectrum, it is difficult to obtain packaged semiconductors owing to the small electrical size, and sophisticated parasitic effects are a formidable challenge. Fortunately, many materials can be utilized as tunable components in terahertz frequencies. Doped semiconductor [
To date, active metasurfaces have been widely used to realize amplitude and/or phase modulation [
Besides, metasurfaces have been utilized to realize dynamic lenses in response to the demands of varifocal applications [
Next, we introduce some metasurfaces that can achieve functional multiplexing dynamically. Making functional multiplexing dynamical can further improve the integration level of various devices and systems, but facing design and manufacturing difficulties is inevitable. In the terahertz and optical spectrum, tiny dimensions and immature technics result in the functional multiplexing facing great difficulties. Although more hardships are confronted compared to merely achieving function tuning, some active metasurfaces with dynamically functional multiplexing have still been implemented [
For instance, Liu et al. have realized a kind of VO2-based metasurface that can be regarded as electrical switching or rewritable memory [
Two concepts of digital characterization of metamaterial were proposed in the same year of 2014, but with essential differences. The concept of “digital metamaterials” [
in which θ and φ are respectively the elevation angle and azimuth angle,
Compared with digital metamaterials, the coding concept is more suitable for simplifying the design of metasurfaces based on abrupt EM responses. More importantly, this concept plays a role in bridging the physical world and the digital world, making programmable metasurface a reality.
In the terahertz spectrum, the coding method has been employed to achieve untrammeled EM manipulations and simplify metasurface designs [
Programmable metasurface, as the active form of coding metasurface, requires flexible unit cells to realize state conversion in a real-time manner. By inputting pre-stored coding sequences, unit cells at specific locations exhibit local EM responses and a global EM response can be timely generated for wave manipulations. In microwave frequencies, programmable metasurfaces have been widely studied and realized. The first microwave programmable metasurface was proposed by Cui et al. [
For example, a MEMS-based programmable metasurface was proposed to perform different logical operations with frequency multiplexing [
As an artificial structure, metasurface exhibits unprecedent capabilities in manipulating wave characteristics, in the microwave, terahertz, and optical frequency bands [
First of all, passive metasurfaces with fascinating functions, especially those with multifunctional properties, are briefly reviewed. Multifunctional characteristic can be acquired by two means, one is frequency multiplexing and the other is polarization multiplexing. Extensive researches on versatility plays an extremely important role in improving device integration. Thereafter, several achievements on active metasurfaces integrated with tunable materials or advanced technics, such as semiconductor, VO2, LC, TCO, graphene, 2DEG, elastomer, perovskite, quantum dot, superconductor, and MEMS, are concisely looked back. Although passive metasurfaces have exhibited high freedom in regulating multiple functions, fixed prototypes make adjustability improbable and limit metasurfaces to restricted scenarios, while active metasurfaces can get rid of this limitation and widen application scopes. In particular, those metasurfaces with dynamically functional multiplexing can offer multifunctional choices in excess of tunability, thus providing the possibility for coexistence of advanced performance and high integration. Finally, some outcomes of coding metasurfaces and programmable metasurfaces are simply retrospect. Characterized by digital bits, coding metasurfaces bond physical EM responses and digital signals without intermediary [
Comparison of several terahertz metasurfaces, including passive metasurfaces, active metasurfaces, coding metasurfaces, and programmable metasurfaces.
Monofunctional passive metasurfaces | Deep subwavelength structure | 0.88 THz | Beam deflection | [ |
Frequency multiplexing metasurfaces | PB phase |
0.5 & 0.63 THz |
Holography |
[ |
Polarization multiplexing metasurfaces | PB phase |
0.6 & 0.8 THz |
Holography |
[ |
Anisotropy | 1 THz | OAM & bessel beam | [ |
|
Active metasurfaces with functional tunability | Superconductor | 0.341 THz | Transmittance modulation | [ |
Perovskite | 0.86 THz/1.12 THz | Transmittance modulation | [ |
|
Quantum dot | 0.5 THz | Transmittance modulation | [ |
|
Graphene | 5 THz | Focus tuning | [ |
|
Active metasurfaces with dynamically functional multiplexing | VO2 |
0.63 & 0.864 THz |
Electrical switching & rewritable memory & ultrafast modulation |
[ |
Coding metasurfaces | 1-bit coding method |
0.8–1.4 THz |
Wave diffusion |
[ |
Programmable metasurfaces | MEMS |
0.56 THz |
Logic operations (XOR, XNOR, PASS & NOT) |
[ |
In summary, multifunctional metasurfaces and programmable metasurfaces play important roles in device integration and information handling, particularly at the fascinating terahertz band. Based on these listed advances, we prospect and highlight a further trend of developing terahertz multifunctional metasurfaces and programmable metasurfaces, which we believe will make unprecedented contributions to the terahertz field.
All authors listed have made a substantial, direct and intellectual contribution to the work, and approved it for publication.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.