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Metamaterials are artificially structured materials designed to control and manipulate physical phenomena such as light and other electromagnetic waves, sound waves and seismic waves in unconventional ways, resulting in exotic behavior that’s not found in nature. This class of micro- and nano-structured artificial media are predicted to be able to protect the building from earthquakes by bending seismic waves around it. Similarly, tsunami waves could be bent around towns, and sound waves could be bent around a room to make it soundproof.
Metamaterials are composed of periodic subwavelength metal/dielectric structures that resonantly couple to the electric and/or magnetic components of the incident electromagnetic fields, exhibiting properties that not found in nature. However, the high losses and strong dispersion associated with the resonant responses and the use of metallic structures, as well as the difficulty in fabricating the micro- and nanoscale 3D structures, have hindered practical applications of metamaterials.
Metasurfaces are thin two-dimensional metamaterial layers that allow or inhibit the propagation of electromagnetic waves in desired directions. For example, metasurfaces have been demonstrated to produce unusual scattering properties of incident plane waves or to guide and modulate surface waves to obtain desired radiation properties. These properties have been employed, for example, to create innovative wireless receivers and transmitters.
The main advantages of metasurfaces with respect to the existing conventional technology include their low cost, low level of absorption in comparison with bulky metamaterials, and easy integration due to their thin profile. Planar metamaterials with subwavelength thickness, or metasurfaces, consisting of single-layer or few-layer stacks of planar structures can be readily fabricated using lithography and nanoprinting methods, and the ultrathin thickness in the wave propagation direction can greatly suppress the undesirable losses.
Metasurfaces have recently been proposed to confine electromagnetic waves, thereby avoiding undesired leakage of energy and increasing the overall efficiency of electromagnetic instruments and devices.
Metasurface based antennas.
In the last few years, metasurfaces have shown possibilities for advanced manipulations of electromagnetic waves, opening new frontiers in the design of antennas. Metasurfaces can be employed to tailor the radiation properties of antennas, their remarkable advantages in comparison with conventional antennas, and the future challenges to be solved.
Conventional approach for antennas results in large and heavy structures, due the bulky reflectors/lenses, gimbals and displaced feed antennas employed.
Recent developments in metasurfaces have opened new opportunities in antenna design. Metasurfaces are surfaces textured at a subwavelength scale to achieve tailored electromagnetic surface properties. They hold promise for the development of low cost, light weight, and compact antennas capable of producing arbitrary aperture fields. Metasurfaces transform wavefronts by imparting field discontinuities across their thin and potentially electrically large and/or conformal surfaces. Their ability to control wavefronts through texture allows for the separation of geometry from electromagnetic functionality.
However, certain challenges lie ahead for their mass deployment. Metasurfaces consist of subwavelength resonators with electromagnetic properties that are often times frequency dispersive. In other words, the resonant nature of a metasurface’s constitutive elements can restrict the usable bandwidth of metasurface-based antennas. Single-layer metasurface antennas with operational bandwidths of up to 8% have been reported. In comparison, a commercially available Ku-band SATCOM antenna covers global receive and transmit bands nearing a total bandwidth of 23%. Furthermore, if simultaneous operation in multiple satellite communication bands is desired with a single antenna, even larger bandwidths or multiband performance are needed.
Recent studies have demonstrated that the properties of metasurfaces are influenced by the symmetries of their constituent elements. Therefore, by controlling the properties of these constitutive elements and their arrangement, one can control the way in which the waves interact with the metasurface. Researchers have analyze the possibilities of combining more than one layer of metasurface, creating a higher symmetry, increasing the operational bandwidth of flat lenses, or producing cost-effective electromagnetic bandgaps.
Reconfigurable and active metasurfaces.
Dynamic metasurfaces are promising new platforms for 5G communications, remote sensing and radar applications. By the insertion of active elements, metasurfaces can break the fundamental limitations of passive and static systems.
Electrically addressable/reconfigurable metasurfaces also pose further challenges. High gain antennas with dynamic beam steering are often needed/desired in wireless applications ranging from satellite, 5G communications to remote sensing, imaging and radar. The introduction of tunability can limit performance (increase loss, limit bandwidth, etc) and result in complex control (tuning, biasing, etc) circuitry.
Electronically tunable Metasurfaces
Researchers at the University of Michigan and City University of New York have recently proposed and experimentally validated a transparent, electronically tunable metasurface. This metasurface, presented in a paper published in Physical Review X, can rotate the polarization of an arbitrarily polarized incident wave without changing its axial ratio.
In recent years, these materials have enabled unprecedented control over electromagnetic waves, opening up interesting possibilities in numerous areas, including wireless communications, imaging and energy harvesting. Despite their numerous advantages, most metasurface designs only offer static functionalities. The new metasurface devised by the researchers, on the other hand, has dynamically tunable properties, and could hence be applied in a broader variety of areas.
“In this work, we integrated tunable devices, varactor diodes, into a metasurface to achieve dynamic control over the metasurface’s response,” Wu explained. “We demonstrated a transparent metasurface with a tunable chiral response, which can rotate the polarization of an incident wave.”
Manipulating the properties of electromagnetic waves (e.g. amplitude, phase and polarization) typically involves a combination of optical components, such as lenses, polarizers and waveplates. The new metasurface introduced by the researchers has a polarization rotator, which consists of a tunable birefringent structure sandwiched between two ±45° rotated metasurface-based quarter-wave plates.
“A conventional tunable polarization rotator is typically multiple wavelengths in size (tens to hundreds) and consists of two rotated quarter wave plates (linear to circular polarizers) placed on either side of a tunable birefringent medium, like a liquid crystal layer,” Anthony Grbic, another researcher who carried out the study, told Phys.org. “Here, we have replaced this bulky device with a cascade of metasurfaces, resulting in a device of subwavelength thickness and equivalent functionality.”
The approach demonstrated by Wu, Grbic and their colleague Younes Ra’di enables ultra-compact designs and could be applied in microwave polarimetric systems for the characterization or detection of objects. For instance, their metasurface-based polarization rotator could be integrated with an antenna element to develop a compact antenna system for polarimetric microwave imaging.
“Our work paves the way for flat/low profile, dynamically tunable antennas and optical/quasioptical systems,” Grbic said. “One can envision replacing bulky electromagnetic or optical setups requiring combinations of conventional components including lenses, tuning elements, phase shifters, spatial light modulators, waveplates, linear polarizers simply with cascaded ultra-thin, tunable metasurfaces.”
The recent study carried out by Wu, Grbic and Ra’di introduced a novel platform that enables full control of the transmitted wave front. The researchers demonstrated their approach by developing a tunable polarization rotator, yet it could also be used to tailor reflected waves. In the future, their method could hence be applied to the design of a tunable metasurface that not only rotates the polarization of the reflected/transmitted wave, but also steers a beam into different directions.
“Our future research plans also include the development stacked metasurface designs for tunable amplitude control, in addition to the phase/polarization control we have demonstrated to date,” Ra’di told Phys.org. “A further goal of ours is to translate such metasurface designs to optical wavelengths.”
An electronically tunable metasurface that rotates polarization
“A few years ago, our research group introduced a pragmatic approach to realizing metasurfaces with tailored bianisotropic responses,” Zhanni Wu, one of the researchers who carried out the study, told Phys.org. “This approach involves cascading patterned (i.e. anisotropic) metallic surfaces across a subwavelength thickness to achieve targeted electric, magnetic and chiral/omega properties. The technique is amenable to planar micro- and nano-fabrication techniques, allowing realization of metasurfaces from RF to visible wavelengths.”
Over the past few years, the same team of researchers designed and realized several metasurfaces, with various functionalities. Although these ultrathin metasurfaces achieved extreme wavefront control, their functionalities remained static and dependent on their fixed geometrical parameters.
“In this work, we integrated tunable devices, varactor diodes, into a metasurface to achieve dynamic control over the metasurface’s response,” Wu explained. “We demonstrated a transparent metasurface with a tunable chiral response, which can rotate the polarization of an incident wave.”
Manipulating the properties of electromagnetic waves (e.g. amplitude, phase and polarization) typically involves a combination of optical components, such as lenses, polarizers and waveplates. The new metasurface introduced by the researchers has a polarization rotator, which consists of a tunable birefringent structure sandwiched between two ±45° rotated metasurface-based quarter-wave plates.
“A conventional tunable polarization rotator is typically multiple wavelengths in size (tens to hundreds) and consists of two rotated quarter wave plates (linear to circular polarizers) placed on either side of a tunable birefringent medium, like a liquid crystal layer,” Anthony Grbic, another researcher who carried out the study, told Phys.org. “Here, we have replaced this bulky device with a cascade of metasurfaces, resulting in a device of subwavelength thickness and equivalent functionality.”
The approach demonstrated by Wu, Grbic and their colleague Younes Ra’di enables ultra-compact designs and could be applied in microwave polarimetric systems for the characterization or detection of objects. For instance, their metasurface-based polarization rotator could be integrated with an antenna element to develop a compact antenna system for polarimetric microwave imaging.
“Our work paves the way for flat/low profile, dynamically tunable antennas and optical/quasioptical systems,” Grbic said. “One can envision replacing bulky electromagnetic or optical setups requiring combinations of conventional components including lenses, tuning elements, phase shifters, spatial light modulators, waveplates, linear polarizers simply with cascaded ultra-thin, tunable metasurfaces.”
The recent study carried out by Wu, Grbic and Ra’di introduced a novel platform that enables full control of the transmitted wave front. The researchers demonstrated their approach by developing a tunable polarization rotator, yet it could also be used to tailor reflected waves. In the future, their method could hence be applied to the design of a tunable metasurface that not only rotates the polarization of the reflected/transmitted wave, but also steers a beam into different directions.
“Our future research plans also include the development stacked metasurface designs for tunable amplitude control, in addition to the phase/polarization control we have demonstrated to date,” Ra’di told Phys.org. “A further goal of ours is to translate such metasurface designs to optical wavelengths.”
Metasurface for Nonlinear Manipulation Could Simplify Wireless Communication
Scientists have developed a metasurface that enables efficient manipulation of spectral harmonic distribution, and have proposed a novel architecture for wireless communication systems based on this time-domain digital coding metasurface. According to the scientists, the metasurface could simplify the architecture of communication systems, while yielding excellent performance for real-time signal transmission. Scientists from the State Key Laboratory of Millimeter Waves, the National Mobil Communication Research Laboratory, and the Photonics Initiative, Advanced Science Research Center located in New York make up the team.
Optical nonlinear phenomena are typically observed in natural materials interacting with light at high intensities, and according to the team, this phenomena can benefit a diverse range of applications, from communication to sensing. However, the team members said, controlling harmonic conversion with high efficiency and flexibility remains a major issue in modern optical and radio-frequency systems.
The scientists found that a metasurface with time-varying reflectivity could respond strongly, in a nonlinear way, to an excitation carrier. The harmonic intensity was dependent on the digital coding sequence of the reflection amplitude and phase. The scientists demonstrated that strong nonlinear processes could be enabled by the temporal modulation of incident waves on the metasurface, with accurate control of both amplitude and phase distributions for all harmonics.
In developing the theory and the design methodology used for the metasurface — and considering its potential for future applications — the team said it was inspired by space-domain digital coding metasurfaces. It employed complex modulation strategies to tailor the wave-matter interactions and frequency spectrum simultaneously. The discrete reflection phase states of the metasurface were controlled by the digital coding sequence.
The metasurface is composed of periodic coding elements loaded with varactor diodes. The scientists said that the metasurface is driven by different combinations of output voltages from a field-programmable gate array (FPGA). By controlling its time-domain digital coding states, the metasurface can be applied to various functions.
As an example of an application, the scientists explored the implementation of a new binary frequency-shift keying (BFSK) wireless communication system, which could simplify the classical heterodyne architectures for wireless network systems. In the BFSK system they propose, the two basic carrier frequencies would be synthesized directly via the metasurface, without the need for a complicated analog-digital converter and microwave filters, mixers, and amplifiers.
The team believes that its proposed concept and method could pave the way for simplified and compact communication and radar systems for a wide frequency range, from acoustic to microwaves and optics. The research was published in National Science Review.
References and Resources also include:
https://iopscience.iop.org/article/10.1088/2040-8986/ab161d/pdf
