2. Nonreciprocal and Active Metamaterials

A particularly active area of recent research progress in the context of metamaterials and metasurfaces, with important opportunities for the next generation of communications technology, is offered by the possibility of breaking reciprocity in compact, ultrathin devices. Reciprocity is the symmetry relations that governs propagation of waves between two points in space. If a radio-wave signal can travel from point A to B, it can travel back with similar properties from B to A. Breaking this symmetry is particularly useful for communication systems: it enables one-way transmission of signals that avoids self-interference and protects sources, and it can be used for implementing full-duplex communications with significant advantages in terms of spectrum efficiency.

Since reciprocity is rooted in the time-reversal symmetry of Maxwell’s equations, it can be broken using a dc bias to the involved materials, imparting spatio-temporal modulation of the channel properties, or through nonlinearities combined with geometrical asymmetries [4]. The most common way of breaking reciprocity is through a dc magnetic bias applied to ferrites or garnets that support magneto-optical phenomena. While this approach is effective in realizing isolators and circulators commonly used in radar and antenna systems, the resulting devices are bulky, expensive and heavy, not amenable for integration. This is why there has been a recent concerted effort to employ metamaterials to realize magnet-free devices that may lead to the next generation of compact, integrated nonreciprocal technologies, with performance metrics comparable and even superior to their magnetized counterparts, but at a fraction of the cost, weight and form factor [5]. Fig. 2 shows recent examples of printed circuits and integrated circuit technologies that implement isolators, circulators and nonreciprocal antennas, opening new opportunities for a broad range of applications, from wireless communications to automotive technologies and quantum computing, which cannot afford magnetic approaches to breaking reciprocity, but that would largely benefit from the implementation of these functionalities in a compact, integrable form factor [4-5].

These technologies exploit modulation and switching schemes to implement tailored spatio-temporal modulation patterns that replace magneto-optical phenomena in a compact device for the purpose of breaking reciprocity, opening truly exciting directions for various technologies. Interestingly, these compact nonreciprocal elements can then also be employed as building blocks of new metamaterials and metasurfaces, with yet even more unusual wave manipulation properties. In metasurfaces, this technology can manipulate the impinging wavefront in nonreciprocal waves, implementing free-space isolation and nonreciprocal radiation [6], while for guided waves arrays of magnet-free subwavelength nonreciprocal elements can realize the electromagnetic version of topological insulators, which provide unusual robustness to disorder, broad bandwidths, and robust multiplexing opportunities [7].

Temporal modulations imparted over metamaterials offer also other interesting opportunities beyond breaking reciprocity. Parametric phenomena enable gain that may pump the impinging signals and compensate insertion loss over a low-noise platform. In addition, suitably tailored spatio-temporal modulations can overcome the limitations of passive, linear, time-invariant devices, with important implications for communications technology. As examples, our group has recently shown that time-modulated antennas may overcome the Chu limit, which conventionally enforces a stringent trade-off between size and bandwidth of an electrically small antenna [8], and that time modulated circuit networks can overcome the trade-off between delay and bandwidth [9], ideally suited for true time delay applications. In this context, the N-path filter platform [10], formed by CMOS compatible time-commutated networks of integrated switches and capacitors, provides an ideal technology to enable many of these principles, and it is an outstanding and mostly untapped opportunity to integrate these concepts into metamaterials and metasurfaces.

The next avenue in this context consists in the possibility of leveraging time modulations to impart reprogrammability and reconfigurability on the fly, with also the option of active feedback and control, based on which active and time-varying metamaterials can reprogram their own function in response to external stimuli or changes in the environment.

3. Conclusions

The field of metamaterials has been progressing at a fast pace in the past years, showing that the conventional limitations of modern technology can be pushed forward and overcome using advanced electromagnetic concepts and nanofabrication techniques. It is the right moment for exploring these features in the context of next-generation communication systems. We may envision smart metasurfaces that can process the impinging signals, focus, steer and route them in desired directions and self-reconfigure their operations on the fly as the environment changes, all with strong robustness to noise and disorder. The implementation of these surfaces can be achieved with standard lithographic or 3D printing techniques. Still of some challenge remains the implementation of reconfigurable and time varying elements in metasurfaces, which may require complex control networks and advanced circuit layouts, especially in the case of fast modulation speeds. Discussions and collaborations between the communications and metamaterial communities may be extremely fruitful to push forward these avenues and converge on a technology platform that holds the potential to revolutionize several application areas for next-generation communication systems.

Acknowledgements

Our work on metamaterials has been funded over the years by the Department of Defense, the National Science Foundation, the Simons Foundation, among several other funding agencies, foundations and companies.

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