Metamaterials Embrace the Temporal Dimension [Special Series: Guest Editorial]

Over the past 20 years, metamaterials have evolved into a powerful platform for wave manipulation, demonstrating a plethora of exciting functionalities relevant to a wide range of technologies. Yet, most of the work on metamaterials to date has been limited to carefully patterning materials in space, enabled by advances in micro- and nano-fabrication and in the understanding of complex wave-matter interactions at subwavelength scales. The complex wave transformations enabled by metamaterials are typically associated with multiple scattering events, driving complex interference and resonance phenomena associated with the multiple spatial interfaces that form their microstructure. Spatial metamaterials are, however, fundamentally limited in the way they can transform and control waves; in the linear regime, the frequency content of the incoming waves is conserved, and time-reversal symmetry, reciprocity, and passivity impose further constraints
[1]
. Recent developments have demonstrated that many of these limitations may be bypassed by adding time as a relevant “fourth” dimension—beyond the three spatial ones—in the design of metamaterials. For instance, tailored time variations can break time-reversal symmetry and reciprocity
[2]
, and they can transform frequencies and impart gain through parametric effects. These concepts have recently been converging into a new paradigm for metamaterials in which space and time can be combined together to drastically expand the palette of degrees of freedom available for wave control
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.