Time-Varying Metamaterials: A Novel Paradigm in Modern Science and Technology
Metamaterials are structures composed of artificial particles or “meta-atoms” exhibiting electromagnetic properties that are not available in nature, such as for instance negative refraction or invisibility cloaking. Research on these artificial materials over the past two decades has advanced multiple frontiers of science and the 21st century will likely see them spark a second industrial revolution.
The quasi-totality of the metamaterials reported to date are time-invariant, which means that their meta-atoms and hence their medium susceptibilities are fixed or slowly (adiabatically) varying in time compared to the oscillation periods of the waves they support. Based on this observation, engineer and physicist Christophe Caloz is exploring the uncharted territory of “living metamaterials” or metamaterials that are parametrically excited so as to exhibit temporal variations at sub-period (diabatic) scales.
Such fundamental differences and complementarities suggest a fertile ground for innovation and place spacetime metamaterials in a top position to represent one of the next big innovations in modern science and technology.
The addition of the time (t) dimension to the space (r), temporal frequency (ω) and spatial frequency (k) dimensions of metamaterial susceptibilities leads to a generalized classification of metamaterials in terms of minkowskian spacetime, leading to a vast diversity of novel potential metamaterial structures and exotic spacetime scattering phenomena. For instance, a medium temporal discontinuity is not the sheer dual of a medium spatial discontinuity: the change in k is indeed mapped into a change in ω, but, since causality prevents reflections back to the past, the reflected wave in the first medium is replaced by a contra-directional wave in the later medium, and a time slab supports only four minkowskian waves whereas a space slab supports an infinite number of multiple reflected waves.
Caloz and his team have started to develop spacetime metamaterials constituted of meta-atoms loaded by tiny semiconductor elements such as diodes and transistors, and are considering other implementation possibilities involving novel nanostructured materials. They are considering an array of novel devices, which include spatio-temporal pulse shapers, magnetless nonreciprocal components, temporal photonic crystal amplifiers, electromagnetic wave “freezers”, tunable harmonic generators, electromagnetic calculators, harmonic-free mixers, spacetime diffraction gratings, and superluminal wave deflectors. It is believed that these novel devices will find applications in areas as diverse as information and communication technologies, transportation, aeronautics, astronomy, defense, security, environment, biology and medicine.