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The advent of ultrathin, dynamically tunable materials has sparked unprecedented interest in the possibility of extending the metamaterial concept to the temporal domain. In electromagnetics, near-zero-index materials offer giant nonlinearities in the near-visible, whereas at mid-IR and below layered materials such as graphene enable efficient modulation of their carrier density. Fuelled by the interest in applications such as nonreciprocity, as well as new amplification and harmonic generation schemes, the concept of time-varying media promises to open new territories across all wave realms. In this contribution, we present a range of avenues opened by temporal inhomogeneities for the trapping, dragging and boosting of electromagnetic waves.
We first present a new strategy for coupling free-space radiation to surface waves. The evanescent character of surface waves implies that their excitation from free space demands the breaking of momentum conservation, and hence the engineering of spatial surface-inhomogeneities. We propose a completely new pathway towards efficient excitation of surface waves, based on the breaking of temporal symmetry. By proposing and modelling a realistic graphene experiment, we demonstrate that surface waves can be excited without the need for any surface inhomogeneities, by modulating in time the carrier density of a material.
We then demonstrate how the conventional concept of motion may be extended by considering spatiotemporal modulation of electromagnetic parameters. We first show how a generalised form of optical drag can be achieved by engineering a travelling-wave modulation of both dielectric and magnetic response parameters. Remarkably, this novel optical drag is not subject to the conventional limitations imposed by special relativity on moving media, as the modulation can be induced by an external pump to sweep the material at superluminal velocities.
When the phase velocity of this travelling-wave modulation approaches the speed of the waves in the pristine medium, light undergoes a localisation transition: waves are trapped within each period of this synthetically moving grating. We conclude by introducing a comprehensive theory describing how how such “luminal media” can grab electromagnetic field lines and compress them, while simultaneously amplifying them, and discuss potential surface-wave implementations.
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