Abstract
Metamaterials and metasurfaces are designed by spatially arranging (periodically or non-periodically) subwavelength geometries, allowing a tailored manipulation of the electromagnetic response of matter. Here, we exploit temporal variations of permittivity inside subwavelength geometries to propose the concept of spatiotemporal meta-atoms having time-dependent properties. We exploit isotropic-to-anisotropic temporal boundaries within spatially subwavelength regions where their permittivity is rapidly changed in time. In so doing, it is shown how resulting scattered waves travel in directions that are different from the direction of the impinging wave, and depend on the values of the chosen anisotropic permittivity tensor. To provide a full physical insight of their performance, multiple scenarios are studied numerically such as the effect of using different values of permittivity tensor, different geometries of the spatiotemporal meta-atom and time duration of the induced isotropic-to-anisotropic temporal boundary. The intrinsic asymmetric response of the proposed spatiotemporal meta-atoms is also studied demonstrating, both theoretically and numerically, its potential for an at-will manipulation of scattered waves in real time. These results may open new paradigms for controlling wave–matter interactions and may pave the way for the next generation of metamaterials and metasurfaces by unleashing their potential using four-dimensional unit cells.
Highlights
Metamaterials and metasurfaces have opened new ways to control and manipulate fields and waves at-will, and have been a hot research topic given their ability to produce artificially engineered media with electromagnetic (EM) properties not available in nature [1,2,3,4]
[∼11.3◦ to ∼76◦] or [∼23.2◦ to ∼83.4◦] when using the same range of εr2z and values of θ1, respectively, by reducing εr2x to εr2x = 5 (see blue lines in figures 2(d)–(f)). Note that this approach, which we have proposed and demonstrated numerically in spatially unbounded temporal metamaterials, is valid for the spatiotemporal meta-atom shown in figure 1(a) where, as it will be shown the newly emitted wave, resulted from applying the first isotropic-to-anisotropic temporal change of εr2, will have the same angle defined by equation (1)
We evaluate the performance of the proposed 2D spatiotemporal meta-atom considering different 2D subwavelength geometries, namely cylinders and squares, of diameter/lateral size of l λ (l = 0.1λ, with λ being the wavelength of the incident wave inside the background medium)
Summary
Metamaterials and metasurfaces (as their 2D version) have opened new ways to control and manipulate fields and waves at-will, and have been a hot research topic given their ability to produce artificially engineered media with electromagnetic (EM) properties not available in nature [1,2,3,4] They have been proposed and experimentally demonstrated in multiple frequency ranges including radio frequencies, microwave and millimeter up to the optical regime [5,6,7,8,9,10] and they have been used in groundbreaking applications such as sensors [11,12,13], quantum devices and technologies [14], antennas and lenses [15,16,17,18,19,20,21,22,23,24], beam steerers [25,26,27], tunable metamaterials and surfaces [28,29,30,31], optical circuits [32, 33], analogue computing [34,35,36] and parity-time-symmetric systems [37, 38], to name a few.
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