Recent studies on fully dielectric multilayered metamaterials have shown that the negligibly small nonlocal effects (spatial dispersion) typically observed in the limit of deeply subwavelength layers may be significantly enhanced by peculiar boundary effects occurring in certain critical parameter regimes. These phenomena, observed so far in periodic and randomly disordered geometries, are manifested as strong differences between the exact optical response of finite-size metamaterial samples and the prediction from conventional effective-theory-medium models based on mixing formulae. Here, with specific focus on the Thue-Morse geometry, we make a first step toward extending the studies above to the middle-ground of aperiodically ordered multilayers, lying in between perfect periodicity and disorder. We show that, also for these geometries, there exist critical parameter ranges that favor the buildup of boundary effects leading to strong enhancement of the (otherwise negligibly weak) nonlocality. However, the underlying mechanisms are fundamentally different from those observed in the periodic case, and exhibit typical footprints (e.g., fractal gaps, quasi-localized states) that are distinctive of aperiodic order. The outcomes of our study indicate that aperiodic order plays a key role in the buildup of the aforementioned boundary effects, and may also find potential applications to optical sensors, absorbers and lasers. * vgaldi@unisannio.it 1 arXiv:1811.01258v1 [physics.optics]

The emerging fields of non-Hermitian optics and photonics are inspiring radically new, unconventional ways of mixing active and passive constituents to attain exotic light–matter interactions. Here, inspired by the concept of parity-time symmetry, we propose and explore a class of non-Hermitian multilayered metamaterials, featuring spatial modulation of gain and loss, which can exhibit extreme anisotropy in the epsilon-near-zero regime. Specifically, via analytic and numerical studies, we investigate the intriguing parameter tunability and wave-propagation effects that can occur in these media due to the delicate interplay between gain and loss. These include, for instance, field canalization, subdiffractive imaging, and reconfigurable waveguiding/radiation, and remarkably, they do not rely on the presence of metallic constituents. Moreover, we show that the extreme-parameter regime of interest is technologically feasible, e.g., in terms of material constituents based on dye-doped indium tin oxide at near-infrared wavelengths. Our outcomes bring about new, largely unexplored dimensionalities and possibilities in the tailoring of the effective properties of non-Hermitian metamaterials and open the door to a wealth of possible developments and applications in reconfigurable nanophotonics scenarios.

It is common understanding that multilayered dielectric metamaterials, in the regime of deeply subwavelength layers, are accurately described by simple effective-medium models based on mixing formulas that do not depend on the spatial arrangement. In the wake of recent studies that have shown counterintuitive examples of periodic and aperiodic (orderly or random) scenarios in which this premise breaks down, we study here the effects of deterministic disorder. With specific reference to a model based on Golay-Rudin-Shapiro sequences, we illustrate certain peculiar boundary effects that can occur in finite-size dielectric multilayers, leading to anomalous light-transport properties that are in stark contrast with the predictions from conventional effective-medium theory. Via parametric and comparative studies, we elucidate the underlying physical mechanisms, also highlighting similarities and differences with respect to previously studied geometries. Our outcomes may inspire potential applications to optical sensing, switching and lasing.

In solid-state physics, “doping” is a pivotal concept that allows controlling and engineering of the macroscopic electronic and optical properties of materials such as semiconductors by judiciously introducing small concentrations of impurities. Recently, this concept has been translated to two-dimensional photonic scenarios in connection with host media characterized by vanishingly small relative permittivity (“epsilon near zero”), showing that it is possible to obtain broadly tunable effective magnetic responses by introducing a single, nonmagnetic doping particle at an arbitrary position. So far, this phenomenon has been studied mostly for lossless configurations. In principle, the inevitable presence of material losses can be compensated via optical gain. However, taking inspiration from quantum (e.g., parity−time) symmetries that are eliciting growing attention in the emerging fields of non-Hermitian optics and photonics, this suggests considering more general gain−loss interactions. Here, we theoretically show that the photonic doping concept can be extended to non-Hermitian scenarios characterized by tailored distributions of gain and loss in either the doping particles or the host medium. In these scenarios, the effective permeability can be modeled as a complex-valued quantity (with the imaginary part accounting for the gain or loss), which can be tailored over broad regions of the complex plane. This enables a variety of unconventional optical responses and waveguiding mechanisms, which can be, in principle, reconfigured by varying the optical gain (e.g., via optical pumping). We envision several possible applications of this concept, including reconfigurable nanophotonics platforms and optical sensing, which motivate further studies for their experimental validation.

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