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Register a new userSurface-Wave Propagation on Non-Hermitian Metasurfaces with Extreme Anisotropy
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Electromagnetic metasurfaces enable the advanced control of surface-wave propagation by spatially tailoring the local surface reactance. Interestingly, tailoring the surface resistance distribution in space provides new, largely unexplored degrees of freedom. Here, we show that suitable spatial modulations of the surface resistance between positive (i.e., loss) and negative (i.e., gain) values can induce peculiar dispersion effects, far beyond a mere compensation. Taking inspiration from the parity-time symmetry concept in quantum physics, we put forward and explore a class of non-Hermitian metasurfaces that may exhibit extreme anisotropy mainly induced by the gain-loss interplay. Via analytical modeling and full-wave numerical simulations, we illustrate the associated phenomenon of surface-wave canalization, explore nonlocal effects and possible departures from the ideal conditions, and address the feasibility of the required constitutive parameters. Our results suggest intriguing possibilities to dynamically reconfigure the surface-wave propagation, and are of potential interest for applications to imaging, sensing and communications.
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Non-Hermitian systems characterized by suitable spatial distributions of gain and loss can exhibit spectral singularities in the form of zero-width resonances associated to real-frequency poles in the scattering operator. Here, we study this intriguing phenomenon in connection with cylindrical geometries, and explore possible applications to controlling and tailoring in unconventional ways the scattering response of sub-wavelength and wavelength-sized objects. Among the possible implications and applications, we illustrate the additional degrees of freedom available in the scattering-absorption-extinction tradeoff, and address the engineering of zero-forward-scattering, transverse scattering, and gain-controlled reconfigurability of the scattering pattern, also paying attention to stability issues. Our results may open up new vistas in active and reconfigurable nanophotonics platforms.
We introduce the concept of nonlinear graphene metasurfaces employing the controllable interaction between a graphene layer and a planar metamaterial. Such hybrid metasurfaces support two types of subradiant resonant modes, asymmetric modes of structured metamaterial elements (metamolecules) and graphene plasmons exhibiting strong mutual coupling and avoided dispersion crossing. High tunability of graphene plasmons facilitates strong interaction between the subradiant modes, modifying the spectral position and lifetime of the associated Fano resonances. We demonstrate that strong resonant interaction, combined with the subwavelength localization of plasmons, leads to the enhanced nonlinear response and high efficiency of the second-harmonic generation.
Artificial magnetic fields are revolutionizing our ability to manipulate neutral particles, by enabling the emulation of exotic phenomena once thought to be exclusive to charged particles. In particular, pseudo-magnetic fields generated by nonuniform strain in artificial lattices have attracted considerable interest because of their simple geometrical origin. However, to date, these strain-induced pseudo-magnetic fields have failed to emulate the tunability of real magnetic fields because they are dictated solely by the strain configuration. Here, we overcome this apparent limitation for polaritons supported by strained metasurfaces, which can be realized with classical dipole antennas or quantum dipole emitters. Without altering the strain configuration, we unveil how one can tune the pseudo-magnetic field by modifying the electromagnetic environment via an enclosing photonic cavity which modifies the nature of the interactions between the dipoles. Remarkably, due to the competition between short-range Coulomb interactions and long-range photon-mediated interactions, we find that the pseudo-magnetic field can be entirely switched off at a critical cavity height for any strain configuration. Consequently, by varying only the cavity height, we demonstrate a tunable Lorentz-like force that can be switched on/off and an unprecedented collapse and revival of polariton Landau levels. Unlocking this tunable pseudo-magnetism for the first time poses new intriguing questions beyond the paradigm of conventional tight-binding physics.
We show that a cylindrical lensing system composed of two metasurfaces with suitably tailored non-Hermitian (i.e., with distributed gain and loss) and nonlocal (i.e., spatially dispersive) properties can perform magnified imaging with reduced aberrations. More specifically, we analytically derive the idealized surface-impedance values that are required for perfect magnification and imaging, and elucidate the role and implications of non-Hermiticity and nonlocality in terms of spatial resolution and practical implementation. For a basic demonstration, we explore some proof-of-principle quasi-local and multilayered implementations, and independently validate the outcomes via full-wave numerical simulations. We also show that the metasurface frequency-dispersion laws can be chosen so as to ensure unconditional stability with respect to arbitrary temporal excitations. These results, which extend previous studies on planar configurations, may open intriguing venues in the design of metastructures for field imaging and processing.
Fundamental interactions induced by lattice vibrations on ultrafast time scales become increasingly important for modern nanoscience and technology. Experimental access to the physical properties of acoustic phonons in the THz frequency range and over the entire Brillouin zone is crucial for understanding electric and thermal transport in solids and their compounds. Here, we report on the generation and nonlinear propagation of giant (1 percent) acoustic strain pulses in hybrid gold/cobalt bilayer structures probed with ultrafast surface plasmon interferometry. This new technique allows for unambiguous characterization of arbitrary ultrafast acoustic transients. The giant acoustic pulses experience substantial nonlinear reshaping already after a propagation distance of 100 nm in a crystalline gold layer. Excellent agreement with the Korteveg-de Vries model points to future quantitative nonlinear femtosecond THz-ultrasonics at the nano-scale in metals at room temperature.
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