We show that by coiling up space using curled perforations, a two-dimensional acoustic metamaterial can be constructed to give a frequency dispersive spectrum of extreme constitutive parameters, including double negativity, a density near zero, and a large refractive index. Such an approach has band foldings at the effective medium regime without using local resonating subwavelength structures, while the principle can be easily generalized to three dimensions. Negative refraction with a double negative prism and tunneling with a density-near-zero metamaterial are numerically demonstrated.

Metamaterials are effectively homogeneous materials that display extraordinary dispersion. Negative index metamaterials, zero index metamaterials and extremely anisotropic metamaterials are just a few examples. Instead of using locally resonating elements that may cause undesirable absorption, there are huge efforts to seek alternative routes to obtain these unusual properties. Here, we demonstrate an alternative approach for constructing metamaterials with extreme dispersion by simply coiling up space with curled channels. Such a geometric approach also has an advantage that the ratio between the wavelength and the lattice constant in achieving a negative or zero index can be changed in principle. It allows us to construct for the first time an acoustic metamaterial with conical dispersion, leading to a clear demonstration of negative refraction from an acoustic metamaterial with airborne sound. We also design and realize a double-negative metamaterial for microwaves under the same principle.

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Inspired by recent demonstrations of metasurfaces in achieving reduced versions of electromagnetic cloaks, we propose and experimentally demonstrate source illusion devices to manipulate flexural waves using metasurfaces. The approach is particularly useful for elastic waves due to the lack of form invariance in usual transformation methods. We demonstrate compact and simple-to-implement metasurfaces for shifting, transforming, and splitting a point source. The effects are measured to be broadband and robust against a change of source positions, with agreement from numerical simulations and the Huygens-Fresnel theory. The proposed method is potentially useful for applications such as nondestructive testing, high-resolution ultrasonography, and advanced signal modulation.

We propose a silicon hollow nanodisk for enhancing magnetic dipole (MD) emission. The Purcell factor can be more than 300, which is one order of magnitude larger than the silicon nanosphere case. It is demonstrated that the silicon hollow nanodisk resembles the function of an azimuthally polarized beam for tailoring the magnetic and electric dipole (ED) emission. It is shown that MD emission can be significantly enhanced, while ED emission will be suppressed when emitters are located in the hollow of the nanodisk. The dependence of the Purcell factor on the geometry parameters is also studied. Our results might facilitate the on-chip engineering of magnetic light emission.

Bianisotropy is common in electromagnetics whenever a cross-coupling between electric and magnetic responses exists. However, the analogous concept for elastic waves in solids, termed as Willis coupling, is more challenging to observe. It requires coupling between stress and velocity or momentum and strain fields, which is difficult to induce in non-negligible levels, even when using metamaterial structures. Here, we report the experimental realization of a Willis metamaterial for flexural waves. Based on a cantilever bending resonance, we demonstrate asymmetric reflection amplitudes and phases due to Willis coupling. We also show that, by introducing loss in the metamaterial, the asymmetric amplitudes can be controlled and can be used to approach an exceptional point of the non-Hermitian system, at which unidirectional zero reflection occurs. The present work extends conventional propagation theory in plates and beams to include Willis coupling, and provides new avenues to tailor flexural waves using artificial structures.

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We introduce a field transformation method for wave manipulation based on completely reciprocal and passive materials. While coordinate transformations in transformation optics (TO) change the size and shape of an object, field transformations give us direct control on the impedance and polarization signature of an object. Using our approach, a new type of perfect conductor can be realized to completely convert between transverse electric and transverse magnetic polarizations at any incidence angles and a perfect magnetic conductor of arbitrary shape can be mimicked by using anisotropic materials. The approach can be further combined with TO to enhance existing TO devices. For example, a dielectric cylinder can become completely transparent for both polarizations using bianisotropic materials.