Zixian Liang

Space-coiling metamaterials with double negativity and conical dispersion

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.

Tailoring electromagnetically induced transparency for terahertz metamaterials: From diatomic to triatomic structural molecules

The coupling effects in electromagnetically induced transparency (EIT) for triatomic metamaterials are investigated at terahertz (THz) frequencies both experimentally and theoretically. We observed enhancement and cancellation of EIT with single transparency window, and also two additional ways to achieve double EIT transparency windows. One is from the hybridization between two dark atoms in a bright-dark-dark configuration. Another is from an averaged effect between absorption of the additional bright atom and the EIT from the original diatomic molecule in a bright-bright-dark configuration. It allows us to control EIT and the associated slow-light effect for THz metamaterials with high accuracy.

Scaling two-dimensional photonic crystals for transformation optics.

We propose a method to manipulate Bloch waves in curved photonic crystals for achieving transformation optical devices in two dimensions. Instead of starting from an effectively homogeneous medium, we transform a regular photonic crystal into a curved one in the physical space. A scaling law is established to construct the curved photonic crystal with similar unit cells and different scales, which is made of dielectrics only. A wave compressor and a bending waveguide are designed using dielectrics with indices only from 1 to 4. The approach will be useful in constructing low-loss transformation media requiring small indices, or large anisotropy which is particularly difficult for E-polarization using the conventional effective medium approach.

Source illusion devices for flexural Lamb waves using elastic metasurfaces

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.

Bending a periodically layered structure for transformation acoustics

Anisotropic acoustic metamaterials have been proved very useful for their high potential in guiding and manipulating sound energy. In this letter, we further develop the idea by using periodically layered structures for transformational acoustics. Such a simple scheme periodically inserts identically bent solid plates in a background fluid. It forms a metamaterial with high refractive index normal to the curved plates and an index near to one along the plates. We show that the periodically layered structure, combined with transformation approach, can be cut into particular device shapes for acoustic cloaking and illusion.

An acoustic beam shifter with enhanced transmission using perforated metamaterials

We experimentally demonstrate an acoustic beam shifter with enhanced transmission based on subwavelength perforated metamaterials with a wide working frequency range from 2.8 to 4.6 kHz. An oblique perforation angle allows a flexible beam shifting distance and negative refraction for one side of incidence angles. While the beam shifting action is broadband due to the geometric nature of design, beam shifting with enhanced efficiency is found at the frequency with Fabry-Perot (FP) resonance through a two-dimensional pressure field mapping. Such a method in combining extraordinary transmission and beam shifting with properly designed metamaterials, enables designing flexible and also transformation acoustic devices with high transmission efficiency in a general context.

Transformation media with variable optical axes

We introduce a general framework in performing transformation optics by purely rotating the optical axis of the same anisotropic dielectric material on the subwavelength scale. The transformation medium realizes any area-preserving maps with maximum anisotropy only limited by the original material. By applying different optical-axis profiles on the same medium, a wave expander and a virtual shifter are constructed as examples. The investigations are potentially useful for designing transformation optical devices using only one type of material.

Artificial Kerr-type medium using metamaterials

We investigated an artificial Kerr-medium realized by actuated THz metamaterials. Instead of directly applying E-field inside the medium, we use micromechanical systems actuated by voltage to tune the phase shift. We established that the combined system can have a relationship between the phase shift and the voltage similar to a Kerr cell. A metamaterial Kerr-cell is designed to modulate the transmission phase difference by 0.99°/V² which is much stronger than natural Kerr crystals. It is attributed to the mechanical tunability of metamaterials with high indices in two orthogonal directions. A Lorentzian model is used in explaining the artificial Kerr cell.

Constructing metamaterials from subwavelength pixels with constant indices product

We investigate a two-dimensional metamaterial template constructed from different pixels through a conservation law of effective indices: If the product of refractive indices along the principal axes is invariant for different anisotropic materials in a two-dimensional space, the product of indices of the effective medium remains constant after mixing these materials. Such effective media of constant indices product can be implemented using metamaterial structures. The orientation of the metamaterial structure in a single pixel controls the direction of the principal axis of the effective medium. Different pixels are assembled into an array to obtain reconfigurable anisotropy of the effective medium. These considerations would be useful for constructing reconfigurable metamaterials and transformation media with area-preserving maps.