Weiren Zhu

Configurable metamaterial absorber with pseudo wideband spectrum.

Metamaterials attain their behavior due to resonant interactions among their subwavelength components and thus show specific designer features only in a very narrow frequency band. There is no simple way to dynamically increase the operating bandwidth of a narrowband metamaterial, but it may be possible to change its central frequency, shifting the spectral response to a new frequency range. In this paper, we propose and experimentally demonstrate a metamaterial absorber that can shift its central operating frequency by using mechanical means. The shift is achieved by varying the gap between the metamaterial and an auxiliary dielectric slab parallel to its surface. We also show that it is possible to create multiple absorption peaks by adjusting the size and/or shape of the dielectric slab, and to shift them by moving the slab relative to the metamaterial. Specifically, using numerical simulations we design a microwave metamaterial absorber and experimentally demonstrate that its central frequency can be set anywhere in a 1.6 GHz frequency range. The proposed configuration is simple and easy to make, and may be readily extended to THz frequencies.

Metamaterial absorber with dendritic cells at infrared frequencies

We present a model of an infrared metamaterial absorber composed of metal dendritic resonators, dielectric substrate, and continuous metal film. Numerical simulation confirms an absorptivity of 98.6% at the infrared wavelength of 2.79 μm. The proposed metamaterial absorber has an excellence of two-dimensional isotropy, and it could be fabricated with a chemical double-template technique. Our simulation shows it could be operated for a wide range of incident angles. The optical metamaterial absorber proposed in this paper has potential applications such as in infrared imaging devices, thermal bolometers, and wavelength-selective radiators.

Tunable terahertz left-handed metamaterial based on multi-layer graphene-dielectric composite

A terahertz fishnet metamaterial, consisting of a gallium arsenide substrate sandwiched between multi-layer graphene-dielectric composites, is theoretically studied. Detailed analysis shows that this metamaterial has a left-handed transmission peak accompanied with an abnormal phase dispersion and a clear negative refractive index which originates from simultaneous magnetic and electric resonances. Our structure is unique because it has no metallic parts to achieve the left-handed properties. The most important utility of this metamaterial comes from the fact that its left-handed features can be dynamically controlled by applying external bias to shift the Fermi level in graphene.

A Compact, Planar, and CPW-Fed Metamaterial-Inspired Dual-Band Antenna

A novel compact, planar, and coplanar waveguide (CPW)-fed dual-band antenna is designed and proposed using composite metamaterial. Such composite metamaterial consists of an inner split-ring resonator (SRR) and an outer closed-ring resonator (CRR). The composite metamaterial can provide dual-band operation at 2.595-2.654 and 3.185-4.245 GHz with reflection coefficient better than -10 dB by the two resonant modes of SRR and CRR, respectively. A CPW-fed line with trapeziform ground plane and tapered impedance transformer line is employed to improve the impedance matching of the antenna. The uniqueness of this design is that the inner SRR with size much smaller than the resonant wavelength is used for obtaining the lower narrow frequency band, which makes the dual-band antenna very compact. Antenna parameters, including reflection coefficient, radiation pattern, radiation efficiency, and gain, are analyzed with numerical simulation and experimental measurement. Good agreement between the simulation and measurement is observed.

Graphene metamaterial for optical reflection modulation

We theoretically demonstrate an advanced optical modulator based on graphene-enabled metamaterial with voltage-controllable reflectance. A significant modulation depth of the reflection coefficient is achieved through a voltage biasing of the graphenes Fermi level, leading to an almost instantaneous change in the effective permittivity of the graphene. We show that even a single graphene layer integrated into the structure of a simple metamaterial absorber enables a relative change in the reflectance as high as 361% at near-infrared frequencies. The designed modulator may be used in optical communication systems and biomedical sensing apparatus.

Multibands of negative refractive indexes in the left-handed metamaterials with multiple dendritic structures

We demonstrate that the multibands of negative refraction indexes in the left-handed metamaterials may be appeared based on multiple dendritic structures cells. Employing microwave transmission experiment, the medium exhibits simultaneous negative refraction at the frequencies of 9.50, 10.24, and 10.78 GHz for the electromagnetic wave parallel incidence, and at the frequencies of 9.24, 9.98, and 10.78 GHz for normal incidence. The multiple dendritic metamaterials here open a way to prepare the cloak suitable for multifrequencies, especially for the cloak of infrared or visible frequency.

A numerical method for designing acoustic cloak with homogeneous metamaterials

Based on the form invariance of Helmholtz equation, we present a rhombic acoustic cloak constructed with homogeneous metamaterials. In free space, the proposed cloak can effectively conceal an object inside under a given incident direction. Another application, namely carpet cloak, was also demonstrated by full wave simulations. The proposed cloak provided great convenience in the fabrication process due to the spatially uniform of relative density and modulus tensors.

Graphene-enabled tunability of optical fishnet metamaterial

We present an effective method for actively controlling intrinsic resonances of optical metamaterials using graphene. Exploiting the Fermi level shift and associated variations in optical transitions of graphene due to voltage biasing, we attain the ability to significantly modulate the intrinsic resonance of the fishnet structure. Despite being atomically thin and having a weak optical response, graphene can be strongly coupled with the left-handed resonance of the fishnet metamaterial. We unambiguously demonstrate that the resonant transmission, absorption, and effective constitutive parameters of the graphene-coupled fishnet metamaterial can be precisely controlled by varying the bias voltage.

Wide-angle 90°-polarization rotator using chiral metamaterial with negative refractive index

We present a novel planar chiral metamaterial (CMM) that can realize the intriguing phenomenon of 90°-polarization rotation to the electromagnetic wave incidence with a linear polarization. This CMM consists of fourfold-symmetrically conjugated metallic pairs, which exhibits a circular dichroism and a giant optical activity. The fascinating 90°-polarization rotation of electromagnetic wave for a wide angle of incidence is demonstrated by both simulations and experiments. To be specific, the rotation angle per wavelength in our design is as large as 3400°/λ, implying that the chirality is significantly stronger than any other planar CMMs reported previously. Due to the giant chirality, the negative refractive index is achieved for circularly polarized waves.

Light amplification in zero-index metamaterial with gain inserts

The concept of light amplification in a system of zero-index metamaterial with gain inserts is studied. We demonstrate that the radiation from a point source embedded in the system can be effectively amplified and the net gain is independent of the positions of gain inserts and the source. We further investigate how the net gain grows with the imaginary part of the inserts’ permittivities and find that it saturates when the imaginary part is sufficiently large. It is also shown that the efficiency of amplification is significantly reduced when lossy inserts are introduced into the system.