Rongzhou Gong

Design, fabrication and measurement of a broadband polarization-insensitive metamaterial absorber based on lumped elements

A broadband metamaterial absorber (MA) based on lumped elements is presented, which is composed of the dielectric substrate sandwiched with metal split-coin resonators (SCR) welded with lumped elements and continuous metal film. We simulated, fabricated, and measured the lumped elements MA. Compared with the single SCR structure MA, the composite MA loaded with lumped elements has a wider absorptivity and works in a lower frequency. The experiment results show that the bandwidth of absorption of 90% is about 1.5 GHz and the full width at half maximum (FWHM) can be up to 50%, the absorptivity is also nearly unchanged for different polarizations. The further simulations of the absorptivity of composite MA with different lumped resistances and capacitances indicate that there exist optimal values for lumped resistances and capacitances, where the absorptivity is the highest and the bandwidth is the widest.

Ultrabroadband reflective polarization convertor for terahertz waves

We design and experimentally demonstrate an ultrathin, ultrabroadband, and highly efficient reflective linear polarization convertor or half-wave retarder operating at terahertz frequencies. The metamaterial-inspired convertor is composed of metallic disks and split-ring resonators placed over a ground plane. The structure exhibits three neighboring resonances, by which the linear polarization of incident waves can be converted to its orthogonal counterpart upon reflection. For an optimal design, a measured polarization conversion ratio for normal incidence is greater than 80% in the range of 0.65–1.45 THz, equivalent to 76% relative bandwidth. The mechanism for polarization conversion is explained via decomposed electric field components that couple with different resonance modes of the structure. The proposed metamaterial design for enhancing efficiency of polarization conversion has potential applications in the area of terahertz spectroscopy, imaging, and communications.

Giant asymmetric transmission of circular polarization in layer-by-layer chiral metamaterials

In this paper, two kinds of bi-layered chiral metamaterials were proposed to enhance the asymmetric transmission effects for circular polarized electromagnetic waves which are only found in planar chiral metamaterials according to previous studies. It was found that the magnitude of the asymmetric transmission parameters mainly depends on the distances between the two metal layers. With appropriate distance, the asymmetric transmission parameter for circular polarized electromagnetic waves can increase to 0.6. Moreover, both proposed configurations show no asymmetric transmission for linear polarized electromagnetic waves which is widely studied in other bi-layered metamaterials.

Electromagnetic manifestation of chirality in layer-by-layer chiral metamaterials.

The three-dimensional (3D) and two-dimensional (2D) chiral metamaterials (CMMs) have been proved to exhibit circular dichroism and circular conversion dichroism, respectively. The layer-by-layer chiral metamaterials, as a category of 3D CMMs, are expected to show the same properties as bulk 3D structures (e.g. helices). However, in this paper, we demonstrated that the layer-by-layer CMMs exhibit circular dichroism and circular conversion dichroism simultaneously by using both theoretical and experimental methods. This work showed that asymmetric transmissions of circular polarizations can also be observed in layer-by-layer CMMs. Moreover, we provided some necessary requirements for the existing of asymmetric transmissions in layer-by-layer CMMs.

Giant Circular Dichroism and Negative Refractive Index of Chiral Metamaterial Based on Split-Ring Resonators

In this paper, a double-layer split-ring resonator structure chiral metamaterial was proposed which could exhibit pronounced circular dichroism (CD) efiect and negative refractive index at microwave frequencies. Experiment and simulation calculations are in good agreement. The retrieved efiective electromagnetic parameters indicate that the lower frequency CD efiect is associated with the negative refractive index property of the left circularly polarized (LCP) wave, and the upper one is to the right circularly polarized (RCP) wave. The mechanism of the giant CD efiect could be further illustrated by simulated surface current and power loss density distributions.

Ultra-Compact Multi-Band Chiral Metamaterial Circular Polarizer Based on Triple Twisted Split-Ring Resonator

An ultra-compact chiral metamaterial (CMM) using triple-layer twisted split-ring resonators (TSRRs) structure was proposed, which can function as a multi-band circular polarizer. This ultra-compact structure CMM can convert an incident linearly y-polarized (x-polarized) wave propagating along thez(+z) direction to the transmitted left circularly polarized (LCP) waves at 7.28 GHz, 13.22 GHz and 15.49 GHz while the right circularly polarized (RCP) waves are at 9.48 GHz simultaneously. In addition, the large polarization extinction ratio (PER) of more than 20 dB across four resonance frequencies can be achieved. The experiment results are in good agreement with the numerical simulation results. The surface current distributions of the structure are analyzed to illustrate this linear to circular polarization conversion. The unit cell structure is extremely small both in longitudinal and transverse directions. Good performances and compact design of this CMM suggest promising applications in circular polarizers that need to be integrated with other compact devices.

Broadband transparent metamaterial linear polarization transformer based on triple-split-ring resonators

In this paper, a chiral metamaterial (CMM) with triple-layer twisted split-ring resonators structure was proposed, which could exhibit a linear polarization conversion as well as asymmetric transmission only for forward and backward propagating linearly polarized waves in a broadband frequency range. The polarization of broadband linearly polarized electromagnetic waves can be rotated in a specific direction by passing it through such a CMM slab with a thickness of about one-twelfth the operational center wavelength. Experiment and simulation calculations are in good agreement, and the polarization conversion rate is above 90% in the frequency range of 5.5–11.1 GHz. The physical mechanism of the broadband linear polarization conversion effect could be further illustrated by simulated electrical field and surface current distributions.

Adjustable low frequency and broadband metamaterial absorber based on magnetic rubber plate and cross resonator

In this paper, the magnetic rubber plate absorber (MRPA) and metamaterial absorber (MA) based on MRP substrate were proposed and studied numerically and experimentally. Based on the characteristic of L-C resonances, experimental results show that the MA composed of cross resonator (CR) embedded single layer MRP could be adjustable easily by changing the wire length and width of CR structure and MRP thickness. Finally, experimental results show that the MA composed of CR-embedded two layers MRP with the total thickness of 2.42 mm exhibit a −10 dB absorption bandwidth from 1.65 GHz to 3.7 GHz, which is 1.86 times wider than the same thickness MRPA.

Giant Optical Activity and Negative Refractive Index Using Complementary U-Shaped Structure Assembly

In this paper, a chiral metamaterial (CMM) with complementary U-shaped structure assembly is proposed. The microwave experimental and simulated results of the proposed complementary structure exhibit giant optical activity. The experimental results are in good agreement with the numerical ones. The retrieval results reveal that negative refractive indices for right- handed and left-handed circularly polarized waves could be easily realized due to strong chirality. The mechanism of the chiral behaviors of resonance frequencies will be illustrated by simulated current distributions. Further, the complementary U-shaped structure assembly also exhibits stronger circular dichroism, giant optical activity, and negative index at near-infrared region by simulations.

Preparation and microwave absorption properties of metal magnetic micropowder-coated honeycomb sandwich structures

Radar absorbing materials with metal magnetic micropowder-coated honeycomb sandwich structures are prepared by a spray process. Metal magnetic micropowder is applied as an absorber which maintains a high absorption, and a honeycomb sandwich structure as a supporter enhancing mechanical strength. The microwave absorption properties are measured by a network analyzer system in the frequency range of 2.6–18 GHz. The concentration of the MMP and the coating thickness of the absorber affect the attenuation properties, a suitable value of them contributing to a broad bandwidth and high loss. A matching layer is introduced to the honeycomb sandwich structure on top, which allows the incident electromagnetic wave to enter and largely get attenuated through the absorbing system, increasing the microwave absorption.