Xiaopeng Zhao

Trapped rainbow effect in visible light left-handed heterostructures

We experimentally demonstrate the trapped rainbow in tapered left-handed heterostructures (LHHs) at visible frequencies. The employed left-handed metamaterials (LHMs) are isotropic with the size of hundreds of square millimeter. Specifically, the LHMs sample at visible frequencies has a broad spectral range and low loss, making it an intrinsic optical response for the LHHs. It is found that the frequency components of the wave packet separate at positions with different guide thicknesses only if the inclination of tapered LHHs is greater than zero and smaller than the critical value.

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.

A frequency-tunable 90°-polarization rotation device using composite chiral metamaterials

The electromagnetic behaviors of the metamaterial can be artificially controlled by changing the effective permittivity of the nearby background medium. Utilizing this fantastic feature, an ultrathin and frequency-tunable polarization rotation device, which can achieve 90° polarization rotation for a linearly polarized incident wave, is constructed via the combination of a composite chiral metamaterial and two auxiliary dielectric slabs. It shows that the operating frequency of the resulting 90°-polarization rotation device can be continuously and reversibly tuned in a wide frequency range by mechanical means. The experimentally measured results are in agreement with the numerical values.

Perfect Absorber Metamaterial for Designing Low-RCS Patch Antenna

Perfect absorber metamaterial (PAM) to reduce antenna radar cross section (RCS) is proposed. The novel low-RCS antenna is based on the concept of the PAM consisting of a periodic array of metallic dendritic structure and metallic square film etched on each face of a single substrate. The PAM processes high electromagnetic wave absorptivity for a wide incident angle. In the present letter, the PAM patterns are printed on the surface of a patch antenna to reduce the antenna RCS. It is demonstrated that, as compared to an antenna without the PAM, the proposed antenna RCS can be reduced drastically due to the fact that the PAM absorbs electromagnetic wave. Meanwhile, the proposed antenna can still maintain good radiation characteristics.

U-shaped multi-band negative-index bulk metamaterials with low loss at visible frequencies

We study bulk negative-index metamaterials made up of U-shaped cells at visible frequencies that can realize multi-band negative refractive index with very low loss based on high-order resonance. The mechanism of multi-band negative refractive index can be interpreted by analyzing the transmission modes, current distribution, effective LC circuit models and kinetic energy of electrons. In the low-frequency region, the multi-band resonances are mainly due to the cell itself; in the high-frequency region, they are mainly due to the interaction between adjacent cells. Compared with cut-wire pairs, U-shaped cells can realize resonances more easily at high frequencies and produce more negative-index transmission bands.

Dual-Frequency and Broadband Circular Patch Antennas With a Monopole-Type Pattern Based on Epsilon-Negative Transmission Line

Dual-frequency and broadband circular patch antennas based on epsilon-negative (ENG) transmission line (TL) with omnidirectional patterns are presented. The ENG TL is achieved by placing mushroom structures along its circumference. Compared to the conventional circular patch antenna whose fundamental operation mode is TM11 mode, the fundamental operation mode of the new antenna is TM01 mode with a monopole-type radiation pattern. With careful design, two omnidirectional modes, TM01 and TM02, can be excited simultaneously. Through this excitation, a dual-frequency antenna with monopole-type pattern can be realized. Frequency ratio can be tuned by changing the number of mushroom unit cells. A broadband antenna can then be implemented by optimizing the frequency ratio. The performances of the proposed antennas are studied numerically and experimentally. The measured and simulated results are in good agreement.

Quasi-phase-matching of the dual-band nonlinear left-handed metamaterial

We demonstrate a type of nonlinear meta-atom creating a dual-band nonlinear left-handed metamaterial (DNLHM). The DNLHM operates at two distinct left-handed frequency bands where there is an interval of one octave between the two center frequencies. Under the illumination of a high-power signal at the first left-handed frequency band corresponding to fundamental frequency (FF), second-harmonic generation (SHG) is observed at the second left-handed band. This means that our DNLHM supports backward-propagating waves both at FF and second-harmonic (SH) frequency. We also experimentally demonstrate quasi-phase-matching configurations for the backward SHG. This fancy parametric process can significantly transmits the SH generated by an incident FF wave.

Investigation of Circularly Polarized Patch Antenna With Chiral Metamaterial

In this letter, a circularly polarized (CP) patch antenna with chiral metamaterial (CM) is proposed. The proposed CP patch antenna is composed of a conventional linearly polarized (LP) patch antenna and a CM. When placing the CM in the presence of a conventional LP patch antenna, the antenna polarization can be changed to CP mode. The antenna performances have been investigated numerically and experimentally. A simple method for realizing CP antenna is provided by using CM in the present letter.

Review Article: The weak interactive characteristic of resonance cells and broadband effect of metamaterials

Metamaterials are artificial media designed to control electromagnetic wave propagation. Due to resonance, most present-day metamaterials inevitably suffer from narrow bandwidth, extremely limiting their practical applications. On the basis of tailored properties, a metamaterial within which each distinct unit cell resonates at its inherent frequency and has almost no coupling effect with the other ones, termed as weak interaction system, can be formulated. The total response of a weak interaction system can be treated as an overlap of the single resonance spectrum of each type of different unit cells. This intriguing feature therefore makes it possible to accomplish multiband or broadband metamaterials in a simple way. By introducing defects into metamaterials to form a weak interaction system, multiband and broadband electromagnetic metamaterials have first been experimentally demonstrated by our group. The similar concept can also be readily extended to acoustic and seismic metamaterials.

Zero index metamaterial for designing high-gain patch antenna

A planar wideband zero-index metamaterial (ZIM) based on mesh grid structure is studied. It is demonstrated that the real part of the index approaches zero at the wideband covering from 9.9 GHz to 11.4 GHz. Two conventional patch antennas whose operating frequencies are both in the range of zero-index frequencies are designed and fabricated. And then, the ZIM is placed in the presence of the conventional patch antennas to form the proposed antennas. The distance between the antenna and the ZIM cover is investigated. Antenna performances are studied with simulations and measurements. The results show that the more directional and higher gain patch antennas can be obtained. The measured results are in good agreement with the simulations. Compared to the conventional patch antenna without the ZIM, it is shown that the beamwidth of antenna with the ZIM cover becomes more convergent and the gain is much higher.