Quanhong Fu

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.

90° polarization rotator with rotation angle independent of substrate permittivity and incident angles using a composite chiral metamaterial

We propose a more efficient way to obtain much stronger polarization rotatory power by constructing a composite chiral metamaterial (CCMM) which is achieved via the combination of the cut-wire pairs (CWPs) and a purely chiral metamaterial (PCMM) composed of conjugated gammadion resonators. Owing to the strong coupling between the CWPs and PCMM, the polarization rotation in our CCMM is more gigantic than that of the PCMM. Furthermore, the CCMM proposed in this paper can function as a wide-angle 90° polarization rotator for different substrate permittivity without needing to adjust its geometric parameters. Due to the unique properties, the CCMM may greatly benefit potential applications including designing a tunable 90°-polarization rotator, microwave devices, telecommunication, and so on.

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.

Three-level dendritic structure with simultaneously negative permeability and permittivity under normal incidence of electromagnetic wave

We present the model of three-level dendritic structure for left-handed metamaterials under normal incidence of electromagnetic radiation and investigate the electromagnetic responses in microwave band by simulation. The influence of branch lengths a , b , and c on the electric and magnetic resonance frequencies is studied by the transmission spectrum of single three-level dendritic structure. Moreover, the transmission of multiple three-level dendritic structures exhibits the interaction between the elements. To confirm the left-handed behaviors of three-level dendritic structures, the permittivity e and permeability μ are calculated by S -parameter retrieval method. The results show that the e and μ are indeed simultaneously negative around 9.2 GHz.

Electrically tunable Fano-type resonance of an asymmetric metal wire pair.

We theoretically and experimentally investigate the electrically tunable Fano-type resonance of asymmetric metal wire pair loaded with varactor diodes. It is illustrated that Fano-type transmission spectrum with high quality factor Q appears as a result of interference between the dipole and quadrupole modes. The ohmic loss of series resistance in varactor diode makes major contribution to absorption. At the Fano-type resonance frequency, both the two metal wires exhibit the strongest electric resonance simultaneously, and the Fano-type resonance manifests a large group delay. As the bias voltage ranges from 0 V to 8 V, the Fano-type resonance frequency exhibits a prominent blueshift of 0.16 GHz and the transmission experiences a modulation with a modulation depth of 97%.

Two-peak property in asymmetric left-handed metamaterials

The transmission and reflection properties of left-handed metamaterials (LHMs) with the local asymmetry are investigated at the X-band frequency range in free space. Experimental results show that two transmission peaks and two reflection dips are exhibited in the presence of the asymmetric configuration of split ring resonators (SRRs). Furthermore, the position and intensity of the peaks and dips are significantly dependent on the asymmetry between the SRRs and the wires. In addition, the numerical calculations indicate that two left-handed frequency regions can exist if another configuration of the SRRs is introduced into the LHMs, which is in agreement with the experimental results.

Panel-allocated defect SRRs effect in X-band LHMs

We experimentally investigated the defect effects of LHMs when panel-allocated defects SRRs are introduced. By measuring the X-band transmission through metamaterial with different sizes and orientations panelallocated defect SRRs, it was found that characters of resonant peak, including resonance frequency, magnitude and band pass, markedly change. And the panel-allocated defects in LHMs have more effect on electromagnetic behavior than that of dot and linear ones. It is thought that the existences of panel-allocated defects break the symmetry of perfect LHMs and result in a new electromagnetic resonance.

Weak coupling between bright and dark resonators with electrical tunability and analysis based on temporal coupled-mode theory

We investigate the electrically tunable Electromagnetic induced transparency (EIT)-like effect of active metamaterial structures composed of a wire and a split ring resonator by the simulation, experiment, and temporal coupled-mode theory. It is illustrated that an EIT-like effect appears as a result of weak coupling between bright and dark resonators. Around the EIT-like peak frequency, the superradiant resonance mode of the bright resonator is highly suppressed by the subradiant resonance mode of the dark resonator, and high transmittance as well as large group delay is manifested. By integrating a varactor diode into the EIT structure and altering the bias voltage, the EIT-like effect can be dynamically tuned. As the bias voltage ranges from 0 V to 8 V, the EIT-like peak frequency exhibits a prominent blueshift of 0.22 GHz and the transmittance experiences a modulation with a modulation depth up to 98%. Using the temporal coupled-mode theory, the transmission spectrum of the EIT structure is predicted ...