Chunrong Luo

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.

High-efficiency broadband and multiband cross-polarization conversion using chiral metamaterial

Metamaterials with tailored electromagnetic properties present incomparable advantages in manipulation and control of electromagnetic polarization states. Here, we report a unique chiral metamaterial (CMM) composed of three layers of metasurfaces. Taking advantage of the Fabry–Perot-like interference effect, the suggested CMM is capable of achieving high-efficiency broadband or multiband cross-polarization conversion for different linearly polarized incident waves, while also showing strong asymmetric transmission effect for the opposite propagation directions. With the specially designed geometry, the present CMM can even simultaneously accomplish nearly 90°-polarization rotation for both the x- and y-polarized incident waves, thereby enabling more opportunities for designing high-performance polarimetric devices.

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.

Double-negative acoustic metamaterial based on hollow steel tube meta-atom

We presented an acoustic “meta-atom” model of hollow steel tube (HST). The simulated and experimental results demonstrated that the resonant frequency is closely related to the length of the HST. Based on the HST model, we fabricated a two-dimensional (2D) acoustic metamaterial (AM) with negative effective mass density, which put up the transmission dip and accompanied inverse phase in experiment. By coupling the HST with split hollow sphere (SHS), another kind of “meta-atom” with negative effective modulus in the layered sponge matrix, a three-dimensional (3D) AM was fabricated with simultaneously negative modulus and negative mass density. From the experiment, it is shown that the transmission peak similar to the electromagnetic metamaterials exhibited in the double-negative region of the AM. We also demonstrated that this kind of double-negative AM can faithfully distinguish the acoustic sub-wavelength details ( λ / 7 ) at the resonance frequency of 1630 Hz.

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.

Meta-atom cluster acoustic metamaterial with broadband negative effective mass density

We design a resonant meta-atom cluster, via which a two-dimensional (2D) acoustic metamaterial (AM) with broadband negative effective mass density from 1560 Hz to 5580 Hz is fabricated. Experimental results confirm that there is only weak interaction among the meta-atoms in the cluster. And then the meta-atoms in the cluster independently resonate, resulting in the cluster becoming equivalent to a broadband resonance unit. Extracted effective refractive indices from reflection and transmission measurements of the 2D AM appear to be negative from 1500 Hz to 5480 Hz. The broadband negative refraction has also been demonstrated by our further experiments. We expect that this meta-atom cluster AM will significantly contribute to the design of broadband negative effective mass density AM.

Ultrathin skin cloaks with metasurfaces for audible sound

We report a design of 2D acoustic skin cloaks by using ultrathin metasurfaces in audible range. The microunit of this metasurface is constructed by a cavity coupled with a membrane. This cloak can completely compensate the wave front discrepancy generated by the scattering of the hidden object because the microunits are capable of arbitrarily modulating the reflected amplitude and phase. The operating frequency ranges from 3.54 kHz to 3.93 kHz. The tolerated maximum incident angle decreases as the height of the hidden object increases. Moreover, the cloaks thickness is only approximately λ/10, so that we can make an object in almost any shape undetectable without obviously increasing the size of the whole system. This intriguing feature forms a sharp contrast to most bulky cloaks on the basis of coordination transformations.

Inverse Doppler Effects in Broadband Acoustic Metamaterials.

The Doppler effect refers to the change in frequency of a wave source as a consequence of the relative motion between the source and an observer. Veselago theoretically predicted that materials with negative refractions can induce inverse Doppler effects. With the development of metamaterials, inverse Doppler effects have been extensively investigated. However, the ideal material parameters prescribed by these metamaterial design approaches are complex and also challenging to obtain experimentally. Here, we demonstrated a method of designing and experimentally characterising arbitrary broadband acoustic metamaterials. These omni-directional, double-negative, acoustic metamaterials are constructed with ‘flute-like’ acoustic meta-cluster sets with seven double meta-molecules; these metamaterials also overcome the limitations of broadband negative bulk modulus and mass density to provide a region of negative refraction and inverse Doppler effects. It was also shown that inverse Doppler effects can be detected in a flute, which has been popular for thousands of years in Asia and Europe.

Slowing down light using a dendritic cell cluster metasurface waveguide

Slowing down or even stopping light is the first task to realising optical information transmission and storage. Theoretical studies have revealed that metamaterials can slow down or even stop light; however, the difficulty of preparing metamaterials that operate in visible light hinders progress in the research of slowing or stopping light. Metasurfaces provide a new opportunity to make progress in such research. In this paper, we propose a dendritic cell cluster metasurface consisting of dendritic structures. The simulation results show that dendritic structure can realise abnormal reflection and refraction effects. Single- and double-layer dendritic metasurfaces that respond in visible light were prepared by electrochemical deposition. Abnormal Goos-Hänchen (GH) shifts were experimentally obtained. The rainbow trapping effect was observed in a waveguide constructed using the dendritic metasurface sample. The incident white light was separated into seven colours ranging from blue to red light. The measured transmission energy in the waveguide showed that the energy escaping from the waveguide was zero at the resonant frequency of the sample under a certain amount of incident light. The proposed metasurface has a simple preparation process, functions in visible light, and can be readily extended to the infrared band and communication wavelengths.

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.