Identification and quantitative detection of two pathogenic bacteria based on a terahertz metasensor.

Bacterial infection can cause a series of diseases and play a vital role in medical care. Therefore, early diagnosis of pathogenic bacteria is crucial for effective treatment and the prevention of further infection. However, restricted by the current technology, bacterial detection is usually time-consuming and laborious and the samples need tedious processing even to be tested. Herein, we present a terahertz metasensor based on the coupling of electrical and toroidal dipoles to achieve rapid, non-destructive, label-free identification and highly sensitive quantitative detection of the two most common pathogenic bacteria. The reinforcement of the toroidal dipole significantly boosts the light-matter interactions around the surface of the microstructure, and thus the sensitivity and Q factor of the designed metasensor reach as high as 378 GHz per refractive index unit (RIU) and 21.28, respectively. Combined with the aforementioned advantages, the proposed metasensor successfully identified Escherichia coli and Staphylococcus aureus and quantitatively detected four concentrations with the lowest detectable concentration being ∼104 cfu mL-1 in the experiment. This work naturally enriches the research on THz metasensors based on the interference mechanism and inspires more innovations to facilitate the development of biosensing applications.

Terahertz meta-biosensor based on high-Q electrical resonance enhanced by the interference of toroidal dipole.

Electrical dipole resonances typically have low Q factor and broad resonant linewidth caused by strong free-space coupling with high radiative loss. Here, we present a strategy for enhancing the Q factor of the electrical resonance via the interference of a toroidal dipole. To validate such a strategy, a metasurface consisting of two resonators is designed that responsible to the electric and toroidal dipoles. According to constructive and destructive hybridizations of the two dipole modes, enhanced and decreased Q factors are found respectively for the two hybrid modes, compared to the one for the conventional electric dipole resonance. As a practical application of such high Q resonance, we further experimentally investigate the sensing performance of the metasurface biosensor by detecting the cell concentration of lung cancer cells (type A549). Moreover, through monitoring both resonance frequency and amplitude variation of the metasurface biosensor, the dielectric permittivity of the lung cancer cells is delicately estimated by the conjoint analysis of both simulated and measured results. Our proposed metasurface paves a promising way for the study of multipole interference in the field of nanophotonics and validates its effectiveness in biomedical sensing.

Graphene-Based Optically Transparent Metasurface Capable of Dual-Polarized Modulation for Electromagnetic Stealth.

Microwave stealth technology with optical transparency is of great significance for solar-powered aircrafts (e.g., satellites or unmanned aerial vehicles) in increasingly complex electromagnetic environments. By coating them with optically transparent absorbing materials or devices, these large-sized solar panels could avoid detection by radar while maintaining highly efficient collection of solar energy. However, conventional microwave-absorbing materials/devices for solar panels suffer from bulky volume and fixed stealth performance that significantly hinders their practicality or multifunctionality. Particularly, dynamic modulation of microwave absorption for dual polarization remains a challenge. In this paper, we propose the design, fabrication, and characterization of an optically transparent and dynamically tunable microwave-absorbing metasurface that enables dual modulations (amplitude and frequency) independently for two orthogonal linearly polarized excitations. The tunability of the proposed metasurface is guaranteed by an elaborately designed anisotropic meta-atom composed of a patterned graphene structure whose electromagnetic responses for different polarizations can be dynamically and independently controlled via bias voltages. The dual tunability in such a graphene-based absorbing metasurface is experimentally measured, which agrees well with those numerical results. We further build an equivalent lumped circuit model to analyze the physical relation between the tunable sheet resistance of graphene and the polarization-independent modulations of the metasurface. Taking into account the advantages of optical transparency and flexibility, the proposed microwave-absorbing metasurface significantly enhances the multitasking stealth performance in complex scenarios and has the potential for advanced solar energy devices.

High-performance meta-absorber for the surface wave under the spoof surface plasmon polariton mode.

Perfect absorbers are highly desired in many engineering and military applications, including radar cross section (RCS) reduction, cloaking devices, and sensor detectors. However, most types of present absorbers can only absorb space propagation waves, yet absorption for the surface wave (SW) has not been researched intensively. In reality, when the space wave illuminates on the metal under large oblique angles, surface waves can be excited on the interface between metal and dielectric and thus would increase the RCS and influence the stealth performance. Here, based on the wave vector and impedance matching theories, we propose a broadband absorber for the surface wave under spoof surface plasmon polariton (SSPP) mode. The former theory ensures that surface waves can enter the absorber efficiently, and the latter guarantees perfect absorption. The experimental results indicate that our absorber can achieve a broadband (9.4-18 GHz) performance with an absorption ratio better than 90%, which is in great agreement with the simulations. Therefore, our device can be applied in RCS reduction for the metal devices, antenna array decoupling and many other applications. Also, this work provides a unique methodology to design new types of broadband surface wave absorbers.

Three-dimensional ultra-broadband absorber based on novel zigzag-shaped structure.

Absorbers have potential applications in the stealth field. However, limited bandwidth and low absorption rate persist in existing methods. Moreover, absorbers working in the low frequency range (1-4 GHz) with small size are much more difficult to realize. In this paper, we propose a novel absorption structure, which combines indium tin oxide film and metal resonator. The former realizes impedance matching with free space in a broad bandwidth at moderate frequency range while the latter shows the resonant property at low frequency. Based on this absorption structure, we design the zigzag-shaped structure to realize high-efficiency and ultra-broadband absorption. To demonstrate the feasibility of our method, we fabricate a sample and perform measurements. The measurement results show that our sample can achieve ultra-broadband absorption with high-efficiency of over 90% from 1 GHz to 18 GHz, which is in good agreement with simulation results. Our findings provide a valuable technique for broadband device design, which could bring about a wide range of applications in cloaking technology.

High-efficiency and ultra-broadband asymmetric transmission metasurface based on topologically coding optimization method.

Achieving asymmetric transmission effects, especially in an ultra-broadband frequency region, is of particular importance in communication systems. Currently available asymmetric transmission metasurfaces are limited to narrow bands and low efficiencies because of the inherently dispersion effects and large transmission fluctuations. In this paper, we propose a new strategy to realize high efficiency and ultra-broadband asymmetric transmission in an ultra-thin profile by using the topologically coding optimization method. The meta-atom consists of two outer orthogonal gratings and a central lattice particle optimized by genetic algorithm. The optimized central lattice suppresses the transmission fluctuations by tuning the coupling among different metallic layers, resulting in very broad band and high transmissions. Experimental results show that our metasurface achieved perfect reflection over 95% and high cross-polarization transmission over 80% for y- and x-polarized incidence from 5.3 GHz to 16.7 GHz, respectively. Our findings pave a way to high-performance and broadband polarization transformers and polarization-controlled devices working in different frequency domains.