Abstract-Terahertz photoconductive antenna based on antireflection dielectric metasurfaces with embedded plasmonic nanodisks

 

Xiao-Qiang Jiang, Wen-Hui Fan, Chao Song, Xu Chen, and Qi Wu

https://www.osapublishing.org/ao/abstract.cfm?uri=ao-60-26-7921

By taking advantage of dielectric metasurfaces and plasmonic nanostructures, a terahertz photoconductive antenna (THz-PCA) is proposed and investigated in detail. The designed dielectric metasurfaces can reduce the optical reflection down to 1.4% and accelerate the switching process (electric conductive to resistive) that broadens the THz spectrum emitted from THz-PCA. Simultaneously, the embedded plasmonic nanostructures can realize 11.2 times enhancement in local electric field without affecting the switching process and the damage threshold of the THz-PCA. Simulated results indicate that the proposed THz-PCA is 70.56 times stronger in THz radiation power than that of the traditional THz-PCA. The significant enhancement ensures the proposed THz-PCA has great prospects in promoting THz technology based on the THz-PCA.

© 2021 Optical Society of America

Abstract-Toroidal dipole bound states in the continuum metasurfaces for terahertz nanofilm sensing

Xu Chen, Wenhui Fan, and Hui Yan

 (a) Schematic view of the proposed THz metasurface sensor, where THz waves are normal incidence with E-field along x-direction and the analyte is ultrathin nanofilm. (b) Top view of the unit cell with structure parameters are P = 64 µm, L = 50 µm, w = 4 µm, g1 = 3 µm, g2 = 3 µm, d = 4 µm, and δ = 13 µm.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-28-11-17102

A novel terahertz nanofilm sensor consisting of toroidal dipole bound states in the continuum (TD-BIC) inspired Fano resonance metasurface is proposed and investigated, which exhibits both the TD character and BIC feature. When the mirror symmetry of the unit cell was broken, the TD resonance was excited and demonstrated by anti-aligned magnetic dipoles and calculated scattering powers and the BIC mode was verified with the quality factor satisfying the inverse square law. Combined with the amplitude difference referencing technique, the TD-BIC inspired Fano resonance was utilized for nanofilm sensing at THz frequencies for the first time. Simulation results show that the amplitude difference can be easily observed by comparing the resonance frequency shift under difference thicknesses of germanium overlayer. Moreover, by coating with a 40 nm-thick analyte overlayer, the sensitivity of amplitude difference can achieve 0.32/RIU, which is a significant value and more suitable for sensing nanofilm analytes than the traditional frequency shift method. These advantages make our proposed structure have potential applications in sensing nanofilm analytes.

© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Network Massive MIMO Transmission Over Millimeter-Wave and Terahertz Bands: Mobility Enhancement and Blockage Mitigation

Li You, Xu Chen, Xiaohang Song, Fan Jiang, Wenjin Wang, Xiqi Gao, Gerhard Fettweis

https://arxiv.org/abs/2005.02650

Mobility and blockage are two critical challenges in wireless transmission over millimeter-wave (mmWave) and Terahertz (THz) bands. In this paper, we investigate network massive multiple-input multiple-output (MIMO) transmission for mmWave/THz downlink in the presence of mobility and blockage. Considering the mmWave/THz propagation characteristics, we first propose to apply per-beam synchronization for network massive MIMO to mitigate the channel Doppler and delay dispersion effects. Accordingly, we establish a transmission model. We then investigate network massive MIMO downlink transmission strategies with only the statistical channel state information (CSI) available at the base stations (BSs), formulating the strategy design problem as an optimization problem to maximize the network sum-rate. We show that the beam domain is favorable to perform transmission, and demonstrate that BSs can work individually when sending signals to user terminals. Based on these insights, the network massive MIMO precoding design is reduced to a network sum-rate maximization problem with respect to beam domain power allocation. By exploiting the sequential optimization method and random matrix theory, an iterative algorithm with guaranteed convergence performance is further proposed for beam domain power allocation. Numerical results reveal that the proposed network massive MIMO transmission approach with the statistical CSI can effectively alleviate the blockage effects and provide mobility enhancement over mmWave and THz bands.

Abstract-Terahertz and infrared characteristic absorption spectra of aqueous glucose and fructose solutions

Chao Song, Wen-Hui Fan, Ling Ding, Xu Chen, Ze-You Chen,  Kai Wang,

https://www.nature.com/articles/s41598-018-27310-7

In this paper, the terahertz (THz) and infrared (IR) characteristic absorption spectra of aqueous glucose solutions and aqueous fructose solutions with different concentrations were measured and studied. The absorption spectra of these two molecules in solid-state and in aqueous solutions were compared and analyzed, the significant effect of molecular adjacent environment on the molecular structure and vibrational mode was revealed. In addition, the THz and IR absorption spectra of these two isomers’ aqueous solutions were also compared and explored. No obvious differences were found from their IR absorption features measured at room temperature, while their THz absorption spectra do have the differences, indicating THz characteristic absorption spectra more suitable for the detection and identification of aqueous glucose and fructose solutions. The results are helpful to understand the influence of aqueous solutions environment on the molecular structures and vibrational modes of the materials, and also provide a theoretical reference for the quantum chemical calculation of biological macromolecules.

Abstract-Ultrasensitive terahertz metamaterial sensor based on spoof surface plasmon

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• Xu Chen & 
• Wenhui Fan

https://www.nature.com/articles/s41598-017-01781-6

A planar terahertz metamaterial sensor consisting of a corrugated metal stripe perforated by three rectangular grooves is proposed and investigated numerically. Due to the formation of Fabry-Perot resonance of the spoof surface plasmons mode on the corrugated metal stripe, the extremely sharp resonance in transmission spectrum associated with strong local field enhancement and high quality factor can be realized and exploited for ultrasensitive sensing. Since the intense interaction between electromagnetic waves and analyte materials, the frequency sensitivity of 1.966 THz per refractive index unit and the figure of merit of 19.86 can be achieved. Meanwhile, the film thickness sensitivity of this metamaterial sensor is higher than 52.5 GHz/μm when the analyte thickness is thinner than 4 μm. More interestingly, we find that the metal thickness has a great effect on the sensor performance. These findings open up opportunities for planar metamaterial structures to be developed into practical sensors in terahertz regime.

Abstract-Study of the interaction between graphene and planar terahertz metamaterial with toroidal dipolar resonance

Xu Chen and Wenhui Fan

https://www.osapublishing.org/ol/abstract.cfm?uri=ol-42-10-2034

A planar terahertz metamaterial consisting of square split ring resonators is proposed, and the excitation of toroidal dipolar resonance is demonstrated. Moreover, we theoretically investigate the strong interaction between graphene and toroidal dipolar resonance of the metamaterial. By varying its Fermi energy, the simulations show that graphene can actively modulate the transmission amplitude of toroidal dipolar resonance and even switch it off. The interaction of the toroidal dipolar resonance with monolayer graphene further highlights the ultrasensitive sensing characteristic of the planar metamaterial, which can be utilized for other graphene-like two-dimensional materials. These intriguing properties of the proposed metamaterial may have potential applications in terahertz modulators and ultrasensitive sensors.

© 2017 Optical Society of America

Abstract-Polarization-insensitive tunable multiple electromagnetically induced transparencies analogue in terahertz graphene metamaterial

Polarization-insensitive tunable multiple electromagnetically induced transparencies analogue in terahertz graphene metamaterial

Xu Chen and Wenhui Fan

https://www.osapublishing.org/ome/abstract.cfm?uri=ome-6-8-2607

A graphene-based metamaterial structure composed of multilayer graphene/dielectric stacking configuration is proposed, which achieves multiple analogue of electromagnetically induced transparencies (EIT) effect at terahertz frequencies. Using the phase-coupling scheme, a theoretical model is established to study the EIT-like effect of the proposed structure, and the theoretical calculations coincide well with the numerical simulated results. By varying the Fermi energy level of the graphene, the EIT-like windows can be dynamically tuned in a wide range of terahertz spectra. Particularly, since the symmetry of the structure, the EIT-like effect is polarization-insensitive and can be performed very well in a large incident angles, nearly 80° for both transverse electric and transverse magnetic waves. The proposed structure has potential applications in tunable terahertz chip-integrated optical devices, especially for dynamic multi-band filters, sensors, modulators and nonlinear devices.

© 2016 Optical Society of America

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Abstract-Ultra-flexible polarization-insensitive multiband terahertz metamaterial absorber

Ultra-flexible polarization-insensitive multiband terahertz metamaterial absorber Xu Chen and Wenhui Fan  »View Author Affiliations

http://www.opticsinfobase.org/ao/abstract.cfm?URI=ao-54-9-2376

Applied Optics, Vol. 54, Issue 9, pp. 2376-2382 (2015)
http://dx.doi.org/10.1364/AO.54.002376

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A thin-flexible and polarization-insensitive multiband terahertz metamaterial absorber (MMA) has been investigated. Each unit cell of the MMA consists of two metallic structures, which include the top metal resonator ring and the bottom metal ground plane, separated by a thin-flexible dielectric spacer. Finite element simulation indicates that this MMA has three high absorption peaks in the terahertz region, with absorptivities of 89% at 0.72 THz, 98% at 1.4 THz, and 85% at 2.3 THz. However, because of its rotationally symmetric structure, this MMA is polarization-insensitive and can perform very well at a wide range of incident angles, namely, 30° for transverse electric waves and 40° for transverse magnetic waves. The thin-flexible device structure and good performance shows that this MMA is very promising to disguise objects and make them less detectable to radar in the terahertz region.

© 2015 Optical Society of America