Abstract-Metasurfaces for Terahertz Wavefront Modulation: a Review

Jingwen He, Tao Dong, Baihong Chi, Yan Zhang,

https://link.springer.com/article/10.1007/s10762-020-00677-3

Metasurface is an artificial material composed of a series of subwavelength structure units and has unique electromagnetic characteristics. Based on the ability of manipulating the phase, amplitude, and polarization of electromagnetic wave, various kinds of metasurfaces are designed to realize wavefront manipulations, such as beam focusing, beam steering, vector beams generating, and holographic imaging. This paper reviews the design methods of metasurfaces for wavefront modulation and evolution of the metasurfaces designed for wavefront manipulation in the terahertz (THz) region. The metasurfaces can be divided into two categories: passive and active metasurfaces. For the passive metasurfaces, the single-functional metasurfaces, multifunctional metasurfaces, and high diffraction efficient metasurfaces designed for various THz wavefront shaping, such as focusing, imaging, and special beams generating, are reviewed. For the active metasurfaces, the metasurfaces with fixed structure and all-optical metasurfaces without fixed structure for THz wavefront modulation are summarized. Furthermore, a comparison on the performance of different kinds of metasurfaces for THz wavefront modulation is presented and the development direction and challenges of the THz wavefront modulation metasurfaces in the future are discussed.

Abstract-Metasurfaces for Terahertz Wavefront Modulation: a Review

Jingwen He, Tao Dong, Baihong Chi,  Yan Zhang,

https://link.springer.com/article/10.1007%2Fs10762-020-00677-3

Metasurface is an artificial material composed of a series of subwavelength structure units and has unique electromagnetic characteristics. Based on the ability of manipulating the phase, amplitude, and polarization of electromagnetic wave, various kinds of metasurfaces are designed to realize wavefront manipulations, such as beam focusing, beam steering, vector beams generating, and holographic imaging. This paper reviews the design methods of metasurfaces for wavefront modulation and evolution of the metasurfaces designed for wavefront manipulation in the terahertz (THz) region. The metasurfaces can be divided into two categories: passive and active metasurfaces. For the passive metasurfaces, the single-functional metasurfaces, multifunctional metasurfaces, and high diffraction efficient metasurfaces designed for various THz wavefront shaping, such as focusing, imaging, and special beams generating, are reviewed. For the active metasurfaces, the metasurfaces with fixed structure and all-optical metasurfaces without fixed structure for THz wavefront modulation are summarized. Furthermore, a comparison on the performance of different kinds of metasurfaces for THz wavefront modulation is presented and the development direction and challenges of the THz wavefront modulation metasurfaces in the future are discussed.

Abstract-Study of 2,4-D Spectral Characteristics and Its Detection in Zizania Latifolia Using Terahertz Time-Domain Spectroscopy

Pengcheng Nie, Chengyong Cai,  Fangfang Qu, Lei Lin, Tao Dong, Yong He,

https://www.mdpi.com/2076-3417/9/11/2248

2,4-Dichlorophenoxyacetic acid (2,4-D) is a common plant growth regulator, which can remain in food and, with long-term consumption, threaten human health. Therefore, it is necessary to propose an effective detection method. Terahertz time-domain spectroscopy technique (THz-TDS) has good advantages in the quantitative and qualitative analysis of most biomolecules due to its rich fingerprint characteristics. In this paper, density functional theory (DFT) was applied to geometry optimization and frequency vibration calculation of 2,4-D, and THz-TDS was used to quantitatively detect 2,4-D in Zizania latifolia. The results showed that there were three characteristic absorption peaks of 2,4-D at 1.36, 1.60, and 2.38 THz, respectively, and the theoretical spectra were in good consistency with experimental spectra, with slight discrepancies. Additionally, the absorption peak at 1.36 THz had the best absorption characteristics and was chosen as the main peak for 2,4-D quantitative analysis. It was demonstrated that the limits of detection (LOD) of 2,4-D in Zizania latifolia were found to be as low as 5%, the absorbance intensity at 1.36 THz showed a good linear relationship (R² = 0.9854) with 2,4-D concentration from 5% to 30%, and the recovery was 93.29%–98.75%. Overall, this work enriched the fingerprint database of pesticide molecules on the basis of terahertz spectroscopy and could provide a technical support for the detection of 2,4-D in food by terahertz spectroscopy

Abstract-Ultrabroadband photosensitivity from visible to terahertz at room temperature

Dong Wu, Yongchang Ma, Yingying Niu, Qiaomei Liu, Tao Dong, Sijie Zhang, Jiasen Niu, Huibin Zhou, Jian Wei,Yingxin Wang, Ziran Zhao,  Nanlin Wang,

http://advances.sciencemag.org/content/4/8/eaao3057

Charge density wave (CDW) is one of the most fundamental quantum phenomena in solids. Different from ordinary metals in which only single-particle excitations exist, CDW also has collective excitations and can carry electric current in a collective fashion. Manipulating this collective condensation for applications has long been a goal in the condensed matter and materials community. We show that the CDW system of 1T-TaS2 is highly sensitive to light directly from visible down to terahertz, with current responsivities on the order of ~1 AW−1 at room temperature. Our findings open a new avenue for realizing uncooled, ultrabroadband, and sensitive photoelectronics continuously down to the terahertz spectral range.

Abstract-Spectral Characterization and Molecular Dynamics Simulation of Pesticides Based on Terahertz Time-Domain Spectra Analyses and Density Functional Theory (DFT) Calculations

Fangfang Qu,  Lei Lin, Yong He,  Pengcheng Nie, Chengyong Cai, Tao Dong,  Yi Pan,  Yu Tang,  Shaoming Luo

http://www.mdpi.com/1420-3049/23/7/1607

This work provides the experimental and theoretical fundamentals for detecting the molecular fingerprints of six kinds of pesticides by using terahertz (THz) time-domain spectroscopy (THz-TDS). The spectra of absorption coefficient and refractive index of the pesticides, chlorpyrifos, fipronil, carbofuran, dimethoate, methomyl, and thidiazuron are obtained in frequencies of 0.1–3.5 THz. To accurately describe the THz spectral characteristics of pesticides, the wavelet threshold de-noising (WTD) method with db 5 wavelet fucntion, 5-layer decomposition, and soft-threshold de-noising was used to eliminate the spectral noise. The spectral baseline correction (SBC) method based on asymmetric least squares smoothing was used to remove the baseline drift. Spectral results show that chlorpyrifo had three characteristic absorption peaks at 1.47, 1.93, and 2.73 THz. Fipronil showed three peaks at 0.76, 1.23, and 2.31 THz. Carbofuran showed two peaks at 2.72 and 3.06 THz. Dimethoate showed three peaks at 1.05, 1.89, and 2.92 THz. Methomyl showed five peaks at 1.01, 1.65, 1.91, 2.72, and 3.20 THz. Thidiazuron showed four peaks at 0.99, 1.57, 2.17, and 2.66 THz. The density functional theory (DFT) of B3LYP/6-31G+(d,p) was applied to simulate the molecular dynamics for peak analyzing of the pesticides based on isolated molecules. The theoretical spectra are in good agreement with the experimental spectra processed by WTD + SBC, which implies the validity of WTD + SBC spectral processing methods and the accuracy of DFT spectral peak analysis. These results support that the combination of THz-TDS and DFT is an effective tool for pesticide fingerprint analysis and the molecular dynamics simulations.

Abstract-Experimental and Theoretical Study on Terahertz Absorption Characteristics and Spectral De-noising of Three Plant Growth Regulators

Fangfang Qu, 

Yi Pan, 

Lei Lin, 

Chengyong Cai, 

Tao Dong, 

Yong He, 

Pengcheng Nie, 

https://link.springer.com/article/10.1007%2Fs10762-018-0507-1
Accurate identification of terahertz (THz) absorption peaks of biological macromolecules is of great significance in practical applications. In this work, the experimental and theoretical fundamentals of detecting three plant growth regulators (PGRs), including 6-Benzylaminopurine (6-BA), paclobutrazol (PBZ), and maleic hydrazide (MH) were investigated by using THz time-domain spectroscopy (THz-TDS). THz absorption coefficient and refractive index in frequencies of 0.06–4 THz were obtained. The wavelet threshold de-noising (WTD) method was used to remove spectral noise and improve the signal-to-noise ratio (SNR). The density functional theory (DFT) was applied to the molecular characterization and theoretical calculation of PGRs. Experimental results showed that the three PGRs had unique characteristic absorption peaks. Based on the sym4 wavelet function and four-layer wavelet decomposition, the de-noising performance of hard threshold WTD was better than that of soft threshold WTD. The spectra processed by hard threshold de-noising achieved higher peak SNR (6-BA: 40.22, PBZ: 37.73, MH: 34.83) and lower root mean square error (6-BA: 0.41, PBZ:0.40, MH:0.54). In addition, the characteristic absorption and anomalous dispersion of 6-BA were found at 2.08 and 3.00 THz, those of PBZ were shown at 0.71, 1.30, 1.88, and 2.67 THz, and those of MH were shown at 2.34 THz. The absorption peaks in THz spectra processed by hard threshold WTD were demonstrated to be in good agreement with the simulation results of DFT. These results show the effectiveness of WTD in THz spectral de-noising and the feasibility of using THz-TDS to detect PGRs.

Abstract-Molecular Characterization and Theoretical Calculation of Plant Growth Regulators Based on Terahertz Time-Domain Spectroscopy

Fangfang Qu  ,Lei Lin ,Chengyong Cai, Tao Dong,Yong He, Pengcheng Nie

http://www.mdpi.com/2076-3417/8/3/420

Terahertz (THz), as an advanced spectral technology, has unique absorption characteristics for most biological macromolecules. In this work, the theoretical fundamentals for the application of THz time-domain spectroscopy (THz-TDS) to molecular characterization and fingerprint peak detection of three plant growth regulators (PGRs), including 2,4-Dichlorophenoxyacetic acid (2,4-D), forchlorfenuron (CPPU) and indole-3-acetic acid (IAA) were researched. Meanwhile, the effects of eight types of window functions on THz spectra were studied when converting time-domain spectra into frequency-domain spectra by Fourier transform. Based on the optimal window function, the THz absorption coefficient and refractive index of PGRs in frequencies of 0.2–3 THz were extracted. The molecule structure and vibration mode of three PGR samples were simulated by using density functional theory (DFT). The results showed that the three PGRs had different fingerprint peaks. Characteristic absorption and anomalous dispersion of 2,4-D were found at 1.35, 1.57 and 2.67 THz, those of CPPU were found at 1.77 and 2.44 THz, and the absorption peak of IAA was located at 2.5 THz. The absorption peaks obtained from THz spectra were identified according to the theoretical calculation results of DFT. These fingerprint peaks in THz spectra were generated by the interior stretching vibration and external deformation vibration of molecular groups. The experimental results revealed the feasibility of identifying PGRs species and detecting residues using THz-TDS