Dongshan Wei

Terahertz Biosensing Based on a Polarization-Insensitive Metamaterial

A polarization-insensitive metamaterial biosensor composed of metallic square ring resonators is proposed and is shown to be especially suitable to work together with the linear polarized femtosecond laser pumped terahertz time-domain spectroscopy. A symmetric slit is distributed on every side of the ring to render the biosensor insensitive to the polarization direction of the incident terahertz wave. The proposed structure is analyzed with the finite-element method and investigated experimentally. The results reveal that a minimal resolution of 17.7 μmol/L in the detection of bovine serum albumin is obtained when the biosensor is oriented arbitrarily in the plane containing the electric field vector.

Molecular dynamics simulations of conformation and chain length dependent terahertz spectra of alanine polypeptides

Terahertz absorption spectra of alanine polypeptides in water were simulated with classical molecular dynamics at 310 K. Vibrational modes and oscillator strengths were calculated based on a quasi-harmonic approximation. Absorption spectra of Alan (n = 5, 15, 30) with different chain lengths and Ala15 in coiled and helical conformations were studied in 10–40 cm− 1 bandwidth. Simulation results indicated both the chain length and the conformation have significant influences on THz spectra of alanine polypeptides. With the increase of chain length, the average THz absorption intensity increases. Compared with the helical Ala15 polypeptide, the THz spectra of coiled one shows stronger absorption peaks. These results were explained from different numbers of hydrogen bonds formed between polypeptides and the surrounding water molecules.

Rapid and label-free detection and assessment of bacteria by terahertz time-domain spectroscopy

Here we demonstrated the potential and applicability of terahertz (THz) spectroscopy to detect four commonly found bacteria in the infectious diseases. Besides the different spectral characteristics between bacterial species, THz absorption differences for living bacteria, dead bacteria and bacterial powder of the same species were also investigated. Our results revealed that small differences in water contents between bacterial cells account for distinct discrepancies of the absorption coefficients, which can be used for bacterial species identification. Furthermore, living and dead bacteria showed different absorption coefficients as a result of their different hydration levels, suggesting that THz spectroscopy can be used to rapidly assess the living state of bacteria under test. Our results clearly demonstrated the ability of THz spectroscopy for time-saving and label-free detection of bacteria with minimal sample preparation, potentially to be utilized for point-of-care tests in the near future. Schematic representation of bacterial detection by THz spectroscopy. Different bacteria have distinctive absorption coefficients as a result of their different water contents.

Terahertz spectroscopy of oligonucleotides in aqueous solutions

Abstract. A terahertz (THz) spectroscopic study is carried out to analyze DNA mutations in a label-free manner. Three newly designed liquid sample cells are considered and the best is selected as the sample carrier for THz transmission spectroscopic analyses. Discrimination based on spectral signatures of single-base mutations on single-stranded 20 nt oligonucleotides has been shown possible experimentally. The results clearly attest the ability of this promising approach for label-free analyses of single-base mutations of DNA molecules. This study has demonstrated that the THz spectroscopic technology can be considered as a potential diagnostic tool for investigating molecular reactions, such as DNA mutations.

Nondestructive Evaluation of Carbon Fiber Reinforced Polymer Composites Using Reflective Terahertz Imaging

Terahertz (THz) time-domain spectroscopy (TDS) imaging is considered a nondestructive evaluation method for composite materials used for examining various defects of carbon fiber reinforced polymer (CFRP) composites and fire-retardant coatings in the reflective imaging modality. We demonstrate that hidden defects simulated by Teflon artificial inserts are imaged clearly in the perpendicular polarization mode. The THz TDS technique is also used to measure the thickness of thin fire-retardant coatings on CFRP composites with a typical accuracy of about 10 micrometers. In addition, coating debonding is successfully imaged based on the time-delay difference of the time-domain waveforms between closely adhered and debonded sample locations.

Spectroscopic study of terahertz reflection and transmission properties of carbon-fiber-reinforced plastic composites

Abstract. Carbon-fiber-reinforced plastic (CFRP) composites are widely used in aerospace and concrete structure reinforcement due to their high strength and light weight. Terahertz (THz) time-domain spectroscopy is an attractive tool for defect inspection in CFRP composites. In order to improve THz nondestructive testing of CFRP composites, we have carried out systematic investigations of THz reflection and transmission properties of CFRP. Unidirectional CFRP composites with different thicknesses are measured with polarization directions 0 deg to 90 deg with respect to the fiber direction, in both reflection and transmission modes. As shown in the experiments, CFRP composites are electrically conducting and therefore exhibit a high THz reflectivity. In addition, CFRP composites have polarization-dependent reflectivity and transmissivity for THz radiation. The reflected THz power in the case of parallel polarization is nearly 1.8 times higher than for perpendicular polarization. At the same time, in the transmission of THz wave, a CFRP acts as a Fabry–Pérot cavity resulting from multiple internal reflections from the CFRP–air interfaces. Moreover, from the measured data, we extract the refractive index and absorption coefficient of CFRP composites in the THz frequency range.

Determination of Critical Micelle Concentrations of Surfactants by Terahertz Time-Domain Spectroscopy

A novel detection method is presented to determine the critical micelle concentrations (CMCs) of surfactants by using a simple and convenient terahertz time-domain spectroscopy (THz-TDS) technology which has been successfully used to study the picosecond dynamics of water in reverse micelles. Absorption coefficients or refractive indices of aqueous solutions of an ionic surfactant sodium dodecyl sulfate and a nonionic surfactant nonaethylene glycol monododecyl ether in 0.2-1.8 THz frequency range at different concentrations are measured through the THz-TDS. A clear intersection of THz absorption coefficient or refractive index caused by the micelle formation is observed. This phenomenon is strongly associated with the change of component ratio between surfactant, bulk water and hydration water during the formation of micelles. The corresponding concentration is determined as the CMC of the surfactant which is in good agreement with the most existing results using traditional methods. For comparison, absorption coefficients of the two surfactants in hydrochloric acid solutions are also measured and differences in contrast to those in aqueous solutions are observed and explained. Our findings indicate that THz spectroscopy technology can be applied to determine the CMCs of surfactants in a sensitive, accurate, and label-free way.

Interaction between metal nanotip and substrate under terahertz wave radiation

In this paper, the interaction between a gold nanotip and a silver substrate under the terahertz (THz) wave radiation is simulated by a finite difference time domain method and theoretically analyzed from the quasi-static charges model. Results from simulations and theoretical analysis show that with the increase in the distance between the nanotip and the substrate, the maximum electric field intensity on the substrate prominently decreases. With the increase in radius of the nanotip, diameter of the focused THz spot almost linearly increases and the intensity of THz spot also has a slight increase. These results are in agreement with the physical principles of the tip-enhanced near-field optical microscopy (TENOM), which indicates the TENOM technique is also suitable for the THz frequency range.

Label-free detection of anti-estrogen receptor alpha and its binding with estrogen receptor peptide alpha by terahertz spectroscopy

Terahertz (THz) spectroscopic techniques were employed to study the hydration shell formation around anti-estrogen receptor alpha (AER-α), an important biomarker in breast cancer diagnosis, and to detect the binding reaction between AER-α and estrogen receptor peptide alpha (ERP-α), in aqueous solutions. A remarkable nonlinear relation between THz absorption and AER-α concentration was demonstrated, shedding some light on the formation process of the hydration shell around AER-α. THz spectroscopic properties of the AER-α were also shown to be significantly affected by the binding with the ERP-α, to the extent that THz transmission and attenuation could be used to investigate the dielectric properties of antibody–antigen binding reactions.

A Terahertz Polarizer Based on Multilayer Metal Grating Filled in Polyimide Film

Terahertz (THz) polarizers with low reflection and absorption loss and high polarization extinction ratio (PER) based on multilayer metallic gratings filled in a polyimide film were studied theoretically and experimentally. In theory, THz polarization transmittance and PER of single-layer and multilayer polarizers at different THz frequencies were simulated, and their variations with the layer number of the metallic grating structure were discussed. With the increase in the layer number, PER increases prominently due to the evanescent wave coupling between different layers. In the experiment, a double-layer polarizer with an effective diameter of 42.3 mm was fabricated via layered imidization processes and evaluated by the THz time-domain spectroscopy measurements. Results showed that an average transmittance of 80.5% and a maximal PER of 70 dB were achieved, which were in good agreement with the simulated results and were compared with or outperformed most reported single- or double-layer polarizers.