Tianying Chang

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.

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.

Millimeter Wave Imaging of Cracks in Bulk Coal

Millimeter wave (mmW) propagation parameters including transmission, attenuation, and reflection coefficients of coal were obtained experimentally in the frequency region of W-band (75-110 GHz). In addition, a raster scanning mmW imaging system including reflection and transmission modalities was developed, which forms images according to the difference between the dielectric parameters of cracks and those of the bulk coal samples, using the spectrum module of a W-band vector network analyzer to radiate the mmW signal to the object and obtain the imaging of the coal cracks from the S-parameters. The electromagnetic simulation and imaging results of various types of cracks are analyzed and the different imaging results are compared. The dependence on frequency and modes of operation of the mmW imaging system was systematically studied. With an appropriate imaging algorithm, we were able to employ appropriate imaging modes and choices of frequency for different types of cracks. This paper forms the foundation of further development of the mmW radar imaging system for underground coal seam crack detection and mapping.

Terahertz low-loss hollow-core pipe waveguides

Abstract. A low-loss terahertz (THz) hollow-core pipe waveguide constructed of plastic material is demonstrated. The structure is designed to be especially effective in transmitting THz waves of 110 GHz, which has important applications in communications, imaging, and sensing. Guiding of the electromagnetic wave is based on the principle of antiresonant reflection. Through careful theoretical analyses and systematic modeling and simulation, followed by a thorough experimental investigation, we show that the proposed structure can successfully transmit THz waves with low attenuation. Furthermore, when the structures of the pipe waveguides are varied for optimization, we find that cladding thickness and the refractive index under antiresonant conditions as well as the core diameter are important physical parameters in designing the low-loss THz waveguide. Considering not just attenuation loss but such factors as volume, weight, and flexibility of the tube waveguide, along with other practical issues such as cost, we arrive at an optimal design of the pipe waveguide, which has an inner diameter of 35 mm and cladding thickness of 5 mm. Teflon is chosen as the material for a guiding 110-GHz THz wave. The attenuation constant is determined by the simulation to be as low as 0.0228  m−1. However, due to nonuniformity of the waveguide wall thickness and random small bending of the waveguide, as well as moisture content in the air filling the pipe core, all of which strongly impede propagation in the waveguide, the experimentally measured attenuation loss (3.65  m−1) of the waveguide is much more significant than the theoretical prediction, with the latter serving as a design benchmark under perfect conditions.

Signal detection techniques for scattering-type scanning near-field optical microscopy

ABSTRACT Scattering-type scanning near-field optical microscopy (s-SNOM) has been playing more and more important roles in investigating electromagnetic properties of various materials and structures on the nanoscale. In this technique, a sharp tip is employed as the near-field antenna to measure the samples properties with a high spatial resolution. As the scattered near-field signal from the tip is extremely weak and contaminated by strong background noise, the effective detection, and subsequent extraction of the near-field information from the detected signals is the key issue for s-SNOM. In this review, we give a systematic explanation of the underlying mechanisms of s-SNOM, and summarize and interpret major signal detection techniques involved, including experimental setups, theories for signal analysis and processing, and exposition of advantages and disadvantages of such techniques. By this, we hope to provide a practical guide and a go-to source of detailed information for those interested in and/or working on s-SNOM.

Dimension-Factorized Range Migration Algorithm for Regularly Distributed Array Imaging

The two-dimensional planar MIMO array is a popular approach for millimeter wave imaging applications. As a promising practical alternative, sparse MIMO arrays have been devised to reduce the number of antenna elements and transmitting/receiving channels with predictable and acceptable loss in image quality. In this paper, a high precision three-dimensional imaging algorithm is proposed for MIMO arrays of the regularly distributed type, especially the sparse varieties. Termed the Dimension-Factorized Range Migration Algorithm, the new imaging approach factorizes the conventional MIMO Range Migration Algorithm into multiple operations across the sparse dimensions. The thinner the sparse dimensions of the array, the more efficient the new algorithm will be. Advantages of the proposed approach are demonstrated by comparison with the conventional MIMO Range Migration Algorithm and its non-uniform fast Fourier transform based variant in terms of all the important characteristics of the approaches, especially the anti-noise capability. The computation cost is analyzed as well to evaluate the efficiency quantitatively.

Void and crack detection of polymethacrylimide foams based on terahertz time-domain spectroscopic imaging

Terahertz reflection imaging is considered as a potential diagnostic tool for the investigation of polymethacrylimide foam defects. Using terahertz time-domain spectroscopy (TDS) and detection methods based on terahertz spectroscopic analysis, the reflection imaging results of different thicknesses of polymethacrylimide foam with two kinds of detections (cracks and voids) are studied. The samples (Degussa Rohacell WF71) are planar slabs of polymethacrylimide foams with thicknesses of 35 mm, 60.5 mm and 10 mm. It is found that the same kinds of polymethacrylimide foam defects with different foam thicknesses have similar spectral characteristics, with marked differences only in the amplitude and phase of the reflected wave. In view of this, we focus our study on the defective spectral characteristics of one of the foams (35 mm thickness). The characteristics of void and crack defects are analyzed in the time domain, which is based mainly on the variation of the reflected waveform. In particular, the imaging and clear identification of voids of less than 2.4 mm in diameter, fine cracks (0.3 mm wide), and the quantification of defects can be readily achieved using the terahertz non-destructive testing technique described here.

Hilbert-Transform-Based Accurate Determination of Ultrashort-Time Delays in Terahertz Time-Domain Spectroscopy

Accurate determination of time delays is crucial for distinguishing a samples different interfaces in terahertz time-domain spectroscopy (THz-TDS), especially for ultrathin samples or interfacial gaps. In this paper, Hilbert transform (HT) is invoked, along with posttransform signal spectral estimation of several varieties, including the conventional, that based on multiple-signal classification spectrum estimation (HT-MUSIC), and that based on autoregressive spectrum estimation using Yule–Walker law (HT-AR), to determine the samples different interfaces. The results are compared with the traditional method of obtaining time delays in THz-TDS, along with analysis of the resolution and accuracy, as well as advantages and deficiencies of the various approaches. It is demonstrated that HT is an effective method for determining different interfaces of an ultrathin sample by simulation and experiments. The simulation results show that MUSIC-HT has the best resolution, at about 0.35 ps and AR-HT has the best accuracy, whose error is less than 0.075 ps. Experimental results for ultrathin air gaps and polymer films confirmed that MUSIC-HT can distinguish time delays as short as 0.44 ps from the THz time-domain spectra.