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This article presents measurements of the permittivity of gelatin hydrogels between 220 and 330 GHz. Hydrated gelatin was treated as a binary mixture of free water and a compound consisting of water bound to collagen. Submillimeter-wave reflectometry was used to estimate the hydrated gelatin permittivity, hydrated gelatin density, and free-water volume fraction in phantoms composed of 62, 67, 72, and 77% water by weight. A hydrated dry/wet density ratio of 0.335 was validated with optical-coherence tomography. A constant nonfreezing bound-water mass of 0.6 g/g was observed and confirmed with differential-scanning calorimetry. Good agreement between results from different modalities supports the dielectric spectroscopy methods and data analysis. Depending on the hydrodynamics at the sample/air interface, measurements indicate a bound-water compound permittivity of 3.77−j2.52 to 3.95−j2.49—contrasting the pure-water average permittivity of 5.16–j5.63. The loss related to bound water was much higher than anticipated and characterization will help reduce uncertainty in measurements of gelatin hydrogel-based tissue phantoms; particularly corneal phantoms where adjacent free water creates complex hydration gradients. This is the first known, submillimeter-wave, frequency domain measurement of complex permittivity of the bound-water component in solid, proteinaceous matter.

In recent decades, terahertz (THz) radiation has been widely applied in many chemical and biomedical areas. Due to its ability to resolve the absorption features of many compounds noninvasively, it is a promising technique for chemical recognition of substances such as drugs or explosives. A key challenge for THz technology is to be able to accurately classify spectral measurements acquired in unknown complicated environments, rather than those from ideal laboratory conditions. Support vector machine (SVM) and deep neural networks (DNNs) are powerful and widely adopted approaches for complex classification with a high accuracy. In this article, we explore and apply the SVM and DNN methods for classifying the frequency spectra of glucose and lactose. We measured 372 groups of independent signals under different conditions to provide a sufficient training set. The classification accuracies achieved were 99% for the SVM method and 89.6% for the DNN method. These high classification accuracies demonstrate great potential in chemical recognition.\ud

Terahertz (THz) phase shifters are an essential component needed to realize many potential applications. Liquid crystals (LC) are commonly used at optical frequencies, yet to achieve an equivalent phase shift at THz frequencies the LC layer needs to be orders of magnitude thicker. Consequently, the time for the LC to relax back to its initial state is prohibitively slow. In this paper, we show for the first time how a thick, nematic phase LC cell can be switched actively in both directions, thereby achieving fast phase shifting of THz light. We call this THz in-plane and THz out-plane (TIP-TOP) switching. To achieve this, we have designed and fabricated a novel electrode structure, able to switch to and from both in-A nd out-plane orientations (TIP-TOP). The performance of the fabricated device provides an actively controllable phase delay with an ON-OFF cycle switching time of approximately 0.5 s: Almost 100 times faster than the usual cycle time which exceeds 40 s. Furthermore, the analysis of the director distributions allows us to understand the causes of the asymmetric switching times. The TIP-TOP cell presents the capability to work as a low insertion loss, fast THz phase shifter and could be scaled up to realize a phased array device.

This paper reports an accurate baseline and reference modification and acquisition algorithm for terahertz (THz) reflection measurements. The algorithm solves the spatial phase variation problem, which is a major accuracy limitation of THz reflection imaging. It also overcomes the sampling error problem without taking multiple measurements by utilizing two-dimensional data fitting. The algorithm records the spatial information and the laser power status as a database to precisely modify the baseline and reference in a measurement, providing a simple, efficient, and accurate real-time baseline and reference acquisition method. The experimental results show that the algorithm significantly improves the accuracy of the extracted optical properties of the sample with only half of the time required in a traditional THz imaging by avoiding additional baseline and reference measurements. The strong robustness of the algorithm is also proved by the experiment, giving a stable performance against laser power fluctuations. Refereed/Peer-reviewed

We report ultrabroadband, easily tunable, and highly efficient metamaterial-based terahertz wave retarders that are able to convert linear polarization into elliptical and circular polarization states. The functional device consists of a metamaterial microstructure and a grating coupler patterned on each side of fused silica substrates. The dielectric response of the metamaterial microstructure and the angular dependent phase dispersion of the grating coupler allow tuning of the phase differences from –110° to 110° within the range of a few terahertz while keeping the magnitudes of the two orthogonally transmitted waves equal. In particular, a high degree of circular polarization (>0.99) can be achieved from 1.78 to 4.88 THz for a specific dielectric value of spacer material 2.8 and angle of incidence –13°. The experimental results in the accessible frequency range of 0.2–2.3 THz show good agreement with the numerical simulations. Our study opens new opportunities for manipulating the broadband polarization responses of terahertz waves. This facilitates the development of new functional devices based on metamaterials for terahertz imaging and spectroscopy.

We propose an adaptive sampling algorithm to improve the acquisition efficiency for terahertz time-domain spectroscopy (THz-TDS). Most THz-TDS measurements scan the delay line with constant speed and the data acquired have constant time steps. Our algorithm exploits the fact that the useful information within THz signals tends to cluster at certain positions: efficient sampling can be done by adaptively increasing the sample rate in regions containing more interesting features. The algorithm was implemented by programming a linear optical delay line. Depending on the experiment parameters, the sampling time of a pulse can be reduced by a factor of 2–3 with only slight degradation in accuracy, possible sources of error are discussed. We show how adaptive sampling algorithms can improve the acquisition time in applications where the main pulse is the primary concern.

Discusses the benefits of mobile health(mHealth) technologies. These technologies empower patients to be more involved in their own care, give healthcare consumers greater access to providers, and vice versa, and allow for better personalized, precise, pervasive, and preventive medicine, in more effective and less costlyways.At the cutting edge of health engineering, the field of emerging mHealth promises a new era of highly individualized treatment of disease and preventive medicine, bringing the health convergence to reality.

A robust thin-film wire-grid terahertz (THz) polarizer was fabricated via a low-cost, mass-producible manufacturing approach. This polarizer is built on a very thin silica layer structurally supported by a silicon substrate. In addition, the metal grating is protected by a polymer thin film, which eliminates the multireflection effect and enhances the robustness of the polarizer for easy packaging. The polarizer can be easily mounted onto a Newport rotation holder for immediate use. A THz time-domain spectrometer is used to characterize its performance, and an excellent agreement is found between the FDTD-simulated results and the experimental results. The polarizer offered 20-40 dB and 0.8 dB of extinction ratio and transmission loss over a frequency range of 0.2-2.0 THz, respectively.