Objective and efficient terahertz signal denoising by transfer function reconstruction

As an essential processing step in many disciplines, signal denoising efficiently improves data quality without extra cost. However, it is relatively under-utilized for terahertz spectroscopy. The major technique reported uses wavelet denoising in the time-domain, which has a fuzzy physical meaning and limited performance in low-frequency and water-vapor regions. Here, we work from a new perspective by reconstructing the transfer function to remove noise-induced oscillations. The method is fully objective without a need for defining a threshold. Both reflection imaging and transmission imaging were conducted. The experimental results show that both low- and high-frequency noise and the water-vapor influence were efficiently removed. The spectrum accuracy was also improved, and the image contrast was significantly enhanced. The signal-to-noise ratio of the leaf image was increased up to 10 dB, with the 6 dB bandwidth being extended by over 0.5 THz

Compressed sensing with near-field THz radiation

We demonstrate a form of near-field terahertz (THz) imaging that is compatible with compressed sensing algorithms. By spatially photomodulating THz pulses using a set of shaped binary optical patterns and employing a 6-μm-thick silicon wafer, we are able to reconstruct THz images of an object placed on the exit interface of the wafer. A single-element detector is used to measure the electric field amplitude of transmitted THz radiation for each projected pattern, with the ultra-thin wafer allowing us to access the THz evanescent near fields to achieve a spatial resolution of ∼9  μm∼9  μm

In vivo terahertz imaging to evaluate scar treatment strategies : silicone gel sheeting

Silicone gel sheeting (SGS) is widely used for scar treatment; however, studies showing its interaction with skin and efficacy of scar treatment are still lacking. THz light is non-ionizing and highly sensitive to changes in water content and thus skin hydration. In this work, we use in-vivo THz imaging to monitor how SGS affects the THz response of human skin during occlusion, and the associated THz reflectivity and refractive index changes are presented. We find that SGS effectively hydrates the skin beneath it, with minimal lateral effects beyond the sheeting. Our work demonstrates that THz imaging is able to detect the subtle hydration changes on the surface of human skin caused by SGS, and it has the potential to be used to evaluate different scar treatment strategies

THz in vivo measurements : the effects of pressure on skin reflectivity

Terahertz (THz) light is non-ionizing and highly sensitive to subtle changes in water concentration which can be indicative of disease. The short THz penetration depth in bio-samples restricts in vivo measurements to be in a reflection geometry and the sample is often placed onto an imaging window. Upon contacting the imaging window, occlusion and compression of the skin affect the THz response. If not appropriately controlled, this could cause misleading results. In this work, we investigate and quantify how the applied pressure affects the THz response of skin and employ a stratified model to help understand the mechanisms at play. This work will enable future THz studies to have a more rigorous experimental protocol, which in turn will facilitate research in various potential biomedical applications under investigation

Simulated and experimental verification for a terahertz specific finite rate of innovation signal processing method

Recently, finite rate of innovation methods have been successfully applied to achieve low sampling rates in many areas, such as for ultrasound and radio signals. However, to the best of our knowledge, there are no journal publications applying this to real terahertz signals. In this work, we mathematically describe a finite rate of innovation method applied specifically to terahertz signals both experimentally and in simulation. To demonstrate our method, we applied it to randomized simulated signals with and without the presence of noise and to simple experimental measurements. We found excellent agreement between the simulated signals and those recreated based on results from our method, with this success also being replicated experimentally. These results were obtained at relatively low sampling rates, compared to standard methods, which is a key advantage to using a finite rate of innovation method as it allows for faster data acquisition and signal processing

Optimized multilayer structure for sensitive THz characterization of thin-film glucose solutions

Terahertz time-domain spectroscopy (THz-TDS) has shown promise in biomedical sample characterization and high characterization sensitivity is in demand due to the thin-film (TF) feature of the sample. This paper proposes an optimized multilayer structure for sensitive characterization of TF aqueous solutions in reflection THz-TDS. Theoretical simulations are conducted for structural optimization and the 75 µm window-sample-mirror structure displays the best sensitivity compared to other sandwich structures and traditional THz measurement geometries. 0-20% TF glucose solutions are then measured; and a spectral peak introduced by the proposed structure is observed to result in the high sensitivity. Our work provides a new way of customizing multilayer structure for THz thin-film characterization

Graphene controlled Brewster angle device for ultra broadband terahertz modulation

Terahertz modulators with high tunability of both intensity and phase are essential for effective control of electromagnetic properties. Due to the underlying physics behind existing approaches there is still a lack of broadband devices able to achieve deep modulation. Here, we demonstrate the effect of tunable Brewster angle controlled by graphene, and develop a highly-tunable solid-state graphene/quartz modulator based on this mechanism. The Brewster angle of the device can be tuned by varying the conductivity of the graphene through an electrical gate. In this way, we achieve near perfect intensity modulation with spectrally flat modulation depth of 99.3 to 99.9 percent and phase tunability of up to 140 degree in the frequency range from 0.5 to 1.6 THz. Different from using electromagnetic resonance effects (for example, metamaterials), this principle ensures that our device can operate in ultra-broadband. Thus it is an effective principle for terahertz modulation

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Terahertz (THz) thin-film total internal reflection (TF-TIR) spectroscopy is shown to have an enhanced sensitivity to the vibrational properties of thin films in comparison with standard THz transmission spectroscopy. This increased sensitivity was used to track photoinduced modifications to the structure of thin films of methylammonium (MA) lead halide, MAPbI3–xBrx (x = 0, 0.5, 1, and 3). Initially, illumination strengthened the phonon modes around 2 THz, associated with Pb–I stretch modes coupled to the MA ions, whereas the 1 THz twist modes of the inorganic octahedra did not alter in strength. Under longer term illumination, the 1 THz phonon modes of encapsulated films slowly reduced in strength, whereas in films exposed to moisture and oxygen, these phonons weaken more rapidly and blue-shift in frequency. The rapid monitoring of environmentally induced changes to the vibrational modes afforded by TF-TIR spectroscopy offers applications in the characterization and quality control of the perovskite thin-film solar cells and other thin-film semiconductors

Terahertz time-domain spectroscopy for the analysis of latex film formation

The subject of latex film formation has been studied for many years and it is known to be affected by many environmental conditions such as evaporation rate, polymer glass transition temperature, T g, and particle size. Understanding latex film formation is particularly relevant to the paint industry, to ensure even coated films. In this study, THz-TDS was used to analyze various latex solutions with different polymer glass transition temperatures and particle sizes. 2D water distribution maps were produced, as a function of drying time, to monitor latex drying processes such as the ‘coffee-ring effect’

Monitoring the terahertz response of skin beneath transdermal drug delivery patches using sparse deconvolution

Terahertz (THz) spectroscopy is a technique proving extremely useful for investigating various biomedical applications by virtue of its high sensitivity in the measurement of water content and non-ionizing nature. By combining this with sparse deconvolution, the THz response of skin directly underneath transdermal drug delivery (TDD) patches was isolated and reconstructed to determine the skin water content in vivo. Verification for this method was given by a comparison of skin measured through patches using sparse deconvolution, and skin measurements immediately following patch removal processed with standard approaches. It was found that patches with a non-permeable film backing hydrated the skin to a greater extent than permeable woven polyester fiber backed patches and that this hydration effect primarily occurs within the first 30 minutes of patch application and lasts for at least 24 hours given that the patch remains applied. We demonstrate the effectiveness of this sparse reconstruction method to track hydration levels through layers such as patches and identify scope for further applications including TDD patch development and wound healing techniques and monitoring