The skin’s interstitial fluid (ISF) represents a versatile platform for non-invasive in vivo biosensing of systemic biomarkers such as glucose. Biomedical applications of THz spectroscopy mostly leverage the strong interaction between THz light and water by mapping frequency-dependent changes in the sample’s dielectric response. We propose a novel THz spectroscopy-based approach for non-invasive detection of glucose in the skin’s ISF in conjunction with machine learning (ML). In this study, we explore advantages and limitations an ex vivo experiment on fresh porcine skin as a proof of concept of our approach. We investigate multiple sources of variation in such a dataset to understand how well our samples represent their in vivo counterpart. We characterize inter-sample and intra-sample variations to rule out undesired bias in our data that may complicate classification or regression tasks for glucose detection. Our results indicate that occlusion during THz contact measurements affects fresh ex vivo porcine skin similarly to what has previously been reported for in vivo human skin. Data processing strategies for ex vivo experiments for THz spectroscopy or imaging should therefore find ways to account for these effects.
Terahertz (THz) sensing of ultrathin layers has been a longstanding challenge due to limitations in conventional detection techniques. In this study, we present a novel approach for sensing sub-1 nm thin dielectric layers based on Fowler-Nordheim (FN) tunneling. Our method exploits the FN tunneling effect at a metal-dielectric interface, enabling sensitive detection of changes in dielectric layer thickness within the THz frequency range. To validate our FN tunneling-based THz sensing technique, we carried out a comprehensive analysis of experimental and simulated data. Our findings demonstrate that this approach exhibits exceptional sensitivity, capable of detecting dielectric layers with thicknesses down to the sub-nanometer scale. Such sensitivity has significant implications for various applications, including nanoscale dielectric characterization, advanced material development, and quality control in microelectronics manufacturing. The FN tunneling-based THz sensing methodology not only overcomes the limitations of traditional detection techniques but also paves the way for novel ultrathin layer sensing capabilities in the rapidly advancing field of terahertz technology. Our study showcases the potential of this groundbreaking technique to revolutionize the THz sensing landscape, offering new opportunities for research and development in various fields.
The skin's interstitial fluid is rich in composition and easily accessible for the monitoring of systemic biomarkers, however, THz-based molecular detection in biological media is challenging. Machine learning can provide solutions, but strict data engineering is required to avoid confounding trends and ensure large training datasets. We propose an experimental framework to mimic interstitial fluid diffusion in ex vivo pig skin to detect analytes via THz-ATR spectroscopy. We evaluate the applicability of the protocol for controlled studies of THz-ATR spectroscopy-based biomolecular detection in skin. Our findings can significantly contribute to the field of ML-reinforced biosensing.
We report on various metasurfaces for the purpose of THz driven electron field emission and subsequent detection using vacuum electronics. The underlying principle is based on strong localised field enhancement at metal and semimetal emission points, which bends the vacuum potential temporarily to allow for field emission of electrons from the parent material. The structures are investigated for varying electric field strength using electron time-of-flight measurements as well as electron multiplication and visualisation on a phosphor screen. Measured properties include the emitted electron energy, their count, and the emission threshold. From the recorded data, the local field enhancement for each structure is extracted and compared to simulated values. Subsequently, optimised metasurfaces are implemented into handheld devices that serve as easy-to-use THz detectors. These devices include photomultiplier tubes which operate at frequencies from THz to infrared, as well as live imaging devices with kilohertz framerates. The investigated metallic structures include standard dipole antennas, double split-ring resonators, bow-tie designs, hybrid split-ring and dipole designs, and logarithmic spirals. Semimetallic structures are based on structured and unstructured graphene, which show different emission characteristics. All samples are investigated using strong-field THz radiation generated using lithiumniobate tilted pulse front setup, as well as commercial THz-TDS instruments. In conclusion, we present a holistic overview of the current state-of-the-art THz-PMTs and image intensifiers.
A THz detector with both high sensitivity and fast time response has been required for industrial applications such as nondestructive testing (NDT), security, and spectroscopy. Through a collaboration with the Technical University of Denmark (DTU), we have recently developed a THz-sensitive point detector and imager based on metasurface and photomultiplier tube (PMT) and image intensifier (I.I.) technologies, respectively. A fast time response is one of the unique characteristics of these devices: the PMT-based point detector provides a nanosecond response time while the I.I.- based imager is capable of frame rates up to 1000 fps. These devices have a double split-ring resonator (DSRR) at the photocathode for THz-electron conversion (metasurface). In this paper, we discuss the two devices and report on the development and results for increasing their sensitivity for ultrafast, broadband THz pulses by sharpening the fieldenhancing antenna tips. This leads to a smaller tip diameter, which increases the electric field confinement and thus intensity at the tip, making the field emission more likely to occur at lower field strengths as a result. Both devices thus offer a sensitive and simple method to detect THz frequencies easily, with the I.I. offering a handheld, 9V batterypowered device.
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