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Funder: Alexander von Humboldt Foundation, Funder: Experienced Researcher Fellowship, Funder: DFG Collaborative, Funder: PAPIIT, Funder: Danish National Research Foundation, Funder: JSPS, Funder: STFC Centre for Instrumentation, Funder: UKSA Centre for Earth, Funder: Leverhulme Trust; doi: http://dx.doi.org/10.13039/501100000275, Funder: NRF, Funder: Royal Society; doi: http://dx.doi.org/10.13039/501100000288, Funder: Australian Government; doi: http://dx.doi.org/10.13039/100015539, Funder: Cancer Research UK; doi: http://dx.doi.org/10.13039/501100000289, Funder: University of Warwick; doi: http://dx.doi.org/10.13039/501100000741, Terahertz (THz) radiation encompasses a wide spectral range within the electromagnetic spectrum that extends from microwaves to the far infrared (100 GHz–∼30 THz). Within its frequency boundaries exist a broad variety of scientific disciplines that have presented, and continue to present, technical challenges to researchers. During the past 50 years, for instance, the demands of the scientific community have substantially evolved and with a need for advanced instrumentation to support radio astronomy, Earth observation, weather forecasting, security imaging, telecommunications, non-destructive device testing and much more. Furthermore, applications have required an emergence of technology from the laboratory environment to production-scale supply and in-the-field deployments ranging from harsh ground-based locations to deep space. In addressing these requirements, the research and development community has advanced related technology and bridged the transition between electronics and photonics that high frequency operation demands. The multidisciplinary nature of THz work was our stimulus for creating the 2017 THz Science and Technology Roadmap (Dhillon et al 2017 J. Phys. D: Appl. Phys. 50 043001). As one might envisage, though, there remains much to explore both scientifically and technically and the field has continued to develop and expand rapidly. It is timely, therefore, to revise our previous roadmap and in this 2023 version we both provide an update on key developments in established technical areas that have important scientific and public benefit, and highlight new and emerging areas that show particular promise. The developments that we describe thus span from fundamental scientific research, such as THz astronomy and the emergent area of THz quantum optics, to highly applied and commercially and societally impactful subjects that include 6G THz communications, medical imaging, and climate monitoring and prediction. Our Roadmap vision draws upon the expertise and perspective of multiple international specialists that together provide an overview of past developments and the likely challenges facing the field of THz science and technology in future decades. The document is written in a form that is accessible to policy makers who wish to gain an overview of the current state of the THz art, and for the non-specialist and curious who wish to understand available technology and challenges. A such, our experts deliver a ‘snapshot’ introduction to the current status of the field and provide suggestions for exciting future technical development directions. Ultimately, we intend the Roadmap to portray the advantages and benefits of the THz domain and to stimulate further exploration of the field in support of scientific research and commercial realisation.

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.

Finite element method (FEM) simulations (employing ANSYS High Frequency Structure Simulator, HFSS) are used to investigate the response of terahertz (THz) frequency range split‐ring resonators (SRRs) integrated with on‐chip THz waveguides to cancerous tissues. Two‐port S‐parameter simulations are performed to obtain the transmission spectra (S21) of a planar Goubau line (PGL) integrated with an SRR. Permittivity and loss tangent of the colonic tissues are both taken into account in the numerical simulation. The transmission spectra of the SRR integrated PGL are obtained for cancerous and healthy tissues in close proximity to the SRR, and it is found that they can be distinguished by the resonant frequency shift of the SRR induced by dielectric loading. The electric field distribution and magnitude near the SRR for various capacitive gap widths of SRR are investigated to understand how the gap width affects the maximum electric field magnitude and the vertical extent of the electric field in the gap area. The simulated imaging of colonic tissue consisting of healthy and cancerous tissues using the SRR integrated PGL device with a protective layer on it is performed, showing how the technique could in principle be used to distinguish tumor margins with realistic THz dielectric parameters.

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

Transdermal drug delivery (TDD) has been widely used in medical treatments due to various advantages, including delivering drugs at a consistent rate. However, variations in skin hydration can have a significant effect on the permeability of chemicals. Therefore, it is essential to study the changes in skin hydration induced by TDD patches for better control of the delivery rate. In this work, in vivo terahertz (THz) spectroscopy is conducted to quantitatively monitor human skin after the application of patches with different backing materials and propylene glycol concentrations. Changes in skin hydration and skin response to occlusion induced by other patches are investigated and compared. Our work demonstrates the potential application of in vivo THz measurements in label-free, non-invasive evaluation of transdermal patches on human skin and further reveals the mechanism behind the effect.

Water content of the skin is an important parameter for controlling the penetration rate of chemicals through the skin barrier; therefore, for transdermal patches designed for drug delivery to be successful, the effects of the patches on the water content of the skin must be understood. Terahertz (THz) spectroscopy is a technique which is being increasingly investigated for biomedical applications due to its high sensitivity to water content and non-ionizing nature. In this study, we used THz measurements of the skin (in vivo) to observe the effect of partially and fully occlusive skin patches on the THz response of the skin after the patches had been applied for 24 h. We were able to observe an increase in the water content of the skin following the application of the patches and to identify that the skin remained hyper-hydrated for four hours after the removal of the fully occlusive patches. Herein, we show that THz spectroscopy has potential for increasing the understanding of how transdermal patches affect the skin, how long the skin takes to recover following patch removal, and what implications these factors might have for how transdermal drug patches are designed and used.

Methods utilizing finite rate of innovation theory have been employed to achieve low sampling rates in ultrasound experiments. Here, we apply this theory to create a terahertz specific method to gain the benefits of faster signal processing speeds and data acquisition compared with current standard processing methods. We verify our method by simulating a THz like signal, adding Gaussian noise, using a low sampling rate, processing it through our code and then comparing the reconstructed output to the original simulated signal to find close agreement.

Terahertz metamaterial sensing (TMS) is a new interdisciplinary technology. A TMS system employs terahertz waves as the pumping source, these then interact with the sample and carry the substance information, e.g., refractive index, absorption spectra. These properties are relevant to the molecular rotation and vibration states produced by a surface-plasmon-polariton-like effect. TMS technology is usually characterized by large penetration depth and high sensitivity. Owing to these advantages, TMS may be used for ultratrace detection and consequently has a wide range of practical applications in biomedicine, food safety, environmental monitoring, industry and agriculture, material characterization, and safety inspection. Furthermore, TMS performance is determined not only by the structural topology of metamaterials, but also by their compositions and substrates. This paper reviews the essential fundamentals, relevant applications, and recent advances in TMS technology with a focus on the influence of material selection on TMS performance. This review is envisaged to be used as a key reference for developing TMS-based functional devices with enhanced characteristics.\ud \ud

Melamine resin sponges have a porous cell structure on the micrometer scale, comparable to the terahertz wavelength. Hydrating the sponge with water introduces a micro-scale rough surface which greatly attenuates the terahertz reflection. When combined with the high absorption of water, the hydrated sponge shows a perfect shielding characteristic by reflecting less than 1% of the light intensity in 0.75-3.5 THz.

Here we present an optimized multilayer structure for sensitive characterization of thin-film aqueous solutions with reflection terahertz time-domain spectroscopy. The theoretical sensitivity is compared with a recently published sensitive structure and other ordinary geometries. Glucose solutions with a 5% concentration gradient are measured for accuracy verification.

Terahertz (THz) frequencies have a strong sensitivity to water, a property which has led to research into ways to utilize this property for non-invasive in vivo diagnostics. This research investigates the ability of measurements using THz light to identify changes induced in the skin by various skin products. These studies enhance the understanding of how THz light could be used for in vivo measurements in the future.

In this work we present three devices based on parallel-plate-waveguides (PPWGs): a demultiplexer; a beam splitter; and an axicon. The PPWGs used in the construction of such devices are made out of stainless steel sheets and vary in longitudinal length (along the beam propagation direction) in a linear fashion. All the proposed quasi-optical elements can be easily tuned to work within different frequency ranges only by changing the spacing between the plates forming the PPWG’s, making this approach attractive for the fabrication of active devices for communication applications.

In this contribution we optimize THz-spatial modulator design, post-processing algorithms and undersampling methods with the aim of minimizing the acquisition of a THz-image using a single-pixel detector and being able to display the image without needing long computational times. Further, we investigate how to synchronize the optical delay line so as to obtain a THz image containing time-of-flight information and then demonstrate that neural networks can improve image quality in real-time.

This work presents an experimental setup to control the way in which pressure interferes with the repeatability of in vivo THz skin measurements. By integrating a pressure sensor circuit into our THz system, it is possible to identify which measurements were taken within a previously specified pressure range. The live response of the pressure sensor helps to acquire data within the desired pressure leading to greater consistency of data between measurements. Additionally, a protocol is proposed to help achieve repeatable results and to remove the effects of the natural variation of the skin through the course of the day. This technique has been shown to be able to quantify the changes induced in the skin following the application of a moisturising skin product and shows the measured result to be significantly different from natural skin variation. This research therefore prepares the way for further studies on the effectiveness of different skin products using in vivo THz measurements.\ud \ud

Terahertz imaging looks set to become an integral part of future applications from semiconductor quality control to medical diagnosis. This will only become a reality when the technology is sufficiently cheap and capabilities adequate to compete with others. Single-pixel cameras use a spatial light modulator and a detector with no spatial-resolution in their imaging process. The spatial-modulator is key as it imparts a series of encoding masks on the beam and the detector measures the dot product of each mask and the object, thereby allowing computers to recover an image via post-processing. They are inherently slower than parallel-pixel imaging arrays although they are more robust and cheaper, hence are highly applicable to the terahertz regime. This chapter dedicates itself to terahertz single-pixel cameras; their current implementations, future directions and how they compare to other terahertz imaging techniques. We start by outlining the competing imaging techniques, then we discuss the theory behind single-pixel imaging; the main section shows the methods of spatially modulating a terahertz beam; and finally there is a discussion about the future limits of such cameras and the concluding remarks express the authors’ vision for the future of single-pixel THz cameras.

Terahertz (THz) in vivo reflection imaging can be used to assess the water content of the surface of the skin. This study presents the results of treating 20 subjects with aqueous, anhydrous and water-oil emulsion samples and observing the changes induced in the skin using THz sensing. These regions were also measured with a corneometer, the present gold standard for skin hydration assessment within the cosmetics industry. We find that THz sensing is effective at observing the presence of oil and water on the surface of the skin, these results can be verified with the measurements of capacitance taken by the corneometer. The THz measurements reveal a distinction between the responses of subjects with initially dry or well hydrated skin, this observation is particularly noticeable with the oil-based samples. Additionally, moderate correlation was found between the THz reflected amplitude and capacitance of untreated skin with a correlation coefficient of r = −0.66, suggesting THz sensing has promising potential for assessing skin hydration.

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.

Terahertz (THz) technology has experienced rapid development in the past two decades. Growing numbers of interdisciplinary applications are emerging, including materials science, physics, communications, and security as well as biomedicine. THz biophotonics involves studies applying THz photonic technology in biomedicine, which has attracted attention due to the unique features of THz waves, such as the high sensitivity to water, resonance with biomolecules, favorable spatial resolution, capacity to probe the water–biomolecule interactions, and nonionizing photon energy. Despite the great potential, THz biophotonics is still at an early stage of development. There is a lack of standards for instrumentation, measurement protocols, and data analysis, which makes it difficult to make comparisons among all the work published. In this article, we give a comprehensive review of the key findings that have underpinned research into biomedical applications of THz technology. In particular, we will focus on the advances made in general THz instrumentation and specific THz-based instruments for biomedical applications. We will also discuss the theories describing the interaction between THz light and biomedical samples. We aim to provide an overview of both basic biomedical research as well as pre-clinical and clinical applications under investigation. The paper aims to provide a clear picture of the achievements, challenges, and future perspectives of THz biophotonics.

We demonstrate the ability of THz imaging to evaluate the efficacy of transdermal drug delivery. Effects of transdermal drug delivery by topical and micro/nano-needle patch methods are measured. Our results prove that THz imaging is able to monitor the improved drug delivery efficacy given by a nanoneedle patch.

Although terahertz waves have been used in various in vivo skin studies, hitherto, very little attention has been given to the skin surface features. Quantitative studies are of great importance to analyze their effects on the characterization of skin and skin modeling. We use Lumerical FDTD and Comsol to simulate the influence on the THz reflectivity for different hydration levels, polarizations and frequencies. Considerably large errors are observed, suggesting that the influence should be carefully considered in THz skin measurements.

In this work we present a 3D printed mux/demux device based on rectangular waveguides. The principle of operation of the proposed technology relies on the coupled mode theory. The proposed device is able to merge and separate two independent broadband input channels into a single output channel. This represents an attractive option for novel passive elements able to process broadband THz signals for communication proposes.

We propose an approach based on support vector machine(SVM) and deep neural networks (DNN) to classify chemical substances under different experimental conditions in terahertz time-domain spectroscopy (THz-TDS). 372 groups of independent signals under different conditions were measured to provide a sufficient training set. 99% accuracy for the SVM and 89.6% for the DNN method are realized in the test set. These excellent classification results show the high potentials in chemical recognition, security detection or clinical diagnosis.

To fully extract useful and accurate information from sample data, signal processing techniques are required. In this work, the finite rate of innovation method is applied to both randomised simulated data and a simple terahertz test scan for verification of the method. The key benefit of this method is the low sampling rate it allows. It is shown that this method produces accurate results whilst being faster than other current methods, such as sparse deconvolution, as the sampling rate is low. This faster data acquisition is especially useful for sampling time varying samples.

Terahertz (THz) imaging is the focus of research regarding a growing number of biomedical applications. In this study the ability of in vivo THz measurements to distinguish the different effects of three commercial skin products composed of water, oil and a water-oil emulsion is investigated. It can be seen that some samples are received in a uniform way by many skin types while others show a broad range of responses.

In this research, we present the design, fabrication, and experimental validation of 3D printed bandpass filters and mux/demux elements for terahertz frequencies. The filters consist of a set of in-line polystyrene (PS) rectangular waveguides, separated by 100 µm, 200 µm, and 400 µm air gaps. The principle of operation for the proposed filters resides in coupled-mode theory. Q-factors of up to 3.4 are observed, and additionally, the experimental evidence demonstrates that the Q-factor of the filters can be improved by adding fiber elements to the design. Finally, using two independent THz broadband channels, we demonstrate the first mux/demux device based on 3D printed in-line filters for the THz range. This approach represents a fast, robust, and low-cost solution for the next generation of THz devices for communications.

Single pixel imaging (SPI) captures images without array detectors or raster scanning. When combined with compressive sensing techniques it enables novel solutions for high-speed optical imaging and spectroscopy. However, when it comes to the real-time capture and analysis of a fast event, the challenge is the inherent trade-off between frame rate and image resolution. Due to the lack of sufficient sparsity and the intrinsic iterative process, conventional compressed sensing techniques have limited improvement in capturing natural scenes and displaying the images in real time. In this work, we demonstrate a novel alternative compressive imaging approach employing an efficient and easy-implementation sampling scheme based on reordering the deterministic Hadamard basis through their total variation. By this means, the number of measurements and acquisition are reduced significantly without needing complex minimization algorithms. We can recover a 128 × 128 image with a sampling ratio of 5% at the signal peak signal-to-noise ratio (PSNR) of 23.8 dB, achieving super sub-Nyquist sampling SPI. Compared to other widely used sampling e.g. standard Hadamard protocols and Gaussian matrix methods, this approach results in a significant improvement both in the compression ratio and image reconstruction quality, enabling SPI for high frame rate imaging or video applications.

In this Letter, we report a broadband frequency/polarization demultiplexer based on parallel-plate waveguides (PPWGs) for terahertz (THz) frequencies. The fabrication and experimental validation of this polarization sensitive demultiplexer is demonstrated for the range from 0.2 to 1 THz. Upgrading the demultiplexer by adding a second demultiplexer stage, a fifty-fifty amplitude splitter is also demonstrated in the same frequency range. The multiplexer is based on a stainless-steel traveling-wave antenna, exhibiting strong mechanical robustness. This unique device exhibits three splitting mechanisms in the same device: amplitude, polarization, and frequency splitting. This is a significant improvement for the next generation of THz passive components for communication purposes.

To promote potential applications of terahertz (THz) technology more advanced functional THz devices with high performance are needed, including modulators, polarizers, lenses, wave retarders and anti-reflection coatings. This article summarizes recent progress in THz components built on functional materials including graphene, vanadium dioxide and metamaterials. Our key message is that while the choice of materials used in such devices is important, the geometry in which they are employed also has a significant effect on the performance achieved. In particular, we review devices operating in total internal reflection geometry and explain how this geometry is able to be exploited to achieve a variety of THz devices with broadband operation

Transdermal drug delivery (TDD) is widely used for painless dosing due to its minimally invasive nature compared to hypodermic needle injection and its avoidance of the gastrointestinal tract. However, the stratum corneum obstructs the permeation of drugs into skin. Microneedle and nanoneedle patches are ways to enhance this permeation. In this work, terahertz (THz) imaging is utilized to compare the efficacy of different TDD methods including topical application and via a needle patch. Our work shows the feasibility and potential of using THz imaging to quantify and evaluate different transdermal application methods.

We report a new strategy to fabricate a core-shell TiO2@g-C3N4 composite for photoelectrochemical water splitting. The heterojunction structure is prepared by chemically wrapping exfoliated thin layer g-C3N4 nanosheets on the surface of anatase TiO2 particles. The TiO2@g-C3N4 sample demonstrates high visible-light photoactivity towards water splitting, resulting in an increase in photocurrent density by a factor of 2.5 times compared to the bare TiO2. This is ascribed to the inhibition of electron-hole pair recombination due to the synergistic effect between TiO2 and g-C3N4, which enhances the charge transfer and separation. The prolonged lifetime of the charges is confirmed by using the transient absorption spectroscopic measurements.