Publications
362 results found
Wu R, Navarro-Cia M, Chekulaev D, et al., 2023, Active control of mid-wavelength infrared non-linearity in silicon photonic crystal slab, Optics Express, Vol: 31, Pages: 35644-35644, ISSN: 1094-4087
Natural materials’ inherently weak nonlinear response demands the design of artificial substitutes to avoid optically large samples and complex phase-matching techniques. Silicon photonic crystals are promising artificial materials for this quest. Their nonlinear properties can be modulated optically, paving the way for applications ranging from ultrafast information processing to quantum technologies. A two-dimensional 15-μm-thick silicon photonic structure, comprising a hexagonal array of air holes traversing the slab’s thickness, has been designed to support a guided resonance for the light with a wavelength of 4-μm. At the resonance conditions, a transverse mode of the light is strongly confined between the holes in the "veins" of the silicon component. Owing to the confinement, the structure exhibits a ratio of nonlinear to linear absorption coefficients threefold higher than the uniform silicon slab of the same thickness. A customised time-resolved Z-scan method with provisions to accommodate ultrafast pump-probe measurements was used to investigate and quantify the non-linear response. We show that optically pumping free charge carriers into the structure decouples the incoming light from the resonance and reduces the non-linear response. The time-resolved measurements suggest that the decoupling is a relatively long-lived effect on the scale comparable to the non-radiative recombination in the bulk material. Moreover, we demonstrate that the excited free carriers are not the source of the nonlinearity, as this property is determined by the structure design.
Azpilicueta L, Schultze A, Celaya-Echarri M, et al., 2023, Diffuse-Scattering-Informed Geometric Channel Modeling for THz Wireless Communications Systems, IEEE Transactions on Antennas and Propagation, Vol: 71, Pages: 8226-8238, ISSN: 0018-926X
Surpassing 100-Gb/s data throughput is a key objective and an active area of research for sixth-generation (6G) wireless networks that can only be met by exploiting the terahertz (THz) frequency band (0.3-10 THz). THz channel modeling faces new challenges given the emerging relevance of scattering and molecular absorption in this frequency range as well as the lack of a reliable library of material properties. In this work, we address these challenges by measuring systematically the dielectric properties of 27 common building and office materials and reporting an in-house 3-D ray-launching (3D-RL) algorithm that uses the created material library and accounts for rough surface scattering and atmospheric attenuation. In order to validate the proposed algorithm, a channel sounder measurement campaign has been performed in a typical indoor environment at 300 GHz. Simulations and measurements show good agreement, demonstrating the need for modeling scattering and atmospheric absorption in the THz band. The proposed channel model approach enables scenarios at THz frequencies to be investigated by simulation, providing relevant knowledge for the development of ultrahigh-speed wireless communication systems.
Hou Y, Navarro-Cia M, 2023, A computationally-inexpensive strategy in CT image data augmentation for robust deep learning classification in the early stages of an outbreak, Biomedical Physics & Engineering Express, Vol: 9, ISSN: 2057-1976
Coronavirus disease 2019 (COVID-19) has spread globally for over three years, and chest computed tomography (CT) has been used to diagnose COVID-19 and identify lung damage in COVID-19 patients. Given its widespread, CT will remain a common diagnostic tool in future pandemics, but its effectiveness at the beginning of any pandemic will depend strongly on the ability to classify CT scans quickly and correctly when only limited resources are available, as it will happen inevitably again in future pandemics. Here, we resort into the transfer learning procedure and limited hyperparameters to use as few computing resources as possible for COVID-19 CT images classification. Advanced Normalisation Tools (ANTs) are used to synthesise images as augmented/independent data and trained on EfficientNet to investigate the effect of synthetic images. On the COVID-CT dataset, classification accuracy increases from 91.15% to 95.50% and Area Under the Receiver Operating Characteristic (AUC) from 96.40% to 98.54%. We also customise a small dataset to simulate data collected in the early stages of the outbreak and report an improvement in accuracy from 85.95% to 94.32% and AUC from 93.21% to 98.61%. This study provides a feasible Low-Threshold, Easy-To-Deploy and Ready-To-Use solution with a relatively low computational cost for medical image classification at an early stage of an outbreak in which scarce data are available and traditional data augmentation may fail. Hence, it would be most suitable for low-resource settings.
Song K, Cao Y, Chen Q, et al., 2023, Frequency and Angle Multiplexed Metadevices with Multifunctional Polarization Modulation, ADVANCED FUNCTIONAL MATERIALS, ISSN: 1616-301X
Song K, Gong X, Cao Y, et al., 2023, Modular design for versatile broadband polarizing metasurfaces with freely switching functions, Advanced Functional Materials, Vol: 33, ISSN: 1616-301X
Polarization is a fundamental property of electromagnetic waves that plays a key role in many physical phenomena and applications. Schemes to manipulate it are revisited with the emergence of metasurfaces, which have brought multi-functionalities straightforwardly. However, this has come at the expense of design complexity that relies strongly on field theory. Here, an ingenious strategy of modular design is proposed to construct subwavelength multifunctional polarization control devices. Chiral metasurfaces with different handedness are first proposed and regarded as modules. The versatile polarization controller can thus be obtained with the combination of different modules. These experiments demonstrate that the well-designed polarization controller possesses reconfigurable functionality, and various broadband polarization and amplitude regulation functions with high efficiency including arbitrary linear polarization rotation, asymmetric transmission effect, neutral-density-like filter, polarization beam splitter, etc., can be readily realized just by changing the cascaded modules. The physical mechanisms of the versatile polarization controller and chiral metasurface modules are both guaranteed by the Fabry–Pérot-like resonances, which are theoretically verified via the transfer matrix method. It is envisioned that the modular concept will be of great benefit to designing compact multifunctional polarization controllers.
Magaway EJY, Farahi Y, Hanham SM, et al., 2023, Silica nanoparticle-based photoresin for THz high-resolution 3-D microfabrication by two-photon polymerization, IEEE Transactions on Terahertz Science and Technology, Vol: 13, Pages: 415-418, ISSN: 2156-342X
Two-photon polymerization is a promising fabrication technique for complex 3-D structures operating at terahertz(THz) given its sub-µm resolution with hundreds of mm3 printvolume capability. However, standard photoresins exhibit unsuitably high THz absorption and have poor mechanical, chemical,and thermal stability. To address the latter three issues, a newphotoresin (commercially known as GP-Silica) based on silicananoparticles dispersed in a photocurable binder matrix has beenrecently developed. To assess its suitability for THz devices, wereport the THz dielectric properties of GP-Silica and comparethem with standard 3-D printable materials. We find that GP-Silicaoutperforms the other photoresins by almost five times in terms ofabsorption, which finally unlocks additive manufacturing for THzapplications.
Tao L, Liu Y, Du L, et al., 2023, Evolution of the edge states and corner states in a multilayer honeycomb valley-Hall topological metamaterial, Physical Review B: Condensed Matter and Materials Physics, Vol: 107, ISSN: 1098-0121
The valley-Hall effect provides topological protection to a broad class of defects in valley-Hall photonictopological metamaterials. Unveiling precisely how such protection is achieved and its implications in practicalimplementations is paramount to move from fundamental science to applications. To this end, we investigatea honeycomb valley-Hall topological metamaterial and monitor the evolution of the topological valley-Halledge states and higher-order corner states under different perturbation δR. The evolutions of the edge statesof the armchair and zigzag interfaces are demonstrated, respectively. By adjusting the geometric parametersand introducing disturbances to break the inversion symmetry, we achieve the edge states with different modesincluding the conventional crossed edge state and the specific gapped edge state. It is found that the edge statesof topological valley kinking will gradually separate with the increase of δR, and finally a complete gap betweenthe edge states appears. The gap has rarely been reported previously in topological materials fabricated byprinted circuit board technology. In addition, the higher-order topological corner states can also be observedin the proposed topological metamaterial. The higher-order topological phase is theoretically characterized bynontrivial bulk polarization and the Wannier centers. Our results show that the corner state localization becomesstronger with the increase of δR. It is expected that our results will provide a platform for the realization ofoptical topological insulators.
Navarro-Cia M, Beaskoetxea U, Teniente J, et al., 2023, Low Sidelobe Level Millimeter-Wave Asymmetric Bull's Eye Antenna with Minimal Profile Feeding, IEEE Antennas and Wireless Propagation Letters, ISSN: 1536-1225
Bull's eye antennas exhibit remarkable directivity considering their low profile, albeit accompanied by high sidelobes. This undesirable radiation characteristic is tackled here by reporting a complementary split ring feeding whereby the broadside space-wave partially responsible for the high sidelobes is cancelled while the leaky-wave is excited effectively. This feeding results into an asymmetric bull's eye antenna with minimal profile (<inline-formula><tex-math notation="LaTeX">$\sim 0.73\lambda _{0}$</tex-math></inline-formula>) and no protrusions on the radiating interface. The fabricated 10 period antenna operating in the Ka band shows a directivity of 23.5 dBi, a sidelobe level of -22.9 dB (<inline-formula><tex-math notation="LaTeX">$>$</tex-math></inline-formula>6 dB improvement compared to other bull's eye antennas) and a beamwidth of 3.7<inline-formula><tex-math notation="LaTeX">$^{\circ }$</tex-math></inline-formula> and 6.7<inline-formula><tex-math notation="LaTeX">$^{\circ }$</tex-math></inline-formula> in the <inline-formula><tex-math notation="LaTeX">$E$</tex-math></inline-formula>- and <inline-formula><tex-math notation="LaTeX">$H$</tex-math></inline-formula>-plane, respectively.
Chopra N, Shaw N, Lotkowska L, et al., 2022, Temperature-dependent dielectric properties of human bone constituents at THz frequencies: contrast mechanisms and bound water dynamics, 47th International Conference on Infrared, Millimeter and Terahertz Waves (IRMMW-THz), Publisher: IEEE, Pages: 1-2, ISSN: 2162-2027
THz time domain spectroscopy (THz-TDS) was used to investigate the dielectric properties of constituents of human bone. We report experimental values for refractive index and absorption coefficient for four major anhydrous constituents of bone: Collagen, Hydroxyapatite, Calcite and Amorphous calcium phosphate. We varied the sample temperature in order to explore the physical mechanisms underpinning the THz dielectric function of these constituents. Experimental data were well described using the Debye model for collagen, or the Lorentz model for mineral compounds, respectively. A fitting procedure based on Particle Swarm Optimization was implemented in order to extract the fitting parameters such as relaxation times and oscillator strengths.
Liu Y, Ren H, Tao L, et al., 2022, Mechanically-reconfigurable edge states in an ultrathin valley-hall topological metamaterial, Advanced Materials Interfaces, Vol: 9, Pages: 1-11, ISSN: 2196-7350
Broadband topological metamaterials hold the key for designing the next generation of integrated photonic platforms and microwave devices given their protected back-scattering-free and unidirectional edge states, among other exotic properties. However, synthesizing such metamaterial has proven challenging. Here, a broadband bandgap (relative bandwidth of more than 43%) Valley-Hall topological metamaterial with deep subwavelength thickness is proposed. The present topological metamaterial is composed of three layers printed circuit boards whose total thickness is 1.524 mm ≈ λ/100. The topological phase transition is achieved by introducing an asymmetry parameter δr. Three mechanically reconfigurable edge states can be obtained by varying interlayer displacement. Their robust transmission is demonstrated through two kinds of waveguide domain walls with cavities and disorders. Exploiting the proposed topological metamaterial, a six-way power divider is constructed and measured as a proof-of-concept of the potential of the proposed technology for future electromagnetic devices.
Alves RA, Pacheco-Pena V, Navarro-Cia M, 2022, Madelung Formalism for Electron Spill-Out in Nonlocal Nanoplasmonics, JOURNAL OF PHYSICAL CHEMISTRY C, Vol: 126, Pages: 14758-14765, ISSN: 1932-7447
Hou Y, Navarro-Cía M, 2022, A Computationally-Inexpensive Strategy in CT Image Data Augmentation for Robust Deep Learning Classification of COVID-19
<jats:p><p>Coronavirus disease 2019 (COVID-19) has spread globally for two years, and chest computed tomography (CT) has been used to diagnose COVID-19 and identify lung damage in long COVID-19 patients. At the beginning of the epidemic, there was a shortage of large and publicly available CT datasets due to privacy concerns. Therefore, it is important to classify CT scans correctly when only limited resources are available, as it will happen again in future pandemics. We followed the transfer learning procedure and limited hyperparameters to use as few computing resources as possible. The Advanced Normalisation Tools (ANTs) were used to synthesise images as augmented/independent data and trained on EfficientNet to investigate the effect of synthetic images. On the COVID-CT dataset, classification accuracy increased from 91.15% to 95.50% and Area Under the Receiver Operating Characteristic (AUC) from 96.40% to 98.54%. We also customised a small dataset to simulate data collected in the early stages of the outbreak and improve accuracy from 85.95% to 94.32% and AUC from 93.21% to 98.61%. This paper provides a feasible solution with a relatively low computational cost for medical image classification when scarce data are available and traditional data augmentation may fail. </p><p><br></p><p>This work has been submitted to the IEEE for possible publication. Copyright may be transferred without notice, after which this version may no longer be accessible. </p></jats:p>
Hou Y, Navarro-Cía M, 2022, A Computationally-Inexpensive Strategy in CT Image Data Augmentation for Robust Deep Learning Classification of COVID-19
<jats:p><p>Coronavirus disease 2019 (COVID-19) has spread globally for two years, and chest computed tomography (CT) has been used to diagnose COVID-19 and identify lung damage in long COVID-19 patients. At the beginning of the epidemic, there was a shortage of large and publicly available CT datasets due to privacy concerns. Therefore, it is important to classify CT scans correctly when only limited resources are available, as it will happen again in future pandemics. We followed the transfer learning procedure and limited hyperparameters to use as few computing resources as possible. The Advanced Normalisation Tools (ANTs) were used to synthesise images as augmented/independent data and trained on EfficientNet to investigate the effect of synthetic images. On the COVID-CT dataset, classification accuracy increased from 91.15% to 95.50% and Area Under the Receiver Operating Characteristic (AUC) from 96.40% to 98.54%. We also customised a small dataset to simulate data collected in the early stages of the outbreak and improve accuracy from 85.95% to 94.32% and AUC from 93.21% to 98.61%. This paper provides a feasible solution with a relatively low computational cost for medical image classification when scarce data are available and traditional data augmentation may fail. </p><p><br></p><p>This work has been submitted to the IEEE for possible publication. Copyright may be transferred without notice, after which this version may no longer be accessible. </p></jats:p>
Attwood T, Adams E, Freer S, et al., 2022, Time and frequency analysis of rough surface scattering in the THz spectrum, 2021 51st European Microwave Conference (EuMC), Publisher: IEEE, Pages: 237-240
The identification and quantification of scattering phenomena is essential for designing indoor wireless communications. From a combination of time domain spectroscopy, analytical modelling and ray tracing simulations, a novel scattering factor is proposed for terahertz frequency bands. These results aim to assist in incorporating scattering effects in ray tracing simulations of indoor environments.
Wu R, Nekovic E, Collins J, et al., 2022, Taming non-radiative recombination in Si nanocrystals interlinked in a porous network, Physical Chemistry Chemical Physics, Vol: 24, Pages: 1-8, ISSN: 1463-9076
A range of the distinctive physical properties, comprising high surface-to-volume ratio, possibility to achieve mechanical and chemical stability after a tailored treatment, controlled quantum confinement and the room-temperature photoluminescence, combined with mass production capabilities offer porous silicon unmatched capabilities required for the development of electro-optical devices. Yet, the mechanism of the charge carrier dynamics remains poorly controlled and understood. In particular, non-radiative recombination, often the main process of the excited carrier's decay, has not been adequately comprehended to this day. Here we show, that the recombination mechanism critically depends on the composition of surface passivation. That is, hydrogen passivated material exhibits Shockley–Read–Hall type of decay, while for oxidised surfaces, it proceeds by two orders of magnitude faster and exclusively through the Auger process. Moreover, it is possible to control the source of recombination in the same sample by applying a cyclic sequence of hydrogenation–oxidation–hydrogenation processes, and, consequently switching on-demand between Shockley–Read–Hall and Auger recombinations. Remarkably, irregardless of the recombination mechanism, the rate constant scales inversely with the average volume of individual silicon nanocrystals contained in the material. Thus, the type of the non-radiative recombination is established by the composition of the passivation, while its rate depends on the degree of the charge carriers’ quantum confinement.
Du L, Liu Y, Zhou X, et al., 2022, Dual-band all-dielectric chiral photonic crystal, Journal of Physics D: Applied Physics, Vol: 55, ISSN: 0022-3727
We present an all-dielectric chiral photonic crystal that guides the propagation of electromagnetic waves without backscattering for dual bands. The chiral photonic crystal unit cell is composed of four dielectric cylinders with increasing inner diameter clockwise or anticlockwise, which leads to chirality. It is demonstrated that the proposed chiral photonic crystal can generate dual band gaps in the gigahertz frequency range and has two types of edge states, which is similar to topologically protected edge states. Hence, the interface formed by the proposed 2D chiral photonic crystal can guide the propagation of electromagnetic waves without backscattering, and this complete propagation is immune to defects (position disorder or frequency disorder). To illustrate the applicability of the findings in communication systems, we report a duplexer and a power divider based on the presented all-dielectric chiral photonic crystal.
DeglInnocenti R, Lin H, Navarro-Cía M, 2022, Recent progress in terahertz metamaterial modulators, Nanophotonics, Vol: 11, Pages: 1485-1514, ISSN: 2192-8606
The terahertz (0.1–10 THz) range represents a fast-evolving research and industrial field. The great interest for this portion of the electromagnetic spectrum, which lies between the photonics and the electronics ranges, stems from the unique and disruptive sectors where this radiation finds applications in, such as spectroscopy, quantum electronics, sensing and wireless communications beyond 5G. Engineering the propagation of terahertz light has always proved to be an intrinsically difficult task and for a long time it has been the bottleneck hindering the full exploitation of the terahertz spectrum. Amongst the different approaches that have been proposed so far for terahertz signal manipulation, the implementation of metamaterials has proved to be the most successful one, owing to the relative ease of realisation, high efficiency and spectral versatility. In this review, we present the latest developments in terahertz modulators based on metamaterials, while highlighting a few selected key applications in sensing, wireless communications and quantum electronics, which have particularly benefitted from these developments.
Nourinovin S, Navarro-Cia M, Rahman MM, et al., 2022, Terahertz metastructures for noninvasive biomedical sensing and characterization in future health care [Bioelectromagnetics], IEEE Antennas and Propagation Magazine, Vol: 64, Pages: 60-70, ISSN: 1045-9243
According to a recent report [1] from the Cancer Research Agency of the World Health Organization, cancer is a dominant cause of mortality worldwide, leading to 10 million deaths in 2020 alone. Diagnosing a patient from the early stages tremendously raises the chance of survival. Current clinical cancer detection approaches including X-ray, magnetic resonance imaging (MRI), and biomarker analysis not only fail to provide a precise border of the malignant tissue, especially in the early stages of cancer, but also can be invasive and lead to tissue damage. Recent progress in EM biosensor technologies has the potential to deliver a point-of-care diagnosis and surpass conventional methods regarding accuracy, time, and cost.
Freer S, Sui C, Grover LM, et al., 2022, Temperature dependent hyperspectral terahertz imaging of human bone for disease diagnosis, Advanced Biomedical and Clinical Diagnostic and Surgical Guidance Systems XX, Publisher: Society of Photo-optical Instrumentation Engineers, Pages: 1-5, ISSN: 0277-786X
Water is a fundamental component of many biological systems. The ability to detect water therefore provides great insight into system functionality, particularly in the development of disease. In this work, the high interaction of terahertz radiation with water, paired with the dependence of the dynamics of water molecules with varying temperature, is utilised to monitor changes in the composition of bone tissue. Heterotopic ossification (HO) bone samples and deionised free water are measured using terahertz time-domain spectroscopy for varying environmental temperatures, for prospective use in disease diagnosis.
Adams E, Attwood T, Freer S, et al., 2021, Broadband characterisation of interior materials and surface scattering using terahertz time-domain spectroscopy, 2021 14th UK-Europe-China Workshop on Millimetre-Waves and Terahertz Technologies (UCMMT), Publisher: IEEE, Pages: 1-3
Indoor wireless communications need to move towards Terahertz (THz) frequencies in order to keep up with society's demand for data transmission, but this change is currently hindered by limited knowledge of material properties and propagation and scattering models at these frequencies. The dielectric properties of common household materials are investigated here with a twofold objective: (1) to extend the library of material properties at THz, and (2) to estimate and disentangle losses in scattering measurements in order to facilitate propagation, scattering and, ultimately, channel models.
Ivanov AV, Tatarenko AY, Gorodetsky AA, et al., 2021, Fabrication of epitaxial W-Doped VO2 nanostructured films for terahertz modulation using the solvothermal process, ACS Applied Nano Materials, Vol: 4, Pages: 10592-10600, ISSN: 2574-0970
We report a feasible and high-throughput method for high-quality W-doped VO2 nanostructured epitaxial films on r-sapphire substrate fabrication. Single-phase, smooth vanadium dioxide thin films with uniform distribution of tungsten (up to 2.3%) are formed using the solvothermal process from ethylene glycol/water V4+ and W6+ solutions. Compositional analysis by X-ray photoelectron and energy-dispersive X-ray spectroscopy (XPS and EDX, respectively); structural analysis (X-ray diffraction, Raman spectroscopy, selected area electron diffraction (SAED)); and detailed analysis of the surface morphology and substrate–film interface using scanning electron microscopy, atomic force microscopy, and high-resolution transmission electron microscopy (SEM, AFM, HRTEM, respectively) confirm the formation of nanoscale (50–60 nm) epitaxial W:VO2 (M1) on r-sapphire with epitaxial relationships (100)VO2∥(101̅2)Al2O3 and [010]VO2∥[011̅0]Al2O3. The nanostructured films demonstrate excellent terahertz (THz) transmission properties: a phase transition temperature of 31 °C, a huge THz modulation depth of over 60%, and broad bandwidth (≥2 THz) operation. Hence, they can be efficiently used as active material for tunable THz manipulation devices.
Liu S, Ma S, Shao R, et al., 2021, Edge state mimicking topological behavior in a one-dimensional electrical circuit, New Journal of Physics, Vol: 23, ISSN: 1367-2630
For one-dimensional (1D) topological insulators, the edge states always reside in the bulk bandgaps as isolated modes. The emergence and vanishing of these topological edge states are always associated with the closing/reopening of the bulk bandgap and changes in topological invariants. In this work, we discover a special kind of edge state in a 1D electrical circuit, which can appear not only inside the bandgap but also outside the bulk bands with the changing of bulk circuit parameters, resembling Tamm states or Shockley states. We prove analytically that the emergence/vanishing of this edge state and its position relative to the bulk bands depends on the intersections of certain critical frequencies. Specifically, the edge mode in the proposed circuit can be mathematically described by polynomials with roots equal to some critical frequencies in the bulk circuit. From this point of view, the transition of the edge state is uniquely determined by the order of the critical frequencies in the bulk circuit. Such topological behaviors shown by the edge state in the proposed electrical circuit may indicate, in a broader sense, the presence of certain type of topology.
Cojocari M, Ospanova AK, Chichkov V, et al., 2021, Pseudo-anapole regime in terahertz metasurfaces, Physical Review B: Condensed Matter and Materials Physics, Vol: 104, ISSN: 1098-0121
We present the numerical, theoretical, and experimental study of a terahertz metasurface supporting a pseudo-anapole. Pseudo-anapole effect arises when electric and toroidal dipole moments both tend to a minimum, instead of destructive interference between electric and toroidal dipole moments in conventional anapole mode. Such overlap allows resonance suppression of electric type radiation. Thus it becomes possible to study the multipoles of other families and higher order excitations. We estimate multipole contribution to the metasurface response via the multipole expansion method. The series is extended with such terms as mean-square radii and multipole interference. We also study the metasurface geometrical tunability. Via scaling, we demonstrate that it is possible to control the metasurface toroidal and electric responses independently. This in turn proves the fact that these multipoles have different physical origin. Moreover, we demonstrate that the proposed metasurface allows excitation of coherent magnetic dipole and electric quadrupole modes, which is crucial for planar cavities and lasing spasers in nanophotonics.
Cojocari MV, Ospanova AK, Chichkov VI, et al., 2021, Pseudo-Anapole Mode Establishment in Planar THz Metamaterial
In this paper, we propose new kind nonradiating state appearing in high Q planar toroidal THz metamaterial. So-called pseudo-anapole regime arises when the trivial solution to the non-radiating state condition is met. Here, both toroidal and electric dipole intensities are suppressed at resonance frequency while their far-field zone intensities tend to zero. The proposed effect is quite different from well-known anapole regime that is established by the condition p =- ikT that leads to nonradiating state [1]. The fundamental difference of pseudo-anapole state is that suppression of both electric and toroidal multipoles providing an opportunity for study of higher order multipoles from different families. This effect has been confirmed both numerically and experimentally in terahertz frequency range.
Freer S, Sui C, Hanham SM, et al., 2021, Hybrid reflection retrieval method for terahertz dielectric imaging of human bone, Biomedical Optics Express, Vol: 12, Pages: 4807-4820, ISSN: 2156-7085
Terahertz imaging is becoming a biological imaging modality in its own right, alongside the more mature infrared and X-ray techniques. Nevertheless, extraction of hyperspectral, biometric information of samples is limited by experimental challenges. Terahertz time domain spectroscopy reflection measurements demand highly precise alignment and suffer from limitations of the sample thickness. In this work, a novel hybrid Kramers-Kronig and Fabry-Pérot based algorithm has been developed to overcome these challenges. While its application is demonstrated through dielectric retrieval of glass-backed human bone slices for prospective characterisation of metastatic defects or osteoporosis, the generality of the algorithm offers itself to wider application towards biological materials.
Freer S, Sui C, Penchev P, et al., 2021, Hyperspectral terahertz imaging for human bone biometrics, Terahertz Emitters, Receivers, and Applications XII, Publisher: SPIE, Pages: 1-6
The realisation of hyperspectral terahertz imaging is a significant step towards understanding of the life sciences on all scales. A key to this understanding is the retrieval of dielectric properties from such images, a task which is plagued by experimental limitations, challenging the terahertz community for more than two decades. In this contribution, we propose a new combined retrieval methodology to overcome misalignments and Fabry-Pérot effects on the extraction of the dielectric properties of human bone samples through the combination of the Kramers-Kronig relations and Fabry-Pérot reflection modelling. Results extracted from ∼100 µm human bone slices composed largely of collagen are consistent with those measured for pristine collagen samples. This represents another stepping-stone towards the adoption of terahertz imaging into pre- and clinical practice.
Sabery SM, Bystrov A, Navarro-Cia M, et al., 2021, Study of low terahertz radar signal backscattering for surface identification, Sensors, Vol: 21, Pages: 1-17, ISSN: 1424-8220
This study explores the scattering of signals within the mm and low Terahertz frequency range, represented by frequencies 79 GHz, 150 GHz, 300 GHz, and 670 GHz, from surfaces with different roughness, to demonstrate advantages of low THz radar for surface discrimination for automotive sensing. The responses of four test surfaces of different roughness were measured and their normalized radar cross sections were estimated as a function of grazing angle and polarization. The Fraunhofer criterion was used as a guideline for determining the type of backscattering (specular and diffuse). The proposed experimental technique provides high accuracy of backscattering coefficient measurement depending on the frequency of the signal, polarization, and grazing angle. An empirical scattering model was used to provide a reference. To compare theoretical and experimental results of the signal scattering on test surfaces, the permittivity of sandpaper has been measured using time-domain spectroscopy. It was shown that the empirical methods for diffuse radar signal scattering developed for lower radar frequencies can be extended for the low THz range with sufficient accuracy. The results obtained will provide reference information for creating remote surface identification systems for automotive use, which will be of particular advantage in surface classification, object classification, and path determination in autonomous automotive vehicle operation.
Nekovic A, Camacho M, Freer S, et al., 2021, Taming extraordinary THz transmission through sub-λ slot arrays via array truncation, slot rotation, polarization and angle of incidence, 2020 45th International Conference on Infrared, Millimeter and Terahertz Waves (IRMMW-THz), Publisher: IEEE, Pages: 1-2
Accurate and time-effective simulation and design optimization of quasi-optical (QO) systems is an extremely challenging electromagnetic problem given the multi-scale dimension of QO components and the need to consider the finite size of such components to account for effects like diffraction. To show that the Method of Moments (MoM) provides an elegant solution for these problems, truncated rectangular arrays of tilted slots are measured in a QO Terahertz (THz) time-domain setup and comparison with MoM is carried out. The extraordinary transmission peaks are modulated by the size of the array and the orientation of the slots with respect to the incident electric field.
Gorodetsky A, Freer S, Navarro-Cia M, 2021, Continuous wave sub-Terahertz lensless holographic reflective imaging, 2020 45th International Conference on Infrared, Millimeter and Terahertz Waves (IRMMW-THz), Publisher: IEEE, Pages: 1-1
We propose a simple setup involving standard commercially available sub-terahertz electronic source and camera in reflection layout. The setup is designed for imaging amplitude and phase objects. The construction allows for quick installation and potentially almost-realtime operation. Initial reconstruction results demonstrate resolution of about three wavelengths.
Freer S, Martinez R, Perez-Quintana D, et al., 2021, Metal 3D printed D-band waveguide to surface wave transition, 2020 45th International Conference on Infrared, Millimeter and Terahertz Waves (IRMMW-THz), Publisher: IEEE, Pages: 1-2
The coupling efficiency between free space waves and surface waves is low, narrowband, or both. Highly efficient broadband (better than 20% fractional bandwidth) coupling from waveguide modes can be achieved through sophisticated transitions whose fabrication can be enabled through additive manufacturing (e.g. selective laser melting). Here, we present alternative metallic transitions designed to couple the fundamental mode of a D-band waveguide to the fundamental transverse-magnetic surface mode supported by a periodic metal corrugated grating. Simulations of the coupling process and initial measurements have been undertaken.
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