The lack of an accessible comprehensive reliable dataset of material properties at terahertz (THz) frequencies is a significant roadblock for the development of accurate propagation models for THz wireless systems. To fill this gap, in this article, we extract the complex permittivity of 75 different home and commercial furnishings categorized into fabric, leather, plastic, stone, and wood from quasi-optical measurements. Given that quasi-optical transmission measurement standards are not developed yet, we utilize both vector network analysis and time-domain spectroscopy under collimated and focused beam illumination to highlight the differences and the challenges that precise material extraction faces at THz, and to compute average complex permittivities with confidence intervals.

Electromagnetic topological optimization holds the promise of the fully automated design of electromagnetic structures such as antennas, waveguides, metasurfaces and metamaterials; however, it can often yield designs that are incompatible with fabrication processes. In this work, we describe a multiphysics topological optimization framework that combines structural finite element analysis and electromagnetic finitedifference time-domain simulation to realize fabricable structures which meet specified electromagnetic design objectives. As a demonstration, the framework is applied towards the design of G-band low-profile leaky lens antennas suitable for future 6G communication applications. The 5λ0 radius, 2λ0 thick leaky lens antenna is compatible with stereolithography 3D printing and displays a realized gain of 23 dBi at 0.2 THz with a low sidelobe level of -20 dB. We foresee the proposed framework being applicable to a wide range of electromagnetic design problems intended for fabrication using additive manufacturing techniques.

Topological optimization has drawn significant interest in the design of antennas because of its ability to design novel nonintuitive designs. However, the optimization algorithm often struggles to produce designs feasible for fabrication. Methods incorporating minimum feature size constraints, connectivity constraints and material filters have been employed in an attempt to make the designs more fabricable. However, these methods do not necessarily guarantee the structural integrity of the design. Here we show the combination of structural and electromagnetic optimizations with adaptive biasing can be utilized to produce structurally feasible and fabricable structures. The two optimization regimes utilize rotational symmetry to optimize a two dimensional domain whose performance was evaluated using axi-symmetric FDTD simulations. As a proof of concept a low cost, lightweight, polymer leaky wave antenna was designed, achieving 30 dBi gain at an operating frequency of 284 GHz. The result was a fully connected antenna with no structural weak points.

This article introduces a millimeter-wave electromagnetic (EM) bandgap (EBG) resonator, commonly referred to as a photonic crystal (PC) resonator (PCR), designed for use in a 45-GHz ultralow phase noise MMIC oscillator. The novel 2-D slab PCR design utilizes a higher order resonant mode, which has been engineered to have a reduced dielectric filling factor (FF) of 47%. These improvements demonstrate a factor of two improvement in unloaded $Q$ -factor (122 000 at 297 K) and halving of the resonant frequency temperature sensitivity (21 ppm/K) compared to a conventional L5 PCR design. The PCR is fabricated using bulk micromachining of high-resistivity silicon (HRS), which has been neutron transmutation doped (NTD) leading to an additional factor of two improvement in quality ( $Q$ -) factor over previous works featuring nonirradiated HRS. Additionally, it is shown that unloaded $Q$ -factors in excess of 200 000 are achievable when the temperature is reduced to below 274 K. The PCR benefits from a planar geometry, integrated waveguides, and fabrication using a mature silicon micromachining process.

A 3D-printed 60 GHz re-entrant cavity resonator for dielectric metrology has been developed for the characterization of low-dielectric loss fluids. A higher-order resonant mode (TM 020 ) at 60 GHz has been utilized owing to its high unloaded measured quality factor of 2150. A full simulation analysis of the cavity resonator for complex permittivity measurement within the V-band frequency range has been demonstrated. The cavity sensor is fabricated using Stereolithography (SLA) printing and coated with copper. The advantage of the resonator geometry over rectangular and cylindrical cavities is that it provides for a concentrated electric field at a defined gap region, enabling stronger interaction between the resonant mode and analyte, and therefore higher sensitivity. The low-cost sensing device has shown good results following a measurement of hexane as a low loss sample. This has demonstrated the potential use of the device in dielectric metrology.

This paper presents an interlaboratory comparison of three widely employed broadband dielectric measurement techniques in the terahertz region - namely, material characterization kit (MCK), conventional free-space method, and time domain spectroscopy (TDS). The comparison was undertaken within the WM-380 band (500-750 GHz), utilizing five common dielectric materials (ABS, HDPE, fused silica, YAG, and high-resistivity silicon). The results are discussed in detail, and generally good agreement in the extracted real part of the complex permittivity and loss tangent between these techniques is demonstrated.

This paper describes robust vector-fitting algorithms for determining the Q-factor and resonant frequency of spectrally-isolated resonances from frequency-swept S-parameter measurements for both one-port (reflection) and two-port (transmission) systems. It also provides guidance on measurement techniques, and gives measurement examples from the electromagnetic and acoustic domains. These include measurements on a LC resonator (unloaded Q-factor Q(o) approximate to 57), a photonic-crystal resonator (Q(o) approximate to 123 000) and a superconducting notch resonator (Q(o) approximate to 1.5 x 10(6)). The vector techniques advocated are often advantageous compared to scalar techniques because they are more informative, and in many cases more precise. Among the most common applications is the measurement of dielectric permittivity and loss by resonance at RF and microwave frequencies by using Vector Network Analysers. The algorithms described, however, are applicable more generally to sensing and imaging applications that use vector instrumentation. This is demonstrated by one of the measurement examples, which shows that acoustic Q-factor can be fitted to vibrational data obtained by Resonant Ultrasound Spectroscopy. Open-source software implementations (Python and Matlab) of the algorithms have been made available.

Despite topological optimization's struggle to produce designs feasible for fabrication, researchers are increasingly turning to topological optimization due to its ability to design novel nonintuitive designs. Previous attempts to combat that drawback included minimum feature size constraints, connectivity constraints and material filters. However, in the electromagnetic regime, these methods do not necessarily ensure the structural integrity of the design. Here, we present a new low cost, low profile 3D printable leaky metalens designed using a novel topological optimization algorithm that incorporates structural and electromagnetic optimizations. The algorithm aimed at producing structurally feasible and fabricable structures. The resulting 0.75 lambda(0) thickness lens was fully connected with no structural weak points. It achieved 20.6 dBi directivity and 11% aperture efficiency at an operating frequency of 295 GHz.

Lens antennas are commonly used for communication and imaging applications in the terahertz band and are often fabricated from high-resistivity silicon due to its low dielectric loss. Drawbacks to silicon lens antennas are that they can be bulky, expensive and require specialized fabrication processes (e.g. DRIE). Additionally, silicon lens antennas are susceptible to Fresnel reflection losses and exhibit a strong Fabry-Perot response, primarily due to the high dielectric constant of silicon. A lower cost solution is to develop a polymeric lens antenna which can be fabricated using 3D printing processes. However, conventional 3D printing techniques (e.g. FDM and SLA) have limited resolution and cannot meet the fabrication tolerances required for operating frequencies above several hundred gigahertz. An alternate approach is to use Direct Laser Writing (DLW) based on Two-Photon Polymerization (2PP) of a photoresin (PR) which combines sub-micron fabrication accuracies with the ability to realize complex 3D geometries. A limitation of this approach is the typically high dielectric loss of the available PRs in the terahertz band.

For the application of geometrically-induced THz surface wave technology for communication and sensing, a critical analysis of the propagation characteristics (i.e. dispersion and attenuation) for different textured surfaces should be studied and benchmarked. For the broadband characterisation of archetypal textured surfaces (e.g. corrugated plane, two-dimensional array of blind holes and bed of nails) supporting THz transverse magnetic (i.e., p-polarized) surface waves, we employ time-domain spectroscopy and edge-diffraction coupling methods. Measurements of laser micromachined prototypes demonstrate strong frequency-dependent dispersion and the large impact that surface roughness of the order of few μm has on the path loss, increasing it by a factor ranging from 1.6 to 4.3 compared to smooth textured surfaces. Together with numerical modelling, we disentangle all loss mechanisms (namely, ohmic, scattering, propagation divergence and phase mismatch) and highlight the challenge of loss estimation due to surface roughness in highly confined THz surface waves.

GP-Silica, a newly developed photoresin embedded with silica nanoparticles useable with two-photon polymerization, aims to address the lack of low-loss material in the THz range. To determine its suitability for THz device fabrication, its dielectric properties were extracted over the range of 0.5 – 1.5 THz. Its refractive index was found to outperform other 3D printable materials by 1.2 – 1.4 times while its absorption coefficient was almost 5 times lower at 1 THz. The data shared cements the potential of not only GP-Silica but also two-photon polymerization for THz device manufacturing.

Two-photon polymerization is a promising fabrication technique for complex 3-D structures operating at terahertz (THz) given its sub- $\mu$ m resolution with hundreds of mm $^{3}$ print volume capability. However, standard photoresins exhibit unsuitably high THz absorption and have poor mechanical, chemical, and thermal stability. To address the latter three issues, a new photoresin (commercially known as GP-Silica) based on silica nanoparticles dispersed in a photocurable binder matrix has been recently developed. To assess its suitability for THz devices, we report the THz dielectric properties of GP-Silica and compare them with standard 3-D printable materials. We find that GP-Silica outperforms the other photoresins by almost five times in terms of absorption, which finally unlocks additive manufacturing for THz applications.

This letter introduces a novel millimeter (mm)-wave oscillator concept based on an electromagnetic bandgap (EBG) resonator (also called a photonic crystal resonator). To realize an ultralow phase noise monolithic microwave integrated circuit (MMIC) oscillator, an EBG resonator is developed by introducing a periodic structure into an ultrahigh resistivity silicon wafer to create an EBG which confines a localized resonant mode with a reduced mode dielectric filling factor of 47%. The measured results of the resonator demonstrate an unloaded Q-factor of 108300 can be achieved at 45 GHz. Measured oscillator phase noise levels of −91.5, −121.5, and −133 dBc/Hz are obtained at offset frequencies of 1, 10, and 100 kHz, respectively.

This letter describes the design, fabrication, and characterization of an air-mode 1-D photonic crystal resonator (PCR) operating at 100 GHz. The PCR was fabricated from an inexpensive cyclic olefin copolymer (COC) using computer numerical control (CNC) milling. It is demonstrated that despite the lack of complete electromagnetic bandgap arising due to the low permittivity of the COC, it remains possible to realize a resonator with a high quality factor ( $Q$ -factor) of 2800 through shaping of the resonant field to minimize radiative loss.

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.

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.

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.

Despite its great potential, the realisation of terahertz dielectric imaging for biomedical application is limited by experimental challenges, such as reference displacements and Fabry-Pérot reflections in thin samples. In this work, an algorithm is developed to overcome such complications for time-domain spectroscopy measurements in reflection configuration. The algorithm is demonstrated on human bone slices without encapsulation, showing great promise for application in extracting biometric information from a broad range of structures.

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.