The development of terahertz (THz) technology exploiting untapped bandwidth is essential for sensing and communications in 6G and beyond. Instrumentation systems for evaluating the performances of THz devices are of increasing interest. Currently, THz device testing with electronics-based vector network analyzers (VNAs) requires a series of frequency extender pairs to cover all individual bands in the THz range. This increases system complexity and cost. Here, we report on an ultrawideband (UWB) photonic THz source, which will enable simpler and cost-effective THz device testing. The source employs a modified unitraveling-carrier photodiode (MUTC-PD) operating from 100 to 600 GHz. The InP-based MUTC-PD is hybrid-integrated with a UWB high-resistivity Si waveguide interface to efficiently couple the optically generated THz waves to the device under test. By utilizing a UWB THz power detector, a waveguide-based scalar transmission measurement system is demonstrated, a crucial step toward a future waveguide-based photonic VNA.

Terahertz frequencies offer excellent bandwidth and high data rates for next-generation communications systems. However, signals at these frequencies experience severe path loss and strong attenuation, requiring strategies to enhance the signal-to-noise ratio (SNR) by continuously directing energy toward mobile targets. This work presents a steerable antenna for terahertz applications using low-temperature co-fired ceramic (LTCC) and employs an effective medium-based lens for beam collimation and mechanical tuning methods for dynamic beam steering. Experimental results demonstrate a maximum gain of 17.5 dBi and a 3-dB bandwidth of 34.5%. A 7$^\circ$ steering angle of the main lobe is observed with a lateral lens offset of 250 μm. This advancement validates the performance of an effective medium based lens for terahertz applications requiring beam steering with low space cost.

We introduce a viable mechanical tuning technique for the dispersion of terahertz dielectric waveguide-based passive devices. Evanescent interaction with a movable conductive surface imposes a null on tangential electrical fields, which affects the dispersion of guided waves. Varying the position of this null allows us to tailor the waveguide's dispersion as-desired. As a proof-of-concept of this effect, we demonstrate motorized frequency-tuning of an integrated terahertz micro-disk resonator, yielding a $\sim 2.5 \%$ blue-shift in the resonance frequency. This span is greater than the frequency-spacing between adjacent resonances, meaning that a resonance can be tuned to any specific frequency within the operation bandwidth.

Terahertz (THz) and millimeter-wave (mmW) spectroscopy provide non-ionizing, label-free access to the electromagnetic response of a wide range of materials, enabling applications in quality control, biomedical diagnostics, and non-destructive evaluation. Conventional free-space THz spectroscopy, however, often suffers from limited interaction length and reduced sensitivity when the sample is thin, spatially localized, available only in small volume, or incompatible with beam-based fixtures. In this work, we present an integrated, non-contact approach for broadband material-property extraction based on the evanescent field of a low-loss high-resistivity silicon dielectric waveguide. The material under test (MUT) is placed in the cladding region, where it perturbs the guided-mode propagation constant and attenuation. By measuring the complex transmission through a reference waveguide and a waveguide loaded with the MUT and by combining these data with mode simulations and a perturbation-based inversion, we extract the complex permittivity over a continuous frequency band. The approach leverages an evanescent-field-based non-destructive method, a compact measurement setup, and feasibility to rescale toward higher frequencies for broadband material screening in the mmW to THz range.

This work presents the first realization of a photonics-based two-dimensional (2D) array architecture concept that combines a three-dimensional (3D) dielectric rod waveguide (DRW) antenna with a $4 \times 4 \text{InP}$-based photodiode (PD) array. Simulations demonstrate that the proposed approach supports 2D beam-steering at carrier frequencies of at least up to 210 GHz. Preliminary characterization results of the monolithically integrated $4 \times 4 \text{InP}$ PD array, which incorporates semiconductor optical amplifiers (SOAs), confirm wideband THz generation. Furthermore, the proposed architecture mitigates microscale assembly challenges through a novel alignment concept for chiplevel integration. This paves the way towards broadband THz sources for next-generation communication systems.

We demonstrate a Bessel beamformer based on multimode interference (MMI) for the terahertz (THz) wave range. Unlike traditional Bessel beam generation methods, which often rely on bulky free-space optical setups, our approach leverages a 3D-printed cyclic-olefin-copolymer (COC) MMI structure fed from a dielectric waveguide (DW) with a lambda/4 slot-waveguide termination (SWT) that provides an index-matched interface for low reflection. Based on this interface and the low losses of the constituent materials, the beamformer achieves a measured transmission efficiency of 93.9% at 275 GHz, with a propagating distance of 20 mm ( similar to 20 lambda ). Even at an extended distance of 35mm ( similar to 35 lambda ), the beam maintains a 50% efficiency. This approach addresses key limitations of existing free-space methods, offering a workable guided-wave approach to launch a THz Bessel beam.

The advancement of terahertz technology is impeded by a lack of viable options for dynamic reconfigurability in compact systems with fixed low‐loss interconnect. To address this absence, we bring conductive walls into proximity with an unclad microscale silicon waveguide core and thereby supply additional boundary conditions to manipulate guided waves through evanescent interaction. This is analogous to a fiber squeezer, in which a dielectric waveguide's dispersion is manipulated via enclosing walls, but in this case, a crucial distinction is that there is no physical contact whatsoever. Analytical and numerical investigations of this phenomenon show that the presence of the conductive walls increases the cutoff frequency and alters the dispersion profile of the waveguide. We implement proof‐of‐concept demonstrations that exploit this effect to realize mechanically tunable terahertz filters of two types: a high‐pass filter and a resonant notch, operating at ∼300 GHz. This experimental demonstration utilizes a featureless straight dielectric waveguide, and the desired frequency‐selective behavior is implemented contactlessly, and hence reversibly, having made no modification to the waveguide core. The capability for on‐demand dispersion tuning of low‐loss terahertz waveguides holds the potential to realize a broad range of practical reconfigurable systems to support diverse applications of terahertz waves.

Terahertz technology is rapidly transitioning from a physics-driven discipline to an engineering-oriented field, with growing emphasis on practical, deployable systems for sensing and communications. For associated antenna technology, key performance metrics, including bandwidth, efficiency, gain, and tunability, have become central design drivers. Nonetheless, progress is constrained by unconventional micro-scale fabrication requirements and the scarcity of low-loss materials and tuning mechanisms at terahertz frequencies. Application goals inform antenna design: near-field systems for imaging and spectroscopy demand flexible beam shaping and high efficiency, whereas far-field links for communications and radioastronomy require extremely high gain to overcome strong free-space path loss. Meeting these requirements under material and fabrication constraints introduces unique challenges that have motivated a wave of unconventional antenna concepts drawing from both microwave engineering and optical design principles. This invited review surveys recent developments in terahertz antennas, covering microwave-inspired structures, optical-inspired and lens-based approaches, dielectric antennas, frequency-diverse architectures, phased arrays, and emerging reconfigurable mechanisms. Looking forward, the field's progress toward dynamic, high-efficiency beamforming will be critical for enabling agile terahertz links and adaptive sensing, provided that such reconfigurability can be achieved without compromising bandwidth or radiation efficiency, requirements that remain particularly stringent in the terahertz domain.

There is a need to increase the physical robustness of substrateless silicon waveguides for terahertz systems. One recently demonstrated approach involves the exploitation of multimode effects to produce field nulls at specific desired edge-locations, enabling strong support beams without disturbing guided waves. However, these effects are frequency-dependent, and if several such supporting structures are deployed in series, i.e. to enhance device strength, then this frequency dependence will become more severe. In this work, we explore this tradeoff between bandwidth and physical robustness that is mediated by the number of multimode support structures that suspend a silicon waveguide core. It is found experimentally that, whilst a two-support waveguide exhibits ∼31% bandwidth, this reduces to 20% if six structures are used.

There is a need for compact low-cost near-field terahertz systems. To this end, we numerically demonstrate that subwavelength-size all-dielectric slot dipoles are capable of near-field sensing, as the presence of a target detunes the matching condition, and this effect can be observed in the antenna’s return spectrum. This holds potential to miniaturize and simplify terahertz near-field sensor devices.

High-frequency antenna testing remains a significant challenge due to the lack of equipment to cover the required far-field distances. For this reason, omnidirectional probe antennas serve as essential tools for evaluating antennas in terms of multipath propagation and signal leakage. In this work, we present an optimized design for a cost-effective monopole probe antenna, incorporating a heterogeneous stack of dielectric resonators to enable broadband operation. Experimental results confirm an enhancement in matching bandwidth, achieving a total bandwidth of 35 GHz and a relative bandwidth of 38%, using a stack of just two resonators. Also, simulation results demonstrate the omnidirectional radiation characteristics of the antenna. These findings highlight the potential of the proposed DRA stack as a broadband probe antenna for testing and evaluating future 5G and 6G broadband antenna systems.

Millimeter-wave (MMW) and Terahertz (THz) sources and receivers are essential in a wide range of applications, from high-resolution radar to spectroscopy, as well as high-capacity wireless links. Current efforts are directed towards their integration, benefiting from their small form factor. However, due to the wide bandwidth available at these frequencies, the challenges that need be addressed include increasing the operating bandwidth to use the available spectrum as well as to increase the total radiated power. Two key factors play a role in this, the intrinsic limitation of the components (i.e. limited f(max)) and the inefficient on-chip radiation. In this work we address how dielectric structures can help improve radiation efficiency.

There is a need for compact, broadband, efficient radiators for terahertz integrated circuits, to serve as feed for large- aperture quasioptics, or as antenna elements in phased arrays. We present a condensed, subwavelength progressive transition between a terahertz microscale dielectric waveguide and a dielectric slot radiating aperture. The result is an endfire point-source radiator spanning > 2.2/1 , from <185 GHz to 410 GHz, which is fabricated in a single etch step from a high-resistivity float-zone intrinsic silicon wafer. The radiation phase center deviates by just sim 500mu*m across the operation bandwidth-less than one free-space wave- length.

Substrate-less all-silicon dielectric waveguide is the natural interface in a unique Terahertz integration platform, which requires an interconnection solution. We demonstrate such interconnects using two different approaches, based on dielectric waveguide with truncated and slot-waveguide dielectric waveguide terminations. Both exhibit a fixed radiation center, which enables consistent coupling point. We experimentally demonstrate interconnection coupling losses of less than 5 dB and 2.5 dB for the truncated and slot-waveguide respectively. We investigate the effects of interconnection misalignments, validating the potential for broadband, contactless power transfer between dielectric waveguide endpoints. The slot-waveguide achieves a coupling efficiency of −3 dB with a misalignment tolerance of $100 \mu \mathrm{m}$. These findings advance terahertz interconnects using slot-waveguides, ensuring efficient coupling despite fabrication, assembly, or environmental offsets.

Microwave photonic components have amongst its advantages providing a large bandwidth, having recently shown up to 500 GHz. These bandwidths are not usually available due to the high losses in current transmission lines and the limited bandwidth of radiofrequency connectors and interconnects. Recently, dielectric waveguide transmission lines have been shown to enable broadband interconnects as well as functional building block structures for signal processing in the electrical domain. This paper presents the potential of a dielectric slab waveguide platform to integrate Terahertz components, having an operating bandwidth that matches with the highest frequency of photonic components.

There is a need to enhance the bandwidth of compact guided-wave devices for reflection-mode applications of terahertz waves including radar, imaging, near-field imaging, reflectometry, and duplex communications. Fundamentally, any reflection-probing device demands a suitable directional coupler, and so the bandwidth of the above-given applications is intrinsically constrained by that of the directional couplers that are available. To this end, we have recently introduced broadband all-dielectric guided-wave quasi-optical directional couplers that are enabled by elementary ray tracing principles. However, the splitting ratio becomes increasingly uneven at lower frequencies. Here, we explore a viable external-loading approach in order to divide power more evenly across the directional coupler’s operational bandwidth.

Antenna miniaturization poses a significant challenge at high frequencies, as we must balance performance against physical viability. Here, we present the realization of a coaxial-W1-fed dielectric resonator antenna (DRA) operating above 100 GHz. The dielectric cavity is fabricated from intrinsic silicon using micromachining. Given its simple design and high-precision manufacture, the proposed DRA simplifies assembly by leveraging a precise alignment concept for realizing a compact probe antenna. At the resonance frequency of 107 GHz, we demonstrate insertion loss of -30 dB and matching bandwidth of 4 GHz. Near-field scanning measurements are also performed to confirm that this probe can be used as a cost-effective mm-wave compact antenna test range. With its omnidirectional radiation and compact structure, this DRA holds the potential as a probe antenna to test and evaluate future 6G communications systems.

Non-destructive testing and evaluation of targets with low contrast and sub-wavelength features tends to be challenging. By illuminating the target off-axis, dark-field imaging techniques can resolve diffracted or scattered features that are typically challenging to discern with conventional bright-field imaging, which relies on specular reflection exclusively. Traditional dark-field methods achieve off-axis illumination by blocking the central portion of the incident beam, but this unfortunately incurs significant power losses of up to 99%. This level of power loss is unfavorable at terahertz frequencies where source power is relatively scarce. To address this, we propose a terahertz dark-field imaging system that makes use of a double axicon beam expander to create an annular beam, which is then focused down upon the target through an objective. The system permits simultaneous bright- and dark-field imaging in reflection without modifying the optical train, making greater use of raster scan time while achieving an average transmission efficiency of 33.15% and bright-field spatial resolution of 0.391 lp mm-1. The proposed dark-field imaging system is able to enhance the detection of fine features such as growth rings in wood, superficial imperfections on bulk materials, and defects in fiberglass. This approach to achieve dark-field imaging will be valuable for biomedical imaging at the terahertz range that harbors interesting molecular vibration activities.

We propose a low-loss one-dimensional cascaded photonic crystal cavity with high-quality factor integrated into an unclad silicon waveguide. This cascaded cavity is conceptually analyzed by temporal coupled mode theory, and its performance is validated using full-wave simulation. The cavity exhibits a high-quality factor combined with high transmission efficiency, which holds potential to precisely select the desired single-frequency component of a local oscillator signal. The simulated results of the proposed cavity show a quality factor of 9,161 at 275 GHz.

Network analysis is foundational to calibrated instrumentation for practical handheld devices, and the directional coupler is the fundamental passive component that makes it possible. In this work, a terahertz-range handheld integrated-photonic directional coupler is reported with broad operation bandwidth spanning more than two octaves. The design adapts the underlying ray-tracing principles of classical parabolic reflectors and optical beam splitters to a monolithic substrateless micro-machined silicon structure. This advance holds the potential to unlock a new generation of terahertz systems, including vector network analyzers, that combine the broad bandwidth of bulky quasi-optical terahertz systems with the handheld form-factor and convenient, repeatable port access of guided-wave approaches.