Journal articles

Recently, terahertz micro-doppler systems have been applied to numerous valuable applications including acoustic and vital-sign sensing, semiconductor probing, and detecting targets with small radar cross-sections. Conventional terahertz systems used for these applications suffer from strong undesirable phase-noise due to the reliance on frequency multipliers, degrading their performance close to the carrier, masking micro-doppler signals. We propose that this troublesome phase-noise can be suppressed through quadrature homodyne detection allowing for exceptional sensitivity close to the carrier. Here we provide a theoretical examination of this system, and a thorough experimental characterization. Furthermore, we demonstrate the ability to measure physical displacements on the order of 0.5 μm, with vibrational sensitivity down to the nanometer scale, 6 orders of magnitude smaller than the carrier wavelength at 275 GHz. We push the sensitivity to the limit, demonstrating the practicality and high signal-to-noise-ratio through acoustic sensing of phone speakers to recover speech. Further, we show the ability to detect changes in integrated circuits in operation. These demonstrations highlight that the sensitivity of the system is high, while the cost and complexity is limited through the use of simple beam space optics and cost-effective diode detectors.

With the continuous advancement of terahertz technology, developing passive components has become crucial for integrating future terahertz systems. Ceramic 3-D printing has emerged as a promising fabrication method for creating low-loss, high-permittivity dielectric devices. However, designing passive components for the WR3 band (220–330 GHz) presents significant challenges due to the relatively small feature sizes required, which demand exceptional print quality. In this study, we design WR3-band photonic crystal slot resonators with high Q factors to evaluate its suitability for 3-D printing. Multiple samples are fabricated in a single run. Their physical dimensions and frequency responses are measured to assess the printing quality and reliability. This work provides valuable insights into the design and fabrication strategies of WR3 band components using ceramic 3-D printing, paving the way for the development of fully integrated terahertz systems.

Silicon photonics has emerged as a key platform for next-generation communication and sensing technologies, enabling compact, scalable, and energy-efficient optical systems compatible with large-volume semiconductor manufacturing. When combined with highly nonlinear organic electro-optic materials, silicon photonics gains functionalities unavailable in silicon alone, opening new opportunities for ultrafast modulation and for direct transduction between high-frequency wireless electromagnetic fields and guided optical signals. In this work, we investigate hybrid silicon–organic devices based on patch-array antennas integrated with silicon waveguides for the detection of free-space terahertz waves. A telecom probe beam is routed through the silicon circuitry and converted into a plasmonic slot mode that interacts with the terahertz field accumulated in the nanoscale gap of the antenna. The latter is filled with a poled organic material providing the Pockels nonlinearity required for electro-optic terahertz modulation. We design various patch-antenna geometries and compare them to more conventional bowtie antennas. We show that patch-array antennas provide enhanced sensitivity at selected frequencies through a stronger resonant field concentration, in contrast to bowtie layouts that instead exhibit broadband responses owing to their tapered dipolar design. We further analyze how the detected signal depends on the relative position of the terahertz beam spot with respect to these two types of antenna on the chip, thus reconstructing the spatial coupling behavior of the two configurations. These results clarify the trade-offs between bandwidth, sensitivity, and spatial tolerance in planar antenna-coupled plasmonic terahertz detectors and provide practical design guidelines for integrated optical sampling of terahertz signals.

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.

Direction-of-arrival (DoA) estimation is one of the key techniques in terahertz communications and sensing, as it provides angular information of incident waves for high-speed links and allows sensing with high angular separability. This paper proposes a simple DoA estimation system that combines a single receiving antenna with a mechanically rotating dielectric lens. The proposed system enables single-frequency DoA estimation by rotating the lens to vary the radiation pattern and exploiting the amplitude variation at each lens rotation angle α. Measurement results show that, when the lens is rotated through 360° in 15° steps, the system achieves a root-mean-square error (RMSE) of 0.22° and a 100% estimation accuracy with an angular tolerance of 0.5°. The estimation performance remains stable over the frequency range of 253–319 GHz (IEEE communication band), regardless of the operation frequency. Furthermore, when using 12 lens rotation angles (corresponding to 180° rotation), the system maintains high estimation performance, enabling two DoA estimations per full rotation. Because the dielectric lens can be fabricated using a 3-D printer without complex antenna arrays, feeding networks, or electronic tuning circuits, the proposed approach provides a low-complexity platform for single-frequency compressive DoA estimation in the sub-terahertz band.

The sub-terahertz frequency region is a promising candidate for realizing 6G-beyond mobile networks. However, sub-terahertz waves are subjected to strong free-path loss, requiring highly directional antennas. Precise direction-of-arrival (DoA) estimation is therefore critical to maintain accurate alignment between transceivers. This paper proposes an all-dielectric frequency-diverse antenna with strongly frequency-dependent radiation patterns, which yield direction-dependent received spectra for DoA estimation. The antenna operates within the IEEE 802.15.3d band and is optimized for improved manufacturability. Beyond single-source detection, the approach is extended to multi-source DoA estimation. A neural network model is trained on the antenna's radiation patterns and evaluated for detecting both single-source and multi-source scenarios. Experimental results indicate that DoA tracking can be achieved for transmitter-receiver distances of up to 900 mm with an average RMSE of 2.53° for single source detection and 4.56° for multiple sources. These results validate the effectiveness of the proposed system as a robust, data-driven solution for DoA estimation in next-generation terahertz communications systems.

All-dielectric effective-medium-clad waveguides have been widely exploited in terahertz communications owing to their extremely low loss, low dispersion, and broad bandwidth. Here, we propose a substrateless effective-medium-slot waveguide. In addition, we introduce a taper-free interface that allows terahertz waves to directly couple from a metallic hollow waveguide without requiring dielectric insertion. We achieve a 3-dB bandwidth of 40% with a maximum coupling efficiency of 90% in the WR-2.2 band (330–500 GHz). By employing a broadband uni-traveling-carrier photodiode transmitter and sub-harmonic mixer receivers, we demonstrate an aggregated data rate of 0.8 Tbit/s with quadrature amplitude modulation schemes across 14 channels from 330 to 600 GHz. The effective-medium-slot waveguide platform yields robust broadband coupling with enhanced mechanical protection, offering reliable interconnects for ultra-high-speed terahertz integrated systems.

A terahertz-wave modulator is a crucial component in achieving high-speed wireless communications and advanced terahertz signal processing. Most previously realized terahertz modulators have primarily focused on wave manipulation in free space, which hardly fulfills the requirements for system-level compactness and miniaturization. This article introduces an optically controlled terahertz modulator on a substrateless silicon platform. This modulator incorporates a dipole resonator and a 1-D photonic crystal cavity to enhance the sensitivity of the terahertz waveguide to optical stimulus, achieving an enhanced modulation depth with a limited optical excitation power. This enabled us to demonstrate a modulation depth of 15.52 dB for the terahertz signal at 275.22 GHz with an optical power of 6.8 mW. While the modulation speed is 74 kHz in this work, which is limited by silicon’s long carrier lifetime, and we anticipate that the modulation frequency can reach gigahertz levels by incorporating a semiconductor with a faster recombination time. The proposed modulator topology holds substantial promise for integration into terahertz systems for wireless communications, where miniaturization, efficiency, and rapid modulation are essential.

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° 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.

Terahertz bands are essential for next‐generation wireless communications, offering ultrabroad bandwidth and unprecedented data throughput. However, realizing compact, low‐cost, broadband, and efficient terahertz transceiver modules remains challenging. Conventional modules that rely on metallic hollow‐waveguide or silicon‐lens packaging suffer from signal loss, bulkiness, and fabrication complexity. Here, we propose a compact terahertz wireless transceiver module enabled by a resonant tunneling diode (RTD) integrated with a photonic‐electronic antenna chain. The RTD, grown on an InP substrate, is coupled to a modified Vivaldi antenna functioning as a broadband mode converter to an all‐silicon effective‐medium‐clad dielectric waveguide, which terminates with a rod antenna directly interfaced with a three‐dimensional (3D)‐printed cyclic olefin copolymer elliptical lens. This configuration enables ultrabroadband and highly directive free‐space radiation without additional matching networks or anti‐reflection coatings. Encased in a low‐cost 3D‐printed polylactic acid package, the module attains realized gains of 28–33 dBi for the E11x mode and 30–33 dBi for the E11y mode across 220–330 GHz. As a receiver, the module exhibits a noise voltage density down to 5.6×10⁻⁹ V/√Hz , a minimum noise equivalent power of 1.8 pW/√Hz , and an average responsivity of 6.8 kV/W in the 300‐GHz band under amplified detection. It supports error‐free transmission (bit error rate (BER) less than 10⁻¹¹) up to 30 Gbit/s for on–off keying (OOK) modulation and 80 Gbit/s (BER < 3.8×10⁻³ below hard‐decision forward error correction limit) for 16‐QAM modulation over 10 cm and enables real‐time uncompressed high‐definition video streaming over 1 m. As a transmitter, the module is demonstrated to support OOK transmission at 332 GHz, achieving error‐free transmission up to 12 Gbit/s. These results establish a compact, lightweight, and multifunctional terahertz transceiver architecture, highlighting the potential of RTD‐based photonic‐electronic integration for 6G and beyond wireless frontends.

The advancement of terahertz sensing and imaging applications is contingent upon the development of detectors that are compact and low-noise and operate reliably at room temperature. Traditional terahertz bolometers are often constrained by limited absorption efficiency, low sensitivity, slow response times, and bulky architecture. We demonstrate an antenna-coupled microelectromechanical system bolometer for terahertz detection, featuring a spiral-shaped niobium antenna suspended above an air cavity on a cyclic olefin copolymer substrate and backed by a gold plate with periodically arranged square apertures. The spiral operating as efficient terahertz absorber, thermal transducer, and mechanical resonator makes the device compact and enhances sensitivity and responsivity. Upon exposure to terahertz radiation near the absorption peak of the structure, thermal expansion induces a redshift in the mechanical resonance frequency of the spiral, which is detected using a laser vibrometer through the rear gold plate. Experimental measurements show a clear resonance shift from 12.25 to 11.16 kHz under incident terahertz radiation at 278 GHz, with power levels ranging from 0 to 0.9 mW. This result corresponds to a thermomechanical responsivity of ∼1200 Hz/mW. Operating at room temperature and relying on monolithic implementation, the proposed device offers a sensitive, compact, and scalable platform for terahertz sensing and imaging applications.

Terahertz waves are sensitive to changes in free carriers within semiconductor materials. To date, research in this area has focused on spectroscopic analysis of exposed wafers and inactive devices. In principle, terahertz waves should be able to probe packaged and active semiconductor devices subjected to bias signals, which induce changes in the carrier density across a PN junction. A challenge lies in a discrepancy between the probing spot size on the order of millimeters at 275 GHz, and the sub-wavelength footprint of a PN junction, which results in small changes of any reflected terahertz waves. Although normally buried underneath the phase noise of the terahertz oscillator, these minuscule changes in carrier density can be observed as a baseband signal in complex reflection measurements using a homodyne quadrature receiver. Due to the high sensitivity of this transceiver, sub-wavelength targets can be effectively monitored with conventional terahertz optics. These experimental results demonstrate the ability of terahertz waves to non-invasively monitor semiconductor device activity. We anticipate that this work will be a precursor for non-destructive inspection of chips during production and the covert monitoring of operational devices.

Efficient mode coupling between dielectric and metallic hollow waveguides is critical in the development of integrated terahertz systems based on all‐silicon platforms. Conventional dielectric linear tapers, while commonly used, require long and thin structures, typically 3 λ₀ to achieve a low coupling loss, resulting in increased device footprint and a high risk of mechanical breakage during fabrication and alignment. In this work, we present two compact all‐silicon couplers, each leveraging the combined principles of dielectric rod antennas and slot waveguides. The proposed designs feature compact footprints of only 0.5 λ₀ in length for operation across 220–330 GHz and demonstrate an average coupling loss of –1.25 dB across this band. Compared to conventional linear tapers, the proposed design reduces footprint, improves mechanical robustness and ease of alignment. Experimental validation through terahertz communications measurements demonstrates bit error rates (BER) below 10⁻¹¹ for the out‐of‐plane polarization and below 10⁻⁷ for the in‐plane polarization at 275 GHz with a 7 Gbit/s data rate, and stable performance at a BER of 10⁻⁴ under high signal‐to‐noise conditions. The proposed couplers facilitate efficient mode conversion between different waveguides, making them valuable components for system‐level integration in future terahertz communications frontends.

As the demand for high-speed terahertz communications grows, bandpass filters become essential for managing the spectrum. However, the development of efficient planar bandpass filters for terahertz frequencies remains limited. This study introduces a new class of terahertz bandpass filters on an all-silicon substrateless platform. The design employs cascaded 2-D and 1-D photonic crystal structures with effective-medium claddings. The proposed approach allows for precise control of the center frequency and bandwidth, providing the flexibility to filter designs that can potentially cover all 69 channels in the IEEE 802.15.3d standard between 252 and 325 GHz. In this work, we demonstrate filter designs for four different channels, with two representative prototypes fabricated and characterized. Experimental results show low insertion loss better than 1.5 dB, high out-of-band rejection beyond 25 dB, flat group delay of 0.25 ns within the passband, and low group delay dispersion around 0.01 ns². Communication experiments confirm the ability of the filter to reshape the terahertz spectrum, suppress out-of-band noise by 17 dB, and improve channel isolation while preserving data integrity. This method provides a promising solution for scalable high-performance filtering in future terahertz communications systems.

In wireless communications, accurate frequency estimation is essential to support reliable demodulation, channel management, and spectrum surveillance. At terahertz frequencies, the higher carrier values and narrow channel spacing demand much finer frequency estimation, which requires high-resolution frequency discrimination. Conventional grating-based approaches are bulky and limited in resolution, making them unsuitable for compact terahertz systems. In this letter, we present a 3D-printed pseudo-random dielectric metasurface that enables subgigahertz frequency discrimination in the 220–330-GHz band through spatially diverse near-field patterns. The design leverages frequency-dependent scattering to create unique intensity distributions captured by a terahertz camera. We demonstrate that these spatial signatures can be used for frequency discrimination by training a convolutional neural network (CNN) to identify the frequency from a single image. To substantiate the reliability of the metasurface response, we employ multiple beam configurations with varying incidence angles in our experimental setup. The results demonstrate high classification accuracy over the operational range, thus corroborating the metasurface as a viable passive frequency analyzer for terahertz communications.

The water status of a plant plays a critical role in its growth and biomass accumulation. It is thus essential to accurately quantify the plant water content to improve the overall productivity. Conventional methods to assessing plant water status are either destructive, contacting, or only qualitative. In this work, we present a study to non-destructively quantify, in real-time, the water status of plant leaves in a non-contact manner using terahertz waves. The presented study correlates the leaf complex permittivity with both the relative water content and water potential in the nearly full range. Therefore, the leaf water status can be quantitatively determined based on the non-destructively extracted permittivity using terahertz waves. Furthermore, single-frequency empirical models linking the imaginary part of the leaf complex permittivity to water status have been developed, enabling the use of high-power narrowband sources in field tests. The simplified measurement and accurate evaluation presented in this study facilitates the application of terahertz technology in supporting sustainable agriculture under climate change and drought.

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 ~500 μm across the operation bandwidth—less than one free-space wavelength.

Dielectric antennas with moderate gain are important in short-range terahertz communication applications. However, achieving moderate gain while maintaining a small antenna footprint is challenging. This letter presents an all-dielectric tapered rod antenna with nonuniformly distributed subwavelength grating operating over 220 GHz to 330 GHz. The antenna is monolithically integrated with a low-loss substrateless silicon effective-medium-clad waveguide platform. The proposed antenna supports two orthogonal fundamental modes, i.e., the in-plane and out-of-plane polarizations. The measured result shows that the realized gain for the two polarizations ranges from 12.7 dBi to 15.5 dBi. Notably, the antenna with grating achieves an average gain enhancement of 1.82 dB compared with the bare tapered rod antenna with the same footprint. This method for improving gain holds great promise for terahertz communications.

All-silicon substrateless integrated systems are suitable to wide-ranging terahertz applications due to their low-loss characteristics. A broadband, high-extinction level polarization filter is in demand for these on-chip systems to select the desired polarization when monomode operation of the dielectric host waveguide is required. Existing photonic designs for polarization filtering cannot cope with a large fractional bandwidth in the terahertz range. To increase the bandwidth and rejection level of the filter, a longitudinal slot is introduced into the host waveguide. Experimental validation shows a high average polarization extinction ratio of 23.5 dB between the TM and TE polarizations and an insertion loss of around 1.5 dB for the TM polarization over a 3-dB bandwidth of 87 GHz covering the whole IEEE communications band between 252 and 325 GHz. The proposed filter will be an essential component for terahertz communications to maintain the polarization purity in substrateless dielectric waveguides.

Terahertz confocal imaging allows 3D see‐through of a non‐metallic object with high resolution. Conventional methods acquiring 3D images of thick objects suffer from limited depth‐of‐field, constrained depth resolution, and/or inconsistent spatial resolution at different depths. To address these limitations, the intrinsic chromatic aberration of a typical focusing metasurface is exploited to achieve frequency‐dependent focal lengths. An object located within this extended focal range can be readily 3D inspected by performing 2D raster scans. A rigorous analysis reveals that the focal spot maintains a constant waist diameter of 2.4 mm (equivalent to 2.2 λ₀ at 275 GHz) and migrates 68.1 mm (equivalent to 62.4 λ₀ , or 16.4 times of Rayleigh length, or 1.4‐fold of the designed focal length at 275 GHz) from 175 to 525 GHz, and thus achieving a consistent spatial resolution and a large depth‐of‐field for 3D imaging. Importantly, this large depth‐of‐field is achieved with a relatively high numerical aperture of around 0.42. Measurements conducted between 220 and 330 GHz exhibit close agreement with the calculation. To demonstrate its imaging functionality, two stacked papers with different texts, a mobile phone, and earphones concealed in a charging case are imaged, where a short‐time Fourier transform is implemented in the time‐domain terahertz images to enhance image contrast. The presented metasurface is technologically significant for imaging systems to rapidly inspect objects in 3D with exceptional resolutions. Its potential applications include in‐situ defect detection and object identification in security screening.

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.

The terahertz communications band in the 252 to 325 GHz range has been recently explored for its potential to meet the stringent requirements for the emerging sixth generation of wireless communications. However, there are several challenges, including noise and nonlinearity, that hinder efficient implementations. This paper aims to address these limitations in terahertz communications through convolutional neural networks (CNN) enhanced by the domain knowledge from traditional Volterra filters. The proposed model achieves a 2.28 dB improvement in total harmonic distortion (THD) compared to the Volterra equalizing method and also shows a significant SNR improvement. In general, this work can significantly enhance the reliability and performance of terahertz communications systems, paving the way for the realization of advanced wireless applications.

Polarization‐division multiplexing is crucial for increasing channel capacity and spectral efficiency in communications. A key component is the polarization multiplexer, which combines or separates signals with orthogonal polarizations. Existing planar multiplexers lack an ultrawide band for terahertz communications. While microwave‐inspired orthomode transducers (OMTs) provide broadband operation, they suffer from high ohmic loss and bulkiness at terahertz frequencies. Optical integrated polarization beam splitters (PBSs) offer low loss and good integrability but have narrow bandwidths. This work presents a substrateless all‐silicon polarization multiplexer based on tapered directional couplers and air‐silicon effective mediums, integrated monolithically on a compact footprint. The device demonstrates a 37.8% fractional bandwidth, an average insertion loss of ≈1 dB, and a polarization extinction ratio above 20 dB over 225–330 GHz. This superior performance is enabled by the anisotropy of the effective medium claddings, affecting the two orthogonal guided modes differently. The multiplexer enables simultaneous two‐channel terahertz fiber communications with polarization diversity. It supports aggregated data rates up to 155 and 190 Gbit s⁻¹ with bit error rates below hard‐ and soft‐decision forward‐error‐correction limits, respectively. This advancement holds promise for 6G terahertz integrated systems with enhanced channel capacities.

At terahertz frequencies, where Ohmic losses are high, all-dielectric waveguides have emerged as the basis of low-loss integrated circuits. Usually, advanced foundry fabrication techniques are leveraged to realize these devices from semiconductor materials, such as silicon, owing to their high permittivities. However, these methods are restrictive when 2.5-D devices are desired, requiring multiple stages of masking and etching. Here, advances in ceramic 3-D printing are exploited to realize single-mode height-varying waveguides that operate over the WR3.4 band, from 220 to 330 GHz. The waveguide is experimentally characterized, showing attenuation on the order of 0.2 dB/cm (±1 dB/cm), comparable to the ubiquitous hollow metallic waveguide at the same frequency range. Capitalizing on design freedom, we demonstrate a liquid-identifying application, using a ring resonator with an integrated well to hold the analyte. The extra design freedom afforded by 3-D printing combined with the properties of ceramics and robust performance of the waveguide lays the groundwork for future integrated passive devices with added functionality.

Although substrateless micro-scale silicon waveguides are a useful and versatile platform for terahertz waves, the fact that modal fields occupy the volume of the core limits the potential to dynamically manipulate guided waves. To address this, we introduce an aperiodic lattice structure to enable the monolithic co-integration of a Vivaldi antenna-like mode converter with a substrateless silicon waveguide. This broadband transition is experimentally confirmed to exhibit ∼2.5 dB average loss for two couplers, from 220 GHz to 330 GHz, and enables a photoexcited variable attenuator as proof-of-concept demonstration. This is an important enabling step to incorporate general-purpose dynamic reconfigurability, sensing, and modulation functionality into terahertz-range silicon-based integrated circuits, which are currently limited to primarily all-passive structures.

All-silicon substrateless waveguides have become a leading contender for terahertz device integration owing to their broad operating bandwidth and exceptional efficiency. However, without metallic confinement, these unshielded waveguides are susceptible to evanescent coupling, requiring that waveguides be placed sparsely to resist unwanted interactions. This sparsity remains a critical obstacle to compact devices and integrated systems. To counter this, we demonstrate substrateless extreme skin-depth waveguides, utilizing a self-supporting anisotropic cladding that markedly reduces the evanescent field penetration into the surrounding cladding, and hence, suppresses cross coupling between waveguides. Here, we achieve 20-dB cross-talk suppression across the WR3.4 band, 220–330 GHz, a 40% fractional bandwidth, with less than 0.5 free-space wavelength separations and coupling lengths exceeding ten free-space wavelengths. In addition, we exploit the low bending loss of this waveguide to realize an efficient and simply designable power divider to realize arbitrary extinction ratios between 1:1 and 10:1. Integrable with existing all-silicon devices, we foresee these techniques enabling the dense integration of terahertz systems with substrateless silicon waveguides.

Nondestructive evaluation is one of the key envisaged applications for terahertz technology due to nonionizing energy levels and the ability to penetrate many optically opaque materials. Conventional terahertz imaging systems typically rely on raster scanning the target with a moving transceiver, or make use of goniometric beam manipulation schemes. As an alternative, we propose the use of a Risley-prism, essentially a cascaded pair of independently rotating prisms, to function as a simple beam-steering mechanism. When deployed in conjunction with an aspheric, telecentric objective, the Risley-prism allows for scanning a focused terahertz beam in two dimensions. We utilize 3D-printed cyclic olefin copolymer, a low-loss and low-dispersion polymer, to minimize material loss and facilitate the construction of these bulk optics. Owing to true time delay, our imaging system operates over 220–330 GHz, making use of this bandwidth for resolving depth features and hidden objects. We achieve a spatial resolution of 0.419 lp/mm over a circular scanning region 27.8 mm in diameter. The proposed Risley-prism imaging system offers a simple solution to the complicated problem of broadband and high resolution imaging, and hence, is readily amenable to widespread adoption and commercial applications.

In terahertz systems, polarization conversion is usually carried out using conventional waveplates. However, limitations in these waveplates arise from inadequate material accessibility, relatively narrow bandwidth, low efficiency, and bulkiness. To overcome these limitations, a transmissive terahertz metasurface with mechanically reconfigurable polarization conversion is proposed. The separation between two identical metasurface plates is varied to switch between quarter-wave plate and half-wave plate functionalities. Measurement results verify that with no separation, the device can perform as a quarter-wave plate providing better than 3-dB axial ratio and an efficiency of over 75% from 246 to 320 GHz. With a separation of 150 μm between the two units, the transmission output becomes a cross-polarized wave with a 18-dB extinction ratio and an efficiency of over 74% across the same band. Altogether, the proposed polarization conversion device provides the advantage of reconfigurability and can be simply installed into existing terahertz systems to achieve multiple functionalities.

Terahertz radars based on leaky-wave antennas are promising for the realization of radar systems with high resolution over short ranges. This type of radar relies on spatial frequency mapping to realize a wide field of view without mechanical actuation or electrical beam steering. Previously, integrated leaky-wave antennas based on metallic wave confinement have been implemented, but the high ohmic losses limit the realized antenna gain, which is essential for extending the range of such a system when limited power is available. Here, we demonstrate an all-dielectric leaky-wave antenna fabricated monolithically from silicon and then apply leaky-wave radar techniques to realize a terahertz radar system capable of real-time data acquisition. Through this all-dielectric approach, we can avoid metallic losses, achieving an experimentally measured maximum realized gain of 25 dBi and a 34.3° 3-dB field-of-view while utilizing established and scalable fabrication techniques essential for the wide-spread adoption of terahertz technologies. We foresee this technique being applied to a variety of real-time radar applications, and here we demonstrate two potential use-cases: multi-object tracking and differentiation of liquids.

We observe that the modal field distribution of a dielectric slot waveguide closely resembles a magnetic dipole antenna. Such an aperture distribution traditionally demands metals, making it ill-suited to high frequencies due to excessive ohmic loss. By terminating a dielectric slot waveguide with a matched free-space interface, a compact all-dielectric radiating magnetic dipole is realized. In this way, we introduce general-purpose dipole antennas, which have long been a mainstay of RF and microwave ranges, into the realm of light wave photonic integrated circuits. The existence of the desired magnetic dipole aperture distribution is experimentally confirmed in the terahertz range, at ∼275 GHz, and good matching is evident in the ∼-25 dB reflection level. This is the electrically smallest radiator to ever be incorporated into an all-dielectric waveguiding platform.

The IEEE 802.15.3d standard for point-to-point wireless terahertz communications is defined to support high-capacity channels. By nature, terahertz signal transmission requires line-of-sight propagation and terahertz communications operates within a challenging power budget limitation. Therefore, accurate and efficient direction-of-arrival (DoA) estimation for maximizing received power becomes paramount to achieve reliable terahertz communications. In this article, we present a frequency-diverse antenna with a machine-learning-based approach utilizing convolutional neural networks (CNNs) to estimate DoA in the terahertz communications band. The antenna is deliberately designed to break symmetry, generating quasi-random radiation patterns, while the CNN captures the relationship between the radiation patterns and their respective angles of arrival. Based on experiments, the DoA estimation results converge to a minimum validation mean squared error of 3.9° and root mean squared error of 1.9°. The estimation efficacy is further substantiated by a consistent performance demonstrated across diverse scenarios, encompassing various obstacles and absorbers around the propagation path. The proposed DoA estimation method shows considerable advantages as a compact, integrable, and cost-effective solution for practical terahertz communications.

Terahertz sources require compact lenses for efficient outcoupling and manipulation of terahertz waves. Traditional terahertz lenses are bulky and require high-precision fabrication techniques, such as micromachining, which limit their flexibility in integrated systems. As an alternative, we present a planar high-gain cavity antenna integrated with a waveguide feed. The antenna is made by laser-etching of high-resistivity float-zone silicon to create nonuniform hole arrays, which function as gradient-index lenses for beamforming. To address the insufficient phase coverage range caused by limitations in feasible etching depth and the intrinsically tapered side walls of the holes, direct laser-etching is performed on both sides of the silicon wafer. A cyclic olefin copolymer sheet is placed on top of the silicon wafer to aid silicon–air impedance matching. The silicon cavity antenna is experimentally validated, with results confirming that a maximum broadside gain of 19 dBi can be achieved at the center frequency of 275 GHz, with a 3 dB bandwidth of around 29%. We further demonstrate that such a high-gain antenna is applicable to error-free point-to-point short-range wireless signal transmission. A demonstration of uncompressed 4K-resolution video transmission is also included. The proposed planar antenna design has the advantages of a significantly reduced device profile and is a cost-effective solution for future practical array-level designs.

Implementing terahertz circuits and system designs relies on integrating reconfigurable devices such as switches, to fulfill a critical role in controlling and manipulating the flow of terahertz signals on the chip. Although there have been several demonstrations of on–off switching in the terahertz range, there have been no demonstrations of 1-to-N switching, to our knowledge. This pronounced lack of dynamically reconfigurable routing has severely limited the achievable complexity of terahertz systems. To address this, we propose 1-to-N switches made of cascaded disk resonators integrated into a substrateless silicon waveguide platform. A single switch can be controlled via photoexcitation using a low-power 658-nm laser focused onto the disk resonator, turning off the resonance and inhibiting coupling into a crossing port. The measurement results demonstrate that the proposed switch has low insertion loss, which can be attributed to the inherently low dissipation of the platform. The proposed on–off switch achieves a maximum insertion loss of 1.2 dB, and the maximum extinction ratio of the switch is 16.1 dB with 1.5 GHz of bandwidth. Furthermore, a 1-to-3 switch is monolithically integrated together with a Luneburg lens in order to project each of its output ports to a different far-field direction and, thereby, translate the switching operation into a form of reconfigurable beam control for future applications.

Traditional microwave design and fabrication techniques have been adopted into the terahertz domain. Understanding the properties of microwave dielectric materials at terahertz frequencies is critical for accurate component design. Nevertheless, terahertz properties for common microwave dielectric materials are largely unknown. Hence, this paper presents the relative permittivity, loss tangent, refractive index, and extinction coefficient for such materials, including microwave substrates and low-temperature co-fired ceramics (LTCCs), within the 0.1 to 3.5 THz range. Terahertz time-domain spectroscopy (THz-TDS) and a vector network analyzer produce accurate material parameter results. The material parameters presented in this paper serve as a valuable resource for component design at terahertz frequencies.

Terahertz communications has been foreseen as a key enabler to the sixth generation (6G) of wireless communications systems. However, the design of spectrally-efficient waveforms and modulation schemes is an ongoing challenge in this regime. In this article, we propose and experimentally demonstrate the transmission of M-ary carrierless amplitude and phase (CAP) modulated signals using a 253 GHz photomixing-based terahertz wireless communications system combined with the optical transmission over a 10-km standard single mode fiber (SSMF). Experimental results show that the CAP modulation technique has a capability to support the high-speed transmission of terahertz signals over a wide range of data rates from 4 Gbit/s (BER >10⁻³) to 96 Gbit/s (BER >10⁻¹), based solely on optical intensity modulation at the transmitter side and terahertz envelope detection at the receiver side. Consequently, M-ary CAP can potentially be adopted as a low-complexity waveform and modulation contender to simplify the transceivers architectures for 6G terahertz communications systems, without sacrificing the high throughput targeted by these systems.

The absence of a suitable standard device platform for terahertz waves is currently a major roadblock that is inhibiting the widespread adoption and exploitation of terahertz technology. As a consequence, terahertz-range devices and systems are generally an ad hoc combination of several different heterogeneous technologies and fields of study, which serves perfectly well for a once-off experimental demonstration or proof-of-concept, but is not readily adapted to real-world use case scenarios. In contrast, establishing a common platform would allow us to consolidate our design efforts, define a well-defined scope of specialization for “terahertz engineering,” and to finally move beyond the disconnected efforts that have characterized the past decades. This tutorial will present arguments that nominate substrateless all-silicon microstructures as the most promising candidate due to the low loss of high-resistivity float-zone intrinsic silicon, the compactness of high-contrast dielectric waveguides, the designability of lattice structures, such as effective medium and photonic crystal, physical rigidity, ease and low cost of manufacture using deep-reactive ion etching, and the versatility of the many diverse functional devices and systems that may be integrated. We will present an overview of the historical development of the various constituents of this technology, compare and contrast different approaches in detail, and briefly describe relevant aspects of electromagnetic theory, which we hope will be of assistance.

Resonant cavities are fundamental to and versatile for terahertz integrated systems. So far, integrated resonant cavities have been implemented in relatively lossy terahertz platforms. In this Letter, we propose a series of integrated disk resonators built into a low-loss substrateless silicon waveguide platform, where the resonances and associated quality factor (Q-factor) can be controlled via an effective medium. The measurement results demonstrate that the Q-factor can reach up to 9146 at 274.4 GHz due to the low dissipation of the platform. Additionally, these resonators show strong tunability of the resonance under moderate optical power. These terahertz integrated disk resonators can be employed in sensing and communications.

For on-site measurements, large, expensive, conventional terahertz pulse time domain systems for non-destructive evaluation will potentially superseded by compact electronic-based terahertz systems. However, common electronic detectors are not capable of detecting the phase of received signals, complicating the characterization of multilayer samples. We propose a practical method for the characterization of in-container liquid samples using swept-source terahertz coherence tomography in reflection configuration. We demonstrate this technique in the 220–330 GHz frequency band to quantify the principle components of common alcoholic beverages, namely, water, sugar, and ethanol, with sensitivity to ethanol concentration below 0.4%v/v, whilst simultaneously recovering information about the container. Additionally, the method allows real-time observation of dilution in the mixtures. The proposed method facilitates measurement with significantly reduced experimental overhead and critically, is compatible with incoherent terahertz cameras for quantitative imaging.

A uniform illumination over a screen is crucial for terahertz imaging. As such, conversion from a Gaussian beam to a flattop beam becomes necessary. Most of the current beam conversion techniques rely on bulky multi-lens systems for collimated input and operate in the far-field. We present a single metasurface lens to efficiently convert a quasi-Gaussian beam from the near-field region of a WR-3.4 horn antenna to a flattop beam. The design process is divided into three sections to minimize simulation time, and the conventional Gerchberg-Saxton (GS) algorithm is supplemented with the Kirchhoff-Fresnel diffraction equation. Experimental validation confirms that a flattop beam with an efficiency of 80% has been achieved at 275 GHz. Such high-efficiency conversion is desirable for practical terahertz systems and the design approach can be generally used for beam shaping in the near-field.

Resonant-tunneling diodes (RTDs) are promising electronic sources for terahertz (THz) waves. A recently proposed slot-based RTD simplifies the fabrication process by removing the metal–insulator–metal capacitor, which is used for dc separation, and introducing an etch stopper layer for simple air bridge formation. The simplified RTD structure, however, exhibits an unexpected large tilt angle in the main lobe of its radiation pattern due to the coupling between the slot antenna and the bias pads. Here, we present a series-fed patch array design that exploits the biasing structure and directs leakage surface waves to the broadside direction. The simulation result shows that the main lobe is along the broadside, with a maximum directivity over 12 dBi and a 3-dB bandwidth around 20%. The radiation efficiency is improved from 22% to 28% at 275 GHz. An experimental validation with a bullet lens indicates that the RTD device with a series-fed patch array antenna has a narrower beamwidth and a sidelobe level much lower compared with the original RTD. With such improved radiation performance, this simple RTD device becomes a potential source for the development of future array-level designs and flat lens systems.

Circular polarizers that selectively transmit only one handedness of circular polarization are useful for imaging and wireless communications. Conventional circular polarizers involve 3D chiral structures, which impose fabrication challenges, while typically introducing chirality within a limited bandwidth. To overcome the limitations associated with conventional non‐planar designs, a three‐layer metasurface‐based planar circular polarizer exhibiting strong and broadband chirality is presented here. Its superiority over existing multilayer designs is derived from a systematic design procedure. Measurement results reveal that the proposed structure maintains a 15‐dB extinction ratio from 251 to 293 GHz for the preferred handedness of circular polarization, leading to a fractional bandwidth of 15.4% with a transmission efficiency above 92.7%. Furthermore, the proposed structure is mechanically tunable to alter its functionality or operation bandwidth. Specifically, through simply rotating the top or bottom metallic layer by 90°, the structure can function as a transmissive quasi‐half‐wave plate that reverses the sense of circular polarization. Moreover, the presented structure can operate at nearby frequency ranges for the aforementioned functionalities by mechanically adjusting the air gap spacings between the metallic layers. Further calculations based on the measured results of each layer suggest that the proposed structure is robust to deviations in the air gap spacings.

Terahertz (THz) systems open up the possibility of new applications of electromagnetic waves. Enormous bandwidths up to several terahertz enable the development of powerful spectroscopy systems that can provide insights into the structure and material of objects with micrometer resolution. New high power and compact THz time-domain spectroscopy (TDS) systems will be presented. The wide bandwidth also enables high-resolution imaging under far-field conditions to analyze arbitrary objects from a distance. In addition, a near-field system-on-a-chip imaging system is presented that achieves a resolution of 10 μm. Additionally, the application of terahertz waves presents challenges due to the low transmit power of the sources, high attenuation in free space, and the high noise figures of the receivers. These challenges can be overcome by antennas with high gain. Since - depending on the application - spatial scanning or focusing in a time-varying direction is required, beam steering is an essential component of many THz systems. In terms of communications, terahertz carrier frequencies offer wide bandwidths, enabling correspondingly high data rates of 100 Gbit/s and more. As a result, the frequency bands between 250 GHz and 450 GHz have already been identified and/or allocated for communication services and are being discussed as a component of 6G mobile communications. Concepts and demonstrations for 6G terahertz mobile communications will be presented. The high bandwidth of terahertz waves can also be used for high-accuracy indoor localization.

Two plasma filaments crossing above the target create a subwavelength window for terahertz microscopy that excludes any subwavelength probe in vicinity.

In order to harness the capabilities of terahertz waves, various metasurface-based functional devices have been developed recently. However, due to the limited usage of systematic optimization methodologies, many existing designs leave room for further bandwidth and efficiency improvement. This article provides an overview on the bandwidth limiting factors associated with metasurfaces and gives a tutorial on a semi-analytical approach to broadband design. The broadband approach incorporates network analysis and genetic algorithm to determine the frequency-independent optimal circuit parameters for multi-layer transmissive metasurfaces, so that targeted complex transmission coefficients can be achieved over a wide bandwidth. The broadband design approach is enabling the configuration and optimization of diverse metasurfaces for wavefront and polarization control of terahertz waves, including quarter- and half-waveplates.

Frequency-tunablefilters are in demand for applications requiring high spectral selectivity. To this end, frequency-selective surfaces (FSSs) have been widely applied for spatial filtering, where varactors well serve the purpose for electronic reconfiguration at microwave frequencies. For terahertz applications, however, varactors are unavoidably lossy owing to their relatively small cross section required to yield small values of capacitance. In this work, we propose a terahertz bandpass multilayer FSS with a finesse around 20, with 80% power transmission. Its operation frequency is mechanically tunable in a 40% frequency range by varying an interlayer spacer thickness. The experimentally validated bandwidth is 31% due to the limitation in measurement apparatus, and the power transmission is above 75%. The filter is insensitive to misalignment and can maintain tunability under oblique angles of incidence. This proposed filter outperforms a series of notable reconfigurable filters available in the literature in terms of tunable range and insertion loss. The proposed design is among the first few reconfigurable terahertz filters, and the design methodology can be readily applied to other FSS-based structures.

Seamless integration of terahertz wireless communications with existing optical access networks can potentially offer tens of gigahertz of channel bandwidth. However, the timing-jitter exhibited by the electronics-based transceivers is a key transmission impairments that tends to ceil the performance of these hybrid access networks, especially at high transmission baud rates. In this work, we propose a sinc-Lorentzian Nyquist pulse shape that outperforms the conventional raised-cosine and the better-than raised-cosine pulse shapes under the influence of timing-jitter up to 35% of the symbol duration. Experimental demonstrations are carried out using a 311 GHz photonic-terahertz system operating at a standard bit rate of 1.44 Gbit/s to investigate the robustness of the three pulse shapes against timing-jitter. It is confirmed that the proposed pulse shape is highly-tolerant to timing-jitter as well as the nonlinearity exhibited by terahertz systems, and hence, can improve the bit error rate performance by an order of magnitude when the average timing-jitter is as large as 0.24 ns. The proposed pulse shape is a critical step for designing ultra-high-speed terahertz communications links with improved robustness to timing-jitter.

All-silicon effective-medium-clad waveguides are a promising candidate for an integrated terahertz platform with high efficiency and broad bandwidth. Waveguide crossings are essential circuit components, allowing for wave routing over shorter paths to increase circuit density. However, the simple intersection of two orthogonal effective-medium-clad waveguides results in terahertz wave scattering, leading to relatively high cross talk. In this work, a low-loss, 40% fractional bandwidth crossing utilizing Maxwell-Garnet effective-medium theory and wavefront planarization techniques is proposed. This monolithic structure is fabricated on a single high-resistivity float-zone silicon wafer using a deep reactive ion etching process with a modest 4.4 mm diameter (4.03λ0) structure footprint. Experimentally verified results show low insertion loss, less than 1 dB, and average cross talk level of -39dB for both E11x and E11y operating modes, over 220-330 GHz with a 40% fractional bandwidth. This waveguide crossing can be foreseen as a useful routing component for terahertz all-silicon integrated circuits. The proposed techniques are applicable to other dielectric waveguide platforms at infrared and optical frequencies.

The terahertz electromagnetic band has been foreseen as a promising candidate to accommodate the ever-increasing wireless data traffic. To this end, the development of novel transceiver architectures and signal processing solutions is crucial to handling massive data volume transmitted over terahertz wireless communications networks. In this paper, we propose a waveform that shows full compliance with the spectral emission mask imposed by the IEEE Standard for terahertz communications. The designed waveform exploits 99.3% of the total in-band energy admissible by this mask and provides an extra degree of freedom for out-of-band interference management. A proof-of-concept experiment is conducted using a 300 GHz photonics-based terahertz communications link to demonstrate the generation and wireless transmission of the proposed waveform. Experimentally validated bit error rates show that the proposed waveform outperforms the widely adopted raised-cosine waveform and the better-than-Nyquist waveform even when the Nyquist criterion for inter-symbol interference-free signaling is not satisfied. Moreover, an error-free transmission at 1.44 Gbit/s is achieved without employing complicated digital signal processing at the receiver side.

Polarization conversion is useful for studies of chiral structures in biology and chemistry, and for polarization diversity in communications. It is conventionally realized with wave plates, which, however, present challenges due to limited material availability, as well as narrow bandwidth and low efficiency at terahertz frequencies. To enhance bandwidth and efficiency, the concept of the Huygens' metasurface is adopted here for a transmissive half-wave plate. The half-wave metasurface is designed following the optimal frequency-independent circuit parameters provided by a broadband semi-analytical approach. Simulation results of an optimal design suggest that a 15-dB extinction ratio can be sustained from 219 GHz to 334 GHz, corresponding to a fractional bandwidth of 41.6%. The measured results indicate that the fabricated structure enables a 15-dB extinction ratio from 220 GHz to 303 GHz, with a cross-polarization transmission efficiency above 76.7% for both linear and circular polarizations. This half-wave metasurface design can be readily integrated into compact terahertz systems for diverse applications.

We propose a series of integrated Bragg grating filters with performance enhancement via the concept of effective medium. The bandstop filters are built in a high-resistivity silicon wafer and operated over the WR-3.4 band (220–330 GHz) with in-plane polarization. The proposed designs use an additional degree of freedom in controlling the effective refractive index so as to fully use the potential of the Bragg grating structures. As a result, the high insertion loss typically observed at the low-frequency bound of the filters due to weak wave confinement can be reduced, while radiation caused by the leaky-wave effect at the high-frequency bound is minimized, allowing for a 40% operation fractional bandwidth. These features are not achievable with conventional waveguide Bragg grating filters. All-silicon prototypes of filter samples are experimentally validated, demonstrating promising performance for a wide range of terahertz applications. The techniques to improve the filter characteristics by controlling the effective medium can be adopted in both microwave and optics domains.

In this letter, we propose an integrated all-silicon 2-D horn antenna operating over 220 to 330 GHz (WR-3 band). This endfire antenna is designed for full integration with a substrateless all-dielectric terahertz frontend. The antenna supports two orthogonal fundamental modes of guided waves with in-plane and out-of-plane polarizations. By introducing a 2-D effective-medium dielectric lens on the same platform, the antenna realized gain together with the free-space impedance matching is enhanced allowing for a broadband operation with a fractional impedance bandwidth of 40% on a compact footprint. The fabricated antenna is experimentally validated with measured realized gain ranging from 11.2 to 14.2 dBi for the in-plane polarization and 10.5–15.0 dBi for the out-of-plane polarization across the entire WR-3 band. We foresee that this all-dielectric horn antenna together with the integrated highly efficient platform can be adopted in beyond 5G communications.

The arrayed waveguide grating (AWG) is a versatile and scalable passive photonic multiplexer that sees widespread usage. However, the necessity of a waveguide array engenders large device size, and gratings invariably commute finite power into undesired diffraction orders. Here, we demonstrate AWG-like functionality without a grating or waveguide array, yielding benefits to compactness, bandwidth, and efficiency. To this end, we exploit optical tunneling from a dielectric waveguide to an adjacent slab in order to realize a slab-confined frequency-scanning beam, which is manipulated using in-slab beamforming techniques that we have developed in order to separate distinct frequency bands. In this way, we devise an all-intrinsic-silicon integrated 4 × 1 frequency-division terahertz multiplexer, which is shown to support aggregate data rates up to 48 Gbit/s with an on–off-keying modulation scheme, operating in the vicinity of 350 GHz. Our investigation targets the terahertz range, to provide a critical missing building block for future high-volume wireless communications networks.

Fibreglass components employed in the marine environment are susceptible to moisture ingress, resulting in a degradation of mechanical properties. The nondestructive testing and evaluation of such materials using acoustic methods is possible under certain conditions, but the detection of internal voids and delaminations is masked by the presence of water in such internal flaws. Herein, we present an investigation into the use of terahertz technology to overcome these limitations, for the detection of damage and water ingress in thick woven glass-fibre composites. This investigation is facilitated by terahertz time-domain spectroscopy, coupled with physical raster scanning to realize object-penetrating imaging. Air-filled defects within the fibreglass volume are clearly identified using this technique. A spectroscopic investigation on the alteration of the terahertz-range dielectric properties of the fibreglass material due to water ingress is performed and a small change is measurable, although likely to be obfuscated by the interaction of terahertz-frequency radiation with the internal fibreglass structure. Moisture-ridden laminates do not impede propagation of terahertz radiation, and therefore wetted materials may be inspected for volumetric defects. A simulation of water-filled volumetric defects shows promise for practical application.

Effective-medium-clad dielectric waveguides are purely built into a single high-resistivity float-zone silicon wafer with their claddings defined by deep subwavelength perforations. The waveguides are substrate-free while supporting both E11x and E11y modes with low loss and low dispersion. This article extends the investigations of the waveguides by analyzing the dispersion, cross-polarization, and crosstalk together with the characteristics of bends and crossings over an operation frequency range of 220–330 GHz (WR-3 band). Taking the E11x mode as an example, the experimental results show an average measured attenuation coefficient of 0.075 dB/cm and a group velocity dispersion ranging from around ±10 ps/THz/mm across the whole band. A crosstalk level below −10 dB is measured for parallel waveguides with a separation of 0.52λ0 at 300 GHz. The realized waveguides show a bending loss ranging from 0.500 to 0.025 dB per bend and a crosstalk at crossing below −15 dB from 220 to 330 GHz. Due to the different dispersion characteristics, the E11y mode has similar performances but with its operation frequency range reduced to 260–330 GHz. Limited by the measurement setup, a cross-coupling between the E11x and E11y modes is measured to be below −20 dB over the whole band. This in-depth investigation of effective-medium-clad waveguides will form a basis for terahertz-integrated platforms.

Frost is estimated to cost Australian grain growers $ 360 million in direct and indirect losses every year. Assessing frost damage manually in barley is labor intensive and involves destructive sampling. To mitigate against significant economic loss, it is crucial that assessment decisions on whether to cut for hay or continue to harvest are made soon after frost damage has occurred. In this paper, we propose a non-destructive technique by using raster-scan terahertz imaging. Terahertz waves can penetrate the spike to determine differences between frosted and unfrosted grains. With terahertz raster-scan imaging, conducted in both transmission and reflection at 275 GHz, frosted and unfrosted barley spikes show significant differences. In addition, terahertz imaging allows to determine individual grain positions. The emergence of compact terahertz sources and cameras would enable field deployment of terahertz non-destructive inspection for early frost damage.

A practical approach to realize substrateless, unclad, micro-scale intrinsic silicon waveguides for the terahertz range is presented. The waveguides are monolithically integrated within a supporting silicon frame, with which they are fabricated together from the same silicon wafer in a single-mask etching process. This establishes an integration platform to house many diverse components and facilitates packaging. Effective medium techniques are deployed to prevent the frame from interfering with the waveguide's functionality. Straight waveguides of this sort are experimentally found to be efficient and broadband. Elementary components including Y-junctions and evanescent couplers are developed, and deployed in demonstrations of applications for terahertz waves including sensing and communications. This is a promising pathway to realize future microphotonic devices for diverse applications of terahertz waves.

Polarization conversion devices are key components in spectroscopy and wireless communications systems. Conventional terahertz waveplates made of natural birefringent materials typically suffer from low efficiency, narrow bandwidth, and substantial thickness. To overcome the limitations associated with conventional waveplates, a terahertz quarter-wave metasurface with enhanced efficiency and wide bandwidth is proposed. The transmissive quarter-wave metasurface is rigorously designed based on an extended semi-analytical approach employing network analysis and genetic algorithm. Simulation results suggest that the design can achieve linear-to-circular polarization conversion with a 3-dB axial ratio relative bandwidth of 53.3%, spanning 205 GHz–354 GHz. The measurement results confirm that the proposed design enables a 3-dB axial ratio from 205 GHz to at least 340 GHz with a total efficiency beyond 70.2%, where the upper frequency bound is limited by the available experimental facility. This quarter-wave metasurface can cover an entire terahertz electronics band and can be scaled to cover other nearby bands under the same convention, which are technologically significant for future portable systems.

Broadband, low‐loss, low‐dispersion propagation of terahertz pulses in compact waveguide chips is indispensable for terahertz integration. Conventional 2D photonic crystals (PCs) based terahertz waveguides are either all‐metallic or all‐dielectric, having either high propagation losses due to the Ohmic loss of metal, or a narrow transmission bandwidth restricted by the range of single‐mode operation in a frequency range defined by the PC bandgap, respectively. To address this problem, a hybrid (metal/dielectric) terahertz waveguide chip is developed, where the guided mode is completely confined by parallel gold plates and silicon PCs in vertical and lateral directions, respectively. A unique multiwafer silicon‐based fabrication process, including gold–silicon eutectic bonding, micropatterning, and Bosch silicon etching, is employed to achieve the self‐supporting hybrid structure. Theoretical and experimental investigations demonstrate that the hybrid waveguide supports a single‐mode transmission covering 0.367–0.411 THz (bandwidth of 44 GHz, over twice wider than that of all‐silicon PC waveguides) with low loss (below 0.05 dB mm−1) and low group velocity dispersion (from −8.4 to −0.8 ps THz−1 mm−1). This work enables more compact, wideband terahertz waveguides and auxiliary functional components that are integratable in chips toward ultra‐high‐density integrated terahertz devices in particular in the field of wireless communications.

Polarization control is shown to enhance functionalities of terahertz waves in various applications. As a step toward introducing efficient practical devices operating in the terahertz band, we propose and experimentally validate a free-standing three-layer polarization converter operating in transmission. The device can efficiently convert a linearly polarized terahertz wave to its orthogonal counterpart. Our device comprises split-ring and H-shaped resonators inserted between two layers of orthogonal wire gratings. The sole dielectric material is a cyclic olefin copolymer (COC) that has exceptionally low-loss to terahertz waves. The measurement demonstrates a conversion efficiency of >80%, over 0.2–1.0 THz, giving an ultra-wide bandwidth of 133%. The wide bandwidth is attributed to multiple resonances of the two resonators in a unit cell. Importantly, an unconventional fabrication process yields a multi-layer low-loss COC that leads to high efficiency. The proposed design can also be tailored for practical applications across the electromagnetic spectrum.

Far-infrared absorbers exhibiting wideband performance are in great demand in numerous applications, including imaging, detection, and wireless communications. Here, a nonresonant far-infrared absorber with ultra-wideband operation is proposed. This absorber is in the form of inverted pyramidal cavities etched into moderately doped silicon. By means of a wet-etching technique, the crystallinity of silicon restricts the formation of the cavities to a particular shape in an angle that favors impedance matching between lossy silicon and free space. Far-infrared waves incident on this absorber experience multiple reflections on the slanted lossy silicon side walls, being dissipated towards the cavity bottom. The simulation and measurement results confirm that an absorption beyond 90% can be sustained from 1.25 to 5.00 THz. Furthermore, the experiment results suggest that the absorber can operate up to at least 21.00 THz with a specular reflection less than 10% and negligible transmission.

This article presents a narrow-band terahertz absorber using miniaturized unit cells. The absorber is made of three metallic layers separated from each other using low-loss cyclic olefin copolymer as dielectric spacers. A high quality factor (Q) is obtained using a first-order resonating structure with additional capacitive loading formed by the top two metallic layers. A circuit model is developed for the analysis and design of the proposed absorber. The designed absorber shows 1% fractional bandwidth for more than 90% absorption around a center frequency of 0.5 THz under normal incidence angle. This is equivalent to a quality factor of Q=31, calculated at 50% absorbance. The structure shows a highly stable absorbance over a wide range of oblique incidence angles. The designed absorber has been fabricated using a microfabrication process. The measured results of the fabricated prototype are in good agreement with the simulations.

Manipulation of terahertz radiation opens new opportunities that underpin application areas in communication, security, material sensing, and characterization. Metasurfaces employed for terahertz manipulation of phase, amplitude, or polarization of terahertz waves have limitations in radiation efficiency which is attributed to losses in the materials constituting the devices. Metallic resonators‐based terahertz devices suffer from high ohmic losses, while dielectric substrates and spacers with high relative permittivity and loss tangent also reduce bandwidth and efficiency. To overcome these issues, a proper choice of low loss and low relative permittivity dielectric layers and substrates can improve field confinement and reduce dissipation. Alternatively, replacing metallic resonators with a moderate relative permittivity dielectric material that supports cavity mode resonances also reduces dissipation due to the absence of conduction current. Herein, an overview of dielectric materials employed as spacers and dielectric resonators is provided, and the fabrication methods employed to realize these devices at the terahertz frequency range are also presented. Material selection guidelines, material‐specific and application‐specific fabrication quality metrics are outlined, and new techniques are proposed.

Terahertz integrated platforms with high efficiency are crucial in a broad range of applications including terahertz communications, radar, imaging and sensing. One key enabling technology is wideband interconnection. This work proposes substrate-less all-dielectric waveguides defined by an effective medium with a subwavelength hole array. These self-supporting structures are built solely into a single silicon wafer to minimize significant absorption in metals and dielectrics at terahertz frequencies. In a stark contrast to photonic crystal waveguides, the guiding mechanism is not based on a photonic bandgap but total internal reflections The waveguides are discussed in the context of terahertz communications that imposes stringent demands on performance. Experimental results show that the realized waveguides can cover the entire 260-400 GHz with single dominant modes in both orthogonal polarizations and an average measured attenuation around 0.05 dB/cm. Limited by the measurement setup, the maximum error-free data rate up to 30 Gbit/s is experimentally achieved at 335 GHz on a 3-cm waveguide. We further demonstrate the transmission of uncompressed 4K-resolution video across this waveguide. This waveguide platform promises integration of diverse active and passive components. Thus, we can foresee it as a potential candidate for the future terahertz integrated circuits, in analogy to photonic integrated circuits at optical frequencies. The proposed concept can potentially benefit integrated optics at large.

Polarization control of electromagnetic waves has wide applications in the field of communications, imaging, and remote sensing. Recent designs of periodic two-dimensional devices or metasurfaces employed for polarization control are limited in efficiency, bandwidth, and allowable incidence angle. This is attributed to high dissipation in the dielectric material used and to less-optimal device configuration. We propose and experimentally validate a reflective linear polarization converter metasurface with high efficiency, wide bandwidth, and wide acceptance angle in the terahertz regime. Our device is composed of three layers: an array of oriented metallic T-shaped resonators, cyclic olefin copolymer (COC) as a low loss dielectric layer, and a ground plane. For the normal and 45° incidence angles, a fabricated sample shows a bandwidth of 95% and 100%, with the average polarization conversion ratio above 80%, covering a frequency range of 0.38–1.07 and 0.36–1.08 THz, respectively. The wide-angle stability is attributed to a phase difference between a single resonance along the T-shaped resonator and a smooth phase response in the low-loss COC dielectric layer. For broad bandwidth performance, a resonator arm extending to adjacent unit cells introduces the fundamental resonance at a lower frequency, while the packed unit cell size shifts the grating lobe onset to a higher frequency. These design aspects can significantly improve the performance of other metasurfaces operating in any frequency range.

Reflectarrays offer unique potential for beamforming at terahertz frequencies as they combine the advantages of low‐profile of phased arrays and high‐efficiency of parabolic antennas. However, one challenge associated with reflectarrays is their bandwidth limitation resulting from the nonlinear phase response. To enhance bandwidth, a single‐layer stub‐loaded resonator is proposed for constructing reflectarrays. This resonator design shows a smooth and near‐linear phase response with a complete 360° phase coverage at and around the design frequency. To demonstrate its capability in realizing beamforming, a focusing reflectarray is then constructed using the proposed resonator as a building block. The measured results reveal that the 3 dB relative bandwidths of the reflectarray for the transverse electric (TE)‐ and transverse magnetic (TM)‐polarized excitations are 23.3% and 23.9%, respectively, while retaining an efficiency of 71.9% for the TE polarization and 71.0% for the TM polarization at the center frequency of 1.00 THz. The simulation bandwidth of this proposed focusing reflectarray is over twice that of an existing dielectric resonator reflectarray. The proposed resonator has a potential to enhance the bandwidth of terahertz reflectarrays for various beamforming functions.

Recent years have seen the emergence of efficient, general-purpose terahertz photonic-crystal waveguides etched from high-resistivity silicon. Systems founded upon this platform will require antennas in order to interface with free-space fields. Multi-beam antennas are desirable to this end, as they are capable of interacting with a number of distinct directions simultaneously. Such functionality can be provided by Luneburg lenses, which we aim to incorporate with the terahertz photonic crystal waveguide. A Luneburg lens requires a precisely defined gradient-index, which we realize using effective medium techniques that are implemented with micro-scale etching of silicon. Thus, the photonic crystal waveguides can be integrated directly with the Luneburg lens and fabricated together from the same silicon wafer. In this way, we develop a planar Luneburg-lens antenna with a diameter of 17 mm and seven evenly spaced ports that cover a 120° field of view. Numerical and experimental characterization confirm that the antenna functions as intended over its operation bandwidth, which spans from 320 to 390 GHz. The Luneburg-lens antenna is subsequently deployed in a demonstration of terahertz communications over a short distance. The device may therefore find applications in terahertz communications, where multiple point-to-point links can be sustained by a given transceiver node. This form of terahertz beam control may also be useful for short-range radar that monitors several directions simultaneously.

The terahertz frequency range locates between 0.1 and 10 THz. This range accommodates atmospheric windows with staggering absolute bandwidth. It holds a potential for point‐to‐point wireless communications with an aggregate capacity reaching terabit per second in a range up to a kilometer. This unique capability is envisaged for backhauls between base stations and for local area networks. To this end, efficiency and compactness of the transceivers are crucial for successful large‐scale adoption. However, state‐of‐the‐art terahertz front ends are based on radio‐frequency or photomixing technologies that are inefficient, bulky, or complicated. In principle, as a neighbor of the microwave and optics domains, the terahertz band can leverage technologies from both sides to overcome those challenges. Recently, low‐loss integrated circuits based on photonic crystal waveguides are developed for routing terahertz waves. Here, a progress report on core components, including waveguides and diplexers, is presented. Additionally, the interfacing of the platform with electronic sources and detectors on one end, and with antennas for free‐space coupling on the other end, is discussed. Currently, the platform can support terahertz communications at a data rate over 10 Gbit s−1. Challenges and opportunities are discussed in the light of future development in this area.

Polarization conversion of terahertz waves is important for applications in imaging and communications. Conventional wave plates used for polarization conversion are inherently bulky and operate at discrete wavelengths. As a substitute, we employ reflective metasurfaces composed of subwavelength resonators to obtain similar functionality but with enhanced performance. More specifically, we demonstrate low-order dielectric resonators in place of commonly used planar metallic resonators to achieve high radiation efficiencies. As a demonstration of the concept, we present firstly, a quarter-wave mirror that converts 45° incident linearly polarized waves into circularly polarized waves. Next, we present a half-wave mirror that preserves the handedness of circularly polarized waves upon reflection, and in addition, rotates linearly polarized waves by 90° upon reflection. Both metasurfaces operate with high efficiency over a measurable relative bandwidth of 49% for the quarter-wave mirror and 53% for the half-wave mirror. This broadband and high efficiency capabilities of our metasurfaces will allow to leverage maximum benefits from a vast terahertz bandwidth.

The terahertz band holds a potential for point-to-point short-range wireless communications at sub-terabit speed. To realize this potential, supporting antennas must have a wide bandwidth to sustain high data rate and must have high gain and low dissipation to compensate for the free space path loss that scales quadratically with frequency. Here we propose an all-dielectric rod antenna array with high radiation efficiency, high gain, and wide bandwidth. The proposed array is integral to a low-loss photonic crystal waveguide platform, and intrinsic silicon is the only constituent material for both the antenna and the feed to maintain the simplicity, compactness, and efficiency. Effective medium theory plays a key role in the antenna performance and integrability. An experimental validation with continuous-wave terahertz electronic systems confirms the minimum gain of 20 dBi across 315–390 GHz. A demonstration shows that a pair of such identical rod array antennas can handle bit-error-free transmission at the speed up to 10 Gbit/s. Further development of this antenna will build critical components for future terahertz communication systems.

The terahertz range possesses significant untapped potential for applications including high-volume wireless communications, noninvasive medical imaging, sensing, and safe security screening. However, due to the unique characteristics and constraints of terahertz waves, the vast majority of these applications are entirely dependent upon the availability of beam control techniques. Thus, the development of advanced terahertz-range beam control techniques yields a range of useful and unparalleled applications. This article provides an overview and tutorial on terahertz beam control. The underlying principles of wavefront engineering include array antenna theory and diffraction optics, which are drawn from the neighboring microwave and optical regimes, respectively. As both principles are applicable across the electromagnetic spectrum, they are reconciled in this overview. This provides a useful foundation for investigations into beam control in the terahertz range, which lies between microwaves and infrared light. Thereafter, noteworthy experimental demonstrations of beam control in the terahertz range are discussed, and these include geometric optics, phased array devices, leaky-wave antennas, reflectarrays, and transmitarrays. These techniques are compared and contrasted for their suitability in applications of terahertz waves.

Splitting circularly polarized waves is desirable for high‐data‐rate wireless communications and study of molecular chirality at terahertz frequencies. Typically, this functionality is achieved using bulk optical systems with limitations in material availability, bandwidth, and efficiency. As an alternative, metasurfaces with spatially varying broadband birefringence are employed to attain the same functionality. It is demonstrated that a metasurface designed with gradually rotated birefringent resonators can deflect normally incident left‐handed circularly polarized and right‐handed circularly polarized waves into different directions. This beam splitting functionality is maintained over an experimentally demonstrated relative deflection bandwidth of 53%, namely, covering the band of 0.58–1.00 THz.