# 18. Waveguide-based THz standards and metrology

Xiaobang Shang<fnref n="1" />, Jeffrey Hesler<fnref n="2" /> and Nick Ridler<fnref n="1" />

## Status

Metrology and standardisation underpin the development of THz science and applications by providing a robust framework of reliable measurement capabilities. In recent years, the field of THz metrology research has become very active, driven by the rapid development and growth of THz test and measurement instrumentation. Measurements at THz frequencies employ two disparate classes of instrumentation: free-space and waveguide-based. Free-space techniques have evolved from optical spectroscopy by expanding the operational range to longer wavelengths, whereas the waveguide-based approach is an extension of electronic RF measurements to higher frequencies. The waveguide-based instrumentation is discussed in this section, focussing on five types of measurement: waveguide, on-wafer, dielectric material characterisation, power and antennas. An example of such instrumentation is shown in figure 28.

Regarding international standardisation activities for waveguides, both IEEE and IEC have been very active [257, 258]: describe frequency bands and sizes of rectangular metallic waveguides used at frequencies to at least 5 THz. For example, table 3 lists selected standardised waveguides for use above 1 THz.

References [259, 260] describe interfaces for use with these waveguides. Whereas the frequency bands and waveguide sizes have generally been adopted by much of the end-user community, this has not been the case for the waveguide interfaces, where adoption of the new interface designs has been less successful. In practice, manufacturers tend to follow their own interface designs to achieve suitable electrical performance. This means that incompatibility of waveguide interfaces can, in principle, still be a problem. This lack of compatibility can lead to poor electrical performance (or, worse still, component damage) for systems comprising components made by different manufacturers.

The development cycle for THz on-wafer semiconductor devices is strongly dependent on the availability of accurate S-parameter measurements from which circuit models can be extracted. Although measuring equipment, such as probes, is commercially available to 1.1 THz [261], improvements in on-wafer measurement accuracy and uncertainties are required to achieve suitable confidence in the results. At present, no metrological traceability has been established for on-wafer measurements above 110 GHz.

Accurate measurement of complex permittivity of dielectric materials is of great importance for many applications. Popular VNA-based measurement techniques include

<figure><img src="image_32.png" /><figcaption>**Figure 28.** An example THz waveguide-based system comprising a dual source vector network analyser (VNA), along with two frequency extender heads shown in the foreground.</figcaption></figure>

free-space methods, open-resonators, and waveguide/planar transmission lines. The VNA-based free-space method represents a mainstream approach over a broad frequency range to 1.1 THz.

Reliable power measurements are also required at these frequencies. Currently, most THz waveguide power measurements use a WR-10 (75–110 GHz) waveguide-based dry calorimeter that uses a DC substitution method to determine THz power [262]. The frequency range is extended using waveguide tapers to other waveguide bands up to WM-164 (1.1–1.7 THz).

There is currently a dramatic growth of interest in THz communications, e.g. for 6G [263]. Since many of these devices have integrated antennas, the need for calibrated THz antenna testing is growing. In addition, regulatory requirements on emissions means over-the-air (OTA) testing is also required. Emissions testing to 600 GHz has already begun, and higher frequency requirements are expected imminently. The test and measurement equipment for these OTA tests is based on waveguide-based technology. There is also much interest in the generation and detection of broadband signals. To address this need, THz extenders relying on heterodyne mixing have been developed that translate signals to and from THz with relatively low distortion.

One key aspect driving the need for THz standards and metrology is the development of active devices such as amplifiers and oscillators, which now exist up to 1 THz [264]. In order to develop these devices, and related systems, the requirement for additional testing increases to include large-signal measurements of compression, distortion, nonlinearity, load-pull and more. End-users need calibrated and traceable measurements of these parameters, and so considerable development in THz test and measurement is now underway. This includes systems with increased test port power and dynamic range. Measurements of active devices require improved harmonic performance and the ability to control the test port power.

<footnote n="1">¹Department of Electromagnetic and Electrochemical Technologies, National Physical Laboratory, Teddington TW11 0LW, United Kingdom</footnote>

<footnote n="2">²Virginia Diodes, Inc., Charlottesville, VA 22902-6172, United States of America</footnote>
<table><caption>**Table 3.** Recommended rectangular waveguide dimensions.</caption><thead><tr><th>IEEE Name</th><th>Width (μm)</th><th>Height (μm)</th><th>Cut-off frequency (THz)</th><th>Suggested minimum frequency (THz)</th><th>Suggested maximum frequency (THz)</th></tr></thead><tbody><tr><td>WM-200</td><td>200</td><td>100</td><td>0.749</td><td>0.9</td><td>1.4</td></tr><tr><td>WM-130</td><td>130</td><td>65</td><td>1.153</td><td>1.4</td><td>2.2</td></tr><tr><td>WM-86</td><td>86</td><td>43</td><td>1.743</td><td>2.2</td><td>3.3</td></tr><tr><td>WM-57</td><td>57</td><td>28.5</td><td>2.629</td><td>3.3</td><td>5.0</td></tr></tbody></table>

## Current and future challenges

Regarding standardised waveguide interfaces, the current challenge is to develop a design which is supported by all the main manufacturers of these waveguides and that gives the required electrical performance at a cost that is acceptable to the end-user community. This has led to IEEE initiating a new standard making activity [265], in December 2021, to develop a waveguide interface for THz frequencies. This IEEE Working Group will also develop an interface for dielectric waveguides used at these frequencies. This should facilitate an increase in use of dielectric waveguides, especially for applications where metallic waveguides are less suitable.

Documentary standards for on-wafer measurements are yet to be developed. Considerable efforts have been devoted to studying factors impacting these measurements, including choice of calibration methods, the design of standards and device interfaces, characterisation of interconnections, etc. A second IEEE Working Group [266] was launched in 2021 to develop a standard for these measurements up to THz frequencies.

For material characterisation, the main challenge for transmission line methods is to provide specimens that fit inside these small waveguides, without introducing air gaps in the waveguide. Some challenges associated with free-space methods are inadequate or oversimplified modelling of beams (e.g. use of plane-wave assumptions) and incorrect positioning of specimens (i.e. the specimen is not situated where the theory assumes it to be).

A significant challenge with THz metrology has been the lack of traceable power standards above 110 GHz. Work is currently underway to extend this to 170 GHz and even higher frequencies [267]. The two basic approaches are: (a) transfer a calibrated optical standard to waveguide; (b) use of dry waveguide calorimeters [268]. The optical approach provides traceability, but there are challenges in linking the optical measurement into waveguide. Regarding the calorimeter approach, scaling a calorimeter for THz frequencies is challenging because of the reduction in size of the waveguide. One alternative approach is to use a WR-10 calorimeter with a series of characterised tapers. One benefit of this approach is the ability to use a sensor head which has good absorption and return loss to 1.5 THz.

Regarding OTA emissions testing, one challenge is the measurement of very small signal levels. The traditional use of high harmonic mixers with relatively low sensitivity is not

sufficient at THz, and so more sensitive receivers are required. Modern receivers can achieve noise levels in the range of -150 dBm Hz⁻¹ to -165 dBm Hz⁻¹, which is key to verification that a device meets the emissions requirements.

A greater challenge relates to the testing of THz communications or automotive radar devices that have built-in antennas. Desired measurements range from antenna testing to radiated power, to nonlinear distortion levels of wideband modulated signals. Accurate and traceable measurements of these parameters are required. Research into these measurements is just beginning, and many challenges remain.

## Advances in science and technology to meet challenges

To address the metrological challenges relating to probe contact in on-wafer measurement, a non-contact probing method has recently been introduced [269]. Such techniques eliminate the need for making physical contact between probe and wafer, resulting in excellent measurement repeatability.

Recently, a guided free-space technique has been introduced for material characterisation up to 750 GHz [270]. This technique requires no focusing elements (mirrors or lenses) that are usually employed to reduce the beam waist radius of a conventional free-space system, resulting in a compact and self-contained system. This new method also simplifies alignment issues, as the samples are clamped between the two ports of the test fixture. Current efforts are focused on establishing measurement traceability for these systems.

Establishing traceability for power measurements to THz frequencies is still in the early stages of development, and so many challenges remain. This includes establishing an infrastructure to allow transfer standards to be calibrated within an international metrological traceability framework. Another area where there has been significant development relates to very broadband VNAs which can now measure in a single sweep and with a single test unit from DC to beyond 200 GHz [271].

## Concluding remarks

The discussions in this section do not represent all advances in measurement science and technology but have provided some examples of advancements for waveguide-based systems.
Recent work has been very successful in establishing traceability and assurance for fundamental quantities such as S-parameters in waveguide. Attention is now turning to establishing this traceability for measurements on-wafer and for

derived measurement quantities such as power, permittivity and antenna characterisations. This will, in turn, enable researchers and manufacturers to be able to fully test and verify the performance of new products and designs.
# 19. Terahertz communication

Cyril C Renaud<fnref n="1" />, Ingmar Kallfass<fnref n="2" /> and Tadao Nagatsuma<fnref n="3" />

## Status

In a world where the transmission of data is seeing an exponential growth, telecommunication technology needs to grow accordingly. While this has happened through the development of optical fibre technologies, the bottleneck now shifts to the wireless domain. Historically, increasing the carrier frequency has been the key solution to meet this challenge and over the last 20 years the THz region has been of particular interest, and is seen as a key enabler for systems beyond 5/6G [272]. This band offers a large amount of bandwidth across several relatively low attenuation windows and could serve several applications within the network (figure 29). It also enables less stringent requirements in terms of alignment compared to free space optical systems leading to less system complexity. Advances in THz wireless communication are being driven by technological progress in photonics and electronics, both capable of realising transmitters and receivers at THz carrier frequencies. Some examples are the development of transistor technology allowing for amplification up to 1 THz, and uni-travelling carrier photodiodes, resonant tuningelling diodes and Schottky barrier-based detectors operating between roughly 0.1–10 THz. All these advances have enabled transmissions above 100 Gbit s⁻<fnref n="1" /> per carrier with distances going close to 1 km [273]. While all these results are impressive, the technology still operates at the limit of the possible system performances in terms of SNR and source phase noise. Although the drive to push the efficiency of devices for signal generation, modulation and amplification in both THz emitters and receivers is still clearly visible and viable, it is believed that the key to the future of THz communication technology will be the integration of individual functional building blocks. Based on photonics and electronics, integration will enable economy of scale and power efficiency in bit-transparent transceivers answering the market needs. Within this roadmap in mind we will explore the challenges corresponding to both individual devices and integration and the possible solutions investigated by the scientific community.

## Current and future challenges

While so far the advances of THz communication were driven by device development, it is now clear that we have reached a point where the market requirements and diverse applications need to drive the choice of transceiver architectures and underlying technologies. From the system point of view and

<figure><img src="image_33.png" /><figcaption>**Figure 29.** (a) Technological solution within the spectrum, (b) Basic Rx/Tx components, (c) THz within a network.</figcaption></figure>

in the different application scenarios (figure 29) we still need to improve the efficiency of all devices. In particular, there is a need to work on minimising both amplitude and phase noise. We will also need to push the operational frequency range of devices further, as the only way to reach 1 Tbit s⁻<fnref n="1" /> and beyond will be by unlocking even higher frequency windows. Another aspect is the need to address nomadic or even mobile applications (figure 29) where beam shaping and steering will be required beyond the small self-alignment adjustment of fixed wireless links. This is typically based on antenna array technology and recently some demonstrations have been made using leaky-wave antennas and integrated arrays [273, 274]. THz signal generation has been typically based on frequency multiplication which naturally increases phase noise. While transistor technology has been pushing cut-off frequencies to improve oscillator phase noise, combination with technologies stemming from optical clocks promise significant improvements [275]. All these demonstrate that the system will require individual device development and a push from different fields of engineering and physics. However, notwithstanding all these key developments, the issue of losses at the interconnection of hybrid integrated transmitters and receivers (figure 29) arises and impedes the overall system performance and throughput. This clearly indicates that the development of high performance yet low-cost integration technologies for both electronic and photonic platforms is one of the key future challenges in THz communications.

<footnote n="1">¹Department of Electronic and Electrical Engineering, University College London, London WC1E 7JE, United Kingdom</footnote>

<footnote n="2">²Institute of Robust Power Semiconductor Systems, University of Stuttgart, 70569 Stuttgart, Germany</footnote>

<footnote n="3">³Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan</footnote>
Advances in science and technology to meet
challenges

Advances are mostly required in the key components which
determine the overall system performance: Low noise figure
and highly linear output power of THz amplifiers, efficient
beam steering, low Vπ optical modulators to re-upconvert
THz signal onto an optical carrier. Active electronic trans-
mit and receive frontends with RF amplification based on
III-V compound semiconductor technologies perform well
up to 850 GHz [276], while Si-CMOS and SiGe-BiCMOS
based active frontends with their invaluable system-on-chip
potential enter the submillimeter-wave range beyond 300 GHz
[277]. For the optical modulator, the key is to push the 3 dB
bandwidth beyond 100 GHz while having a Vπ lower than
2 V to enable THz detection and remodulation on an optical
beam with minimal electrical amplification. Here, works on
plasmonics-enabled modulators are encouraging [278], while
the introduction of organic polymers with high electro-optic
coefficients could also prove interesting.

The antenna technology has multiple aspects, but the key is
to efficiently couple the signal of the THz oscillator/generator
to the emitting antenna. Several developments offer interest-
ing routes towards more efficiency and improved connectiv-
ity such as the use of photonic bandgap-inspired structures
or development in metamaterials combined with intelligent
reflecting surfaces [279].

Finally, we need to discuss the underlying key development
that is integration technologies. While there has been some
success in integrating both electronic and photonic systems at
frequencies around 100 GHz there is still a clear need to push
the technology beyond. In respect to photonic integration the

optical losses need to be managed in a scenario where cascades
of multiple photonic elements are placed in 2-dimensional
arrays. While the most efficient photonic components at THz
frequencies use III–V semiconductor technology, this is not a
low optical loss material and seamless integration with lower
optical loss platforms such as SOI is essential [280]. Further-
more, the manipulation of the THz signal on chip mandates a
strong integrated waveguide and coupling technology. In that
case several potential routes have shown promises, such as
low-loss dielectrics [281] and MEMS-based metallic hollow
waveguides [282]. Further, we see development in metamater-
ials, plasmonic design and 2D materials as worthwhile explor-
ation routes.

Concluding remarks

It is clear that THz communication is poised to make the
step from laboratory-grade to real-world application scen-
arios, and soon to become driven by the market and the
system requirements to replace the device-technology driven
phase. To life up to this expectation several key technolo-
gical challenges are highlighted such as improved device
efficiency, beam steering capacity and integration at both
transmitter and receiver. We now experience a positive
gradient of advances from both electronics and photon-
ics that promise to access the different transmission bands,
and the next step will be to combine these electronic and
photonic solutions with system-enabled functionality into full-
scale transceivers and to address the key requirements for
each system application of wireless networks beyond 6 G,
reaching 1 Tbit s⁻¹.
## 20. THz technology in industry

J Axel Zeitler¹ and Don Arnone²

¹Department of Chemical Engineering and Biotechnology, University of Cambridge, Cambridge CB3 0AS, United Kingdom

²Teraview, 1, Enterprise, Cambridge Research Park, Cambridge CB25 9PD, United Kingdom

### Status

With the emergence of turn-key terahertz sources and detectors, industrial applications were explored in the first decade of the 21st century. Initially, the most promising areas included medical imaging of skin and breast cancer, security applications such as stand-off detection of weapons and explosives, non-destructive imaging for quality control and spectroscopy and imaging in the pharmaceutical industry for the identification of crystalline polymorphs and film coatings on tablets [283]. The key characteristics of terahertz radiation that formed the basis for the developments were the ability to penetrate and resolve the internal structure of a wide range of optically opaque materials, including semiconductors, polymers, and ceramics, at a sub-millimetre spatial resolution as well as the ability to tune into vibrational and dielectric spectroscopic signatures that govern the interaction of organic molecular materials, particularly in their crystalline state. In the following decade, progress was made to establish commercial viability and advance the technological maturity of the early applications. The initial excitement around security applications waned owing to the progress in the development of more millimetre wave scanners that provide sufficient spatial resolution imaging over large areas in a matter of seconds. The lack of a distinct spectral response of nitrogen-based explosives in the range of 0.3–4 THz was also a limitation. An overview of current and emerging applications of terahertz technology is presented in figure 30. The medical imaging applications, in particular for cancer detection, dermatology, and assisted surgery, show significant promise but are likely to require considerable research input and clinical testing and trials before they are ready for clinical application in routine use [284]. One area where terahertz technology has reached commercial maturity is the semiconductor industry for failure analysis of advanced integrated circuits where pulses of terahertz radiation are used for interconnect failure identification and isolation [285]. The pulsed nature of the time-domain methods is also exploited successfully for the analysis of high-value coating structures. Initially, development focused on polymer film coatings of pharmaceutical tablets. However, the technique has recently found growing success in the analysis of automotive paints, aerospace and thermal barrier coatings as well as other speciality coatings such as low-density foam structures [286]. In lower-value coating products, terahertz technology is successfully being used for gauge measurements on coated wires as well as in paper mills [287, 288]. In the pharmaceutical field, non-destructive measurement of tablet porosity by determining its effective refractive index

<figure><img src="image_34.png" /><figcaption>**Figure 30.** Existing and emerging applications.</figcaption></figure>

is an emerging application [289]. Given the inherent mechanistic link between porosity and the disintegration of the tablet when water enters the tablets’ pores this method has the potential to replace the time and cost-intensive dissolution testing procedure required in medicine manufacture. Another emerging application is the quantitative mapping of the electrical conductivity of large areas of graphene and other substrates [290].

### Current and future challenges

An increasing diversification of commercially available terahertz systems that are provided by different suppliers is supporting the growing market for terahertz technology. A critical aspect is to provide not only the means of generating and detecting terahertz radiation, but also the methods and algorithms for analysing measured terahertz waveforms and extracting information relevant to the customer. For example, in automotive paints, the wavelength of terahertz light is frequently less than the thickness of the layers to be measured, necessitating accurate modelling of the multilayer system, and fitting this model to measured data. In addition, the reproducibility and robustness, of measurements is becoming increasingly important. With this there is a need for appropriate, easily available, and traceable metrology standards as well as International Organization for Standardization (ISO) certification to support the industrial implementation of the existing and emerging applications in the field.

To increase the number of applications and overall size of the terahertz market, decreases in measurement time and concomitant increases in measurement throughput of terahertz systems would further enable the shift from laboratory and at-line deployments to in-line measurements in production environments as well as allowing for higher fidelity to support quantitative analytical methods.

A further reduction in the cost of instruments, their packaging in smaller form factor devices and enhanced ruggedisation to withstand harsh operating environments in terms of
<figure><img src="image_35.png" /><figcaption>**Figure 31.** Applications and key performance criteria.</figcaption></figure>

environmental conditions, dust and vibrations would further
benefit industrial sensing requirements.

Advances in science and technology to meet
challenges

Terahertz technology is currently targeted mainly at high value
industrial applications where conventional technologies using
other parts of the spectrum are not effective. By their nature,
these high value applications demand high performance stand-
ards, e.g. measurements based on achieving 3–6 sigma per-
formance in terms of accuracy and reproducibility. Recent
advances in semiconductor and laser technology to produce
more stable and reproducible sources and detectors have thus
benefited the acceptance of terahertz applications. With the
advent of 6G wireless networks which will require highly
accurate characterization as well as test and measurement tools
in the terahertz range, these requirements are set to grow [263].
Similarly, efforts to improve the signal-to-noise ratio of tera-
hertz sensors to enable single shot measurements, as well as
efforts to develop arrays of sensors for parallel processing, will
increase the speed and throughput for industrial applications
(figure 31) [291].

Frequency modulation techniques applied to continuous
wave (cw) solutions are important steps towards broadening

the market potential for terahertz application. The ability to
produce lower cost and more compact systems based on coher-
ent cw laser diode technology opens a number of new indus-
trial applications in imaging and gauge thickness applications
amongst others.

Concluding remarks

Over the past two decades, through a combination of intense
research efforts as well as trial-and-error efforts to identify
commercially-viable applications, terahertz markets have
been identified and commercialisation has commenced. Whilst
this has taken time, historically it represents an improve-
ment on the timescales needed to commercialise other mod-
alities such as ultrasound and infrared technologies. There
remain both technology challenges and potential solutions
expanding existing applications and identifying new ones.
The applications and technology requirements cited above
are focussed on the use of terahertz methods as an inspec-
tion and quality control technique. To move terahertz tech-
nology from inspection into everyday consumer applications
will require considerable development effort to miniatur-
ize, ruggedize and cost-reduce the terahertz sensors whilst
maintaining many of the key performance requirements cited
above.
# Acknowledgments

## 2. THz time-domain quantum optics

Alfred Leitenstorfer, Andrey S Moskalenko

Funded by the Deutsche Forschungsgemeinschaft (DFG)—Project No. 425217212—SFB 1432. A S M is also supported by the National Research Foundation of Korea (NRF) Grant funded by the Korea government (MSIT) (2020R1A2C1008500).

## 3. Terahertz spintronics and magnetism

Tobias Kampfrath, Junichiro Kono

T K acknowledges funding by the DFG Collaborative Research Center TRR 227 ‘Ultrafast spin dynamics’ (project ID 328545488, projects B02, A05 and B05), the DFG priority program SPP2314 INTEREST (project ITISA), and the ERC H2020 through Projects CoG TERAMAG/Grant No. 681917. T K also acknowledges financial support from the Horizon 2020 Framework Program of the European Commission under FET-Open Grant No. 863155 (s-Nebula). J K acknowledges support from the U.S. Army Research Office (Grant No. W911NF-17-1-0259) and the National Science Foundation through the Center for Dynamics and Control of Materials: an NSF MRSEC under Cooperative Agreement No. DMR-1720595.

## 4. Terahertz time-domain polarimetry and ellipsometry

Kun Peng, Naser Qureshi, Enrique Castro-Camus

E C C would like to acknowledge the Alexander von Humboldt Foundation for financial support through an Experienced Researcher Fellowship and the Philipps-Universität Marburg through a Guest Professorship. N Q acknowledges support from PAPIIT, Universidad Nacional Autónoma de México IG100521.

## 5. Nonlinear terahertz spectroscopy

Dmitry Turchinovich, Koichiro Tanaka

D T acknowledges the financial support from European Union’s Horizon 2020 research and innovation programme (Grant Agreement No. 964735 EXTREME-IR), and Deutsche Forschungsgemeinschaft (DFG) within the Project 468 501 411–SPP2314 INTEGRATECH under the framework of the priority programme SPP2314—INTEREST.

## 6. THz Biological Effects and Sensing

A G Markelz, Martina Havenith, Cameron Hough

A G M acknowledges funding by National Science Foundation (DBI 1556359 and MCB 1616529); U.S. Department of Energy (DE-SC0016317). M H acknowledges funding by the ERC Advanced Grant 695437 ‘“THz Calorimetry”’, the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany’s Excellence Strategy–EXC

2033–390677874–RESOLV, and the MERCUR Project Pe-2016-0048. We acknowledge Prof. Matthias Heyden and Prof. Joerg Tatzelt for providing pictures.

## 7. Terahertz spectroscopy of emerging materials

Hannah J Joyce, Willie J Padilla

H J J gratefully acknowledges funding from ERC Starting Grant ACrossWire (Grant No. 716471). W J P acknowledges support from the US Department of Energy (DOE) (DESC0014372).

## 8. THz air photonics

Binbin Zhou, Ki-Yong Kim, Xi-Cheng Zhang, Peter Uhd Jepsen

This research of University of Rochester is supported by the Army Research Office [Grant No. W911NF-17-1-0428], Air Force Office of Scientific Research [Grant No. FA9550-18-1-0357], and National Science Foundation [Grant No. ECCS-1916068]. The research at DTU is supported by Innovation Fund Denmark [Project No. 8057-00010B, TRIM], Danish National Research Foundation [Project No. DNRF103, CNG], Velux Foundation [Project No. 00023215, THZ-HANDSHAKE], and Independent Research Fund Denmark (Project No. Danish National Research Foundation [Project No. 9040-00360B, ULTRA-TED].

## 9. Recent progress in terahertz intersubband devices

S S Dhillon, M S Vitiello, E H Linfield, A G Davies

S S D acknowledges funding from European Union’s Horizon 2020 research and innovation programme under Grant Agreement No 964735 (FET-OPEN EXTREME-IR). M S V acknowledges financial support from the European Research Council (681379, SPRINT). E H L and A G D acknowledge financial support from the EPSRC (‘HyperTerahertz’, EP/P021859/1).

## 10. Laser-based terahertz sources

Matthias C Hoffmann, Roger Lewis

M C H is supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award No. 2018-SLAC-100499-Funding.

## 11. Terahertz Emission Spectroscopy and Imaging

Masayoshi Tonouchi, Tom S Seifert, and Pernille Klarskov

T S S acknowledges funding by the DFG Collaborative Research Center TRR 227 ‘Ultrafast spin dynamics’ (Project ID 328545488, Project A05). MT acknowledges funding by JSPS KAKENHI, Grant Number 18KK0140. P K acknowledges funding from the Innovation Fund Denmark (Project No. 0177–00035B, RePlast) and the Independent Research Fund Denmark (Project No. 1032–00309B, OpSky).
## 12. Terahertz scanning tunnelling microscopy

Yaroslav A Gerasimenko, Dragan Mihailovic, Rupert Huber

Financial support by the Deutsche Forschungsgemeinschaft through Grant Numbers HU1598/8 and HU1598/9 is gratefully acknowledged.

## 13. Near field THz imaging

Jessica Boland, Oleg Mitrofanov, Pernille Klarskov, Paul Dean

P D and O M acknowledge support from the EPSRC Programme Grant ‘HyperTerahertz’ (EP/P021859/1). J L B acknowledges support from EPSRC via EP/S037438/1 and the NAME Programme Grant (EP/V001914/1) and UKRI via MR/T022140/1. She also thanks the Leverhulme Trust for funding via Philip Leverhulme Prize. The authors are grateful to Tom Siday, who provided graphics for figure 19.

## 14. Terahertz Instrumentation for Satellites

Brian N Ellison, Peter G Huggard, Simon P Rea

The authors acknowledge support for THz technology development from the STFC Centre for Instrumentation and the UKSA Centre for Earth Observation Instrumentation.

## 15. Status of THz astronomy

C K Walker, D Leisawitz, and J R Gao

We stand on the shoulders of giants and thank all those who contributed to the previously flown FIR/THz missions and the detector technologies cited here, as well as those who contributed to the recently concluded mission concept studies, such as Origins Space Telescope (OST) and Orbiting Astronomical Satellite for Investigating Stellar Systems (OASIS).

## 16. Multipixel THz imaging

Chong Li, Gintaras Valušis, Qin Chen

We highly appreciate Ignas Grigelionis and Alvydas Lisauskas for useful discussions and Domas Jokubauskis for technical assistance.

## 17. Medical Applications

Vincent P. Wallace and Emma Pickwell-MacPherson

V P W is supported by the Australian Research Council via a Future Fellowship award (Project Number FT180100683) funded by the Australian Government. E P M is with the University of Warwick and is supported by a Royal Society Wolfson Award, Cancer Research UK, and EPSRC funding (Projects EP/S021442/1 and EP/V047914/1).

## 19. Terahertz Communication

Cyril C Renaud, Ingmar Kallfass, Tadao Nagatsuma

Professor Cyril Renaud acknowledges funding from the UKRI (EP/R042578, P021859, P003990/1) and the EU (761579, 956857), and Professor Tadao Nagatsuma acknowledges funding from MIC (SCOPE 195010002) and NICT

(Beyond 5G Promotion Project 00901), and Professor Kallfass acknowledges funding from DFG (KA 3062/15-1), BMBF (16KISK019) and EU (814523).

## Data availability statement

No new data were created or analysed in this study.

## ORCID iDs

Andrey S Moskalenko https://orcid.org/0000-0002-0028-4459

Tobias Kampfrath https://orcid.org/0000-0001-8854-8713

Enrique Castro-Camus https://orcid.org/0000-0002-8218-9155

Kun Peng https://orcid.org/0000-0002-2111-3341

Naser Qureshi https://orcid.org/0000-0002-5333-9126

Dmitry Turchinovich https://orcid.org/0000-0003-0054-7092

Cameron Hough https://orcid.org/0000-0001-8008-7561

Hannah J Joyce https://orcid.org/0000-0002-9737-680X

Peter Uhd Jepsen https://orcid.org/0000-0003-3915-1167

Miriam Vitiello https://orcid.org/0000-0002-4914-0421

Edmund Linfield https://orcid.org/0000-0001-6912-0535

A Giles Davies https://orcid.org/0000-0002-1987-4846

Roger Lewis https://orcid.org/0000-0002-4598-7553

Masayoshi Tonouchi https://orcid.org/0000-0002-9284-3501

Tom S Seifert https://orcid.org/0000-0001-8163-482X

Yaroslav A Gerasimenko https://orcid.org/0000-0002-5337-284X

Jessica L Boland https://orcid.org/0000-0002-6351-5699

Peter G Huggard https://orcid.org/0000-0002-6613-5683

David T Leisawitz https://orcid.org/0000-0003-1000-2547

Jian Rong Gao https://orcid.org/0000-0001-7306-6607

Vincent P Wallace https://orcid.org/0000-0003-3814-5400

Emma Pickwell-MacPherson https://orcid.org/0000-0001-6062-5959

Xiaobang Shang https://orcid.org/0000-0002-0298-4184

J Axel Zeitler https://orcid.org/0000-0002-4958-0582

Michael B Johnston https://orcid.org/0000-0002-0301-8033

John Cunningham https://orcid.org/0000-0002-1805-9743

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