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Funder: Alexander von Humboldt Foundation, Funder: Experienced Researcher Fellowship, Funder: DFG Collaborative, Funder: PAPIIT, Funder: Danish National Research Foundation, Funder: JSPS, Funder: STFC Centre for Instrumentation, Funder: UKSA Centre for Earth, Funder: Leverhulme Trust; doi: http://dx.doi.org/10.13039/501100000275, Funder: NRF, Funder: Royal Society; doi: http://dx.doi.org/10.13039/501100000288, Funder: Australian Government; doi: http://dx.doi.org/10.13039/100015539, Funder: Cancer Research UK; doi: http://dx.doi.org/10.13039/501100000289, Funder: University of Warwick; doi: http://dx.doi.org/10.13039/501100000741, Terahertz (THz) radiation encompasses a wide spectral range within the electromagnetic spectrum that extends from microwaves to the far infrared (100 GHz–∼30 THz). Within its frequency boundaries exist a broad variety of scientific disciplines that have presented, and continue to present, technical challenges to researchers. During the past 50 years, for instance, the demands of the scientific community have substantially evolved and with a need for advanced instrumentation to support radio astronomy, Earth observation, weather forecasting, security imaging, telecommunications, non-destructive device testing and much more. Furthermore, applications have required an emergence of technology from the laboratory environment to production-scale supply and in-the-field deployments ranging from harsh ground-based locations to deep space. In addressing these requirements, the research and development community has advanced related technology and bridged the transition between electronics and photonics that high frequency operation demands. The multidisciplinary nature of THz work was our stimulus for creating the 2017 THz Science and Technology Roadmap (Dhillon et al 2017 J. Phys. D: Appl. Phys. 50 043001). As one might envisage, though, there remains much to explore both scientifically and technically and the field has continued to develop and expand rapidly. It is timely, therefore, to revise our previous roadmap and in this 2023 version we both provide an update on key developments in established technical areas that have important scientific and public benefit, and highlight new and emerging areas that show particular promise. The developments that we describe thus span from fundamental scientific research, such as THz astronomy and the emergent area of THz quantum optics, to highly applied and commercially and societally impactful subjects that include 6G THz communications, medical imaging, and climate monitoring and prediction. Our Roadmap vision draws upon the expertise and perspective of multiple international specialists that together provide an overview of past developments and the likely challenges facing the field of THz science and technology in future decades. The document is written in a form that is accessible to policy makers who wish to gain an overview of the current state of the THz art, and for the non-specialist and curious who wish to understand available technology and challenges. A such, our experts deliver a ‘snapshot’ introduction to the current status of the field and provide suggestions for exciting future technical development directions. Ultimately, we intend the Roadmap to portray the advantages and benefits of the THz domain and to stimulate further exploration of the field in support of scientific research and commercial realisation.

Realizing packaged state-of-the-art performance of monolithic microwave integrated circuits (MMICs) operating at millimeter wavelengths presents significant challenges in terms of electrical interface circuitry and physical construction. For instance, even with the aid of modern electromagnetic simulation tools, modeling the interaction between the MMIC and its package embedding circuit can lack the necessary precision to achieve optimum device performance. Physical implementation also introduces inaccuracies and requires iterative interface component substitution that can produce variable results, is invasive and risks damaging the MMIC. This paper describes a novel method for in situ optimization of packaged millimeter-wave devices using a pulsed ultraviolet laser to remove pre-selected areas of interface circuit metallization. The method was successfully demonstrated through the optimization of a 183 GHz low noise amplifier destined for use on the MetOp-SG meteorological satellite series. An improvement in amplifier output return loss from an average of 12.9 dB to 22.7 dB was achieved across an operational frequency range of 175–191 GHz and the improved circuit reproduced. We believe that our in situ tuning technique can be applied more widely to planar millimeter-wave interface circuits that are critical in achieving optimum device performance.

The work done by B. M. Courtier, A. Battaglia, and C. D. Westbrook was funded by the UK NERC project GRACES (G-band RAdar for Cloud and prEcipitation Studies, Grant No. RP16G1219). The instrument development work by D. A. Robertson, R. J. Wylde, H. Wang, G. Howells, B. N. Ellison, and P. G. Huggard was funded by the UK-CEOI project GRaCE. The work by K. Mroz was performed at the University of Leicester under Grant No. RP1890005 with the National Centre for Earth Observation. C. D. Westbrook's contribution was also supported by a Research Endowment Trust Fund award from the University of Reading. Operation of the Chilbolton Ka-band and W-band radars was funded as part of NERC National Capability. We would like to thank Karina McCusker and Emal Rumi for coordinating the collection of 35 and 94 GHz radar data as part of the NERC PICASSO project, Grant No. NE/P012426/1. The first Doppler spectra ever acquired by an atmospheric radar at 200 GHz (G-band) are presented. The observations were taken during a light precipitation event in May (rain rates

We have developed precision micromachining techniques for integration of terahertz quantum cascade lasers (QCLs) with waveguides and feedhorns for use as local oscillators in satellite-borne receivers. We demonstrate these techniques using QCLs at 3.4 THz and 4.7 THz, as well as the first QCL with a monolithically integrated power modulator.

The Heterodyne Receiver for Origins (HERO) is the first detailed study of a heterodyne focal plane array receiver for space applications. HERO gives the Origins Space Telescope the capability to observe at very high spectral resolution (R = 107) over an unprecedentedly large far-infrared (FIR) wavelengths range (111 to 617 μm) with high sensitivity, with simultaneous dual polarization and dual-frequency band operation. The design is based on prior successful heterodyne receivers, such as Heterodyne Instrument for the Far-Infrared /Herschel, but surpasses it by one to two orders of magnitude by exploiting the latest technological developments. Innovative components are used to keep the required satellite resources low and thus allowing for the first time a convincing design of a large format heterodyne array receiver for space. HERO on Origins is a unique tool to explore the FIR universe and extends the enormous potential of submillimeter astronomical spectroscopy into new areas of astronomical research.

A 345 GHz room-temperature single-pixel heterodyne receiver using sub-harmonic Schottky barrier diode mixers has been installed at the Large Millimeter Telescope (LMT) on the Sierra Negra in Mexico. The receiver was developed at the Science and Technology Facilities Council (STFC) Rutherford Appleton Laboratory (RAL) in the UK in 2013 to perform ground-based atmospheric studies between 312 GHz and 360 GHz. With support from the STFC Global Challenge Research Fund (GCRF) project “Astronomical System Training, Engineering and Collaboration (ASTEC)” the instrument has been reconfigured to support astronomical research and installed on the 50-meter LMT to be used as a pathfinder for sub-millimeter wavelength observations. This new receiver, CHARM (Collaborative Heterodyne Astronomical Receiver for Mexico), has exchanged an originally implemented single-sideband mixer design for a double-sideband device. In addition, a broader bandwidth intermediate frequency (IF) chain, additional digital sampling spectrometers and appropriate interface quasi-optics have been installed. The modifications have resulted in a turnkey receiver system with a double sideband (DSB) receiver noise temperature (Trec) of ~1200 K as measured in the laboratory. The inclusion of a wider IF and a total of four digital spectrometers the instrument encompasses a 12 GHz IF bandwidth with 1.46 MHz resolution. Use of Schottky mixers allows room temperature operation and whilst both of these attributes sacrifice noise performance, and thus detection sensitivity, when compared with cryogenic superconducting systems, they allow a relatively simple system architecture to be implemented that has objective of establishing the LMT sub-millimeter wave performance potential and related quality local atmospheric ‘seeing’ conditions. The detailed design of the instrument, description of the optical system for the LMT adaptation and first-light results are presented.

A room temperature sub-millimetre-wave Schottky barrier diode heterodyne receiver system (the Collaborative Heterodyne Astronomical Receiver for Mexico aka CHARM) operating at ~360GHz, has been deployed at the Large Millimetre Telescope, Mexico. The receiver is fully integrated as a turn-key system, and uses a double-sideband subharmonic mixer with an average noise equivalent system temperature of 1,000K. The system back-end signal processing incorporates four wide-band digital spectrometers, with a combined instantaneous IF bandwidth of 10.4GHz for both upper and lower side-bands, and a channel spectral resolution of 1.2MHz. The system has been used to observe first light with the LMT in the sub-millimetre-wave range. It will be further used to perform estimates of the telescope system performance in support of future sub-millimetre-wave observations from the LMT.

We demonstrate a terahertz gas spectroscopy technique based on self-mixing in a multimode quantum cascade laser (QCL). The spectral parameters of the H/D exchange reaction between water (H 2 O) and deuterated water (D 2 O) have been measured between 3.36-3.37 THz.

We present a self-mixing terahertz-frequency gas spectroscopy technique using a multimode quantum cascade laser. A precision-micro-machined external waveguide module and a double-metal quantum cascade laser device are used to increase the optical feedback and the laser’s frequency tuning range. Methanol spectra are obtained using two laser modes at 3.362 THz and 3.428 THz simultaneously, with more than 8 absorption peaks resolved over a 17-GHz bandwidth, which provide minimum detectable absorption coefficients of 2.7×10-4 cm−1 and 4.9×10−4 cm−1, respectively. In contrast to all previous self-mixing spectroscopy, our multimode technique expands the sensing bandwidth significantly. This broadband spectroscopy technique can potentially be used for the identification and analysis of chemical, biological radiological and nuclear (CBRN) agents and explosives.

An airborne heterodyne radiometer has been developed at the Rutherford Appleton Laboratory in order to observe oxygen spectral signatures around 60 GHz at an altitude above 40 km with high spectral resolution. It comprises low noise millimeter-wave amplifiers and a diplexer to split the channel into the lower side band (50.3 GHz - 57.3 GHz) and the upper side band (63.3 GHz - 67.9 GHz). A Schottky mixer in each band dowconverts the incoming signal into the intermediate frequencies (IF) and is then processed using a novel wideband fast Fourier transform digital spectrometer, which has an instantaneous bandwidth of 8 GHz and spectral resolution of 3 MHz. For this radiometer, we have measured a double sideband system noise temperature of less than 200 K and a noise equivalent differential temperature of 0.3 K for a spectral bandwidth of 10 MHz and an integration time of 300 milliseconds. The sideband rejection measured between the upper and lower sidebands is better than -45 dB.

we present an electromagnetic-field analysis of a terahertz-frequency quantum-cascade laser (THz QCL) integrated with a mechanically micro-machined waveguide cavity and diagonal feedhorn. A hybrid finite-element/Fourier transform approach enables analysis of both the near-field and far-field regions and is shown to agree well with experimental observations. The far-field antenna patterns show enhancement of the beam profile when compared with an unmounted QCL, in terms of beam divergence and side-lobe suppression ratio. Furthermore, we demonstrate integration of the QCL with dual diagonal feedhorns, enabling simultaneous access to both facets of the QCL, underpinning future integration with a satellite-based receiver and frequency-stabilization subsystem.

Rutherford Appleton Laboratory Space Department (RAL Space) and Radiometer Physics GmbH are responsible for the provision of millimetre-wave front-end receivers operating from 165 GHz to 664 GHz for three instruments on board the MetOp Second Generation (MetOp-SG) satellites. Qualification of the Engineering Qualification Models (EQMs) is recently completed. This paper presents the results of the EQM qualification for receivers operating at 165 GHz, 183 GHz and 229 GHz for the MWS and MWI instruments.

Terahertz-frequency quantum-cascade lasers (THz QCLs) are compact, electrically-driven sources of narrowband radiation in the ~2–5-THz band. Numerous scientifically important gas-phase species within the Earth’s upper atmosphere have distinctive spectral features within this band, making QCLs attractive sources for spectroscopic and radiometric atmospheric studies. In this paper, we demonstrate the integration of a QCL with a satellite-compliant cryocooler, precision-micromachined waveguide, a pair of diagonal feedhorns and a Cassegrain telescope, as key steps toward a complete 3.5-THz integrated receiver system.

Accurate pre-launch calibration of the MetOp-SG (Meteorological-Operations Second-Generation) Microwave Sounder (MWS) remote sounding instrument is critical in achieving an operational radiometric temperature uncertainty of 0.1 K. The RAL Space department at the STFC Rutherford Appleton Laboratory is working with Airbus Defence and Space (UK) to a develop calibration system for MWS. This system sets a new level of required calibration accuracy for (sub-) millimetre wave space-borne instruments, and needs blackbody targets with a temperature stability of 0.05 K over 5 minutes and 0.5 K over 1 hour for the temperature range 80 K to 335 K, with a return loss requirement of 45 dB. Achieving the required target diameters of 0.5 m presents significant design and manufacturing challenges.

We present antenna-pattern measurements obtained from a double-metal supra-terahertz-frequency (supra-THz) quantum cascade laser (QCL) mounted within a mechanically micro-machined waveguide cavity and dual diagonal feedhorn assembly. With the QCL operating in continuous-wave mode at 3.5 THz, and at an ambient temperature of ∼60 K, emission from both laser facets has been simultaneously directed to a suitable supra-THz detector mounted on a multi-axis linear scanner. Comparison of simulated and measured far-field antenna patterns shows an excellent degree of correlation between beamwidth (full-width-half-maximum) and sidelobe content. Furthermore, when compared with unmounted equivalents, very substantially enhanced QCL beam profiles are observed. Our novel device demonstrates the effectiveness of diagonal feedhorns intended for use in future spaceborne Earth-observing supra-THz heterodyne radiometers.

We present the first antenna power measurements from a double metal terahertz-frequency quantum cascade laser (THz QCL) mounted within a waveguide cavity and incorporating dual diagonal feedhorns that enable directional output coupling of THz radiation from both laser facets. The feedhorn antenna patterns have been measured simultaneously at a frequency of 3.5 THz and with the QCL operating at an ambient temperature of 60 K in continuous wave mode. Comparison with a feedhorn theoretical model shows good pattern correlation and implies that the QCL signal is likely propagating within the waveguide in a fundamental mode.

Radio emission emanating from celestial sources was first detected by Karl Jansky during the 1930s. Using primitive radio instrumentation coupled to a relatively small aperture telescope, Jansky mapped portions of the Milky Way and established the field of radio astronomy. Thus was born a branch of modern astrophysics that has allowed the study of radio emission from cosmic sources and revolutionized our understanding of the universe.

We report the development of terahertz frequency quantum cascade lasers for applications as local oscillators. A range of active region designs and waveguide structures have been characterised in order to develop the devices for operation at high temperatures, with high output power and low dissipated power. Quantum cascade lasers based on a LO-phonon bound-to-continuum design emitting at 3.5 THz, suitable for the detection of hydroxyl, were fabricated with a double-metal (gold-gold) waveguide structure. These devices operated in continuous-wave up to 94 K, with an output power of 0.4 mW and dissipated power of 1.7 W at 10 K. A new, mechanically robust packaging and waveguide-integration scheme is also presented for operation outside laboratory environments, which further allows integration of quantum cascade lasers with terahertz waveguides, mixers and other system components. This integration scheme yielded a better beam quality, with a divergence of

Terahertz-frequency quantum cascade lasers (THz QCLs) have numerous potential applications as 1–5 THz radiation sources in space science, biomedical and industrial sensing scenarios. However, the key obstacles to their wide-scale adoption outside laboratory environments have included their poor far-field beam quality and the lack of mechanically robust schemes that allow integration of QCLs with THz waveguides, mixers and other system components. A block integration scheme is presented, in which a continuous-wave ∼3.4 THz double-metal QCL is bonded into a precision-machined rectangular waveguide within a copper heat-sink block. This highly reproducible approach provides a single-lobed far-field beam profile with a divergence of ≲20°, and with no significant degradation in threshold current or in the range of operating temperatures.

A limiting factor of GaAs Schottky diodes used as mixers in heterodyne receivers is their high intrinsic noise when compared to SIS or HEB devices. This work presents results on the numerical optimization of fundamental Schottky diode mixers, results which predict a significant improvement in the noise performance of current Schottky mixer technology in the THz band. The optimization of the Schottky mixers has been carried out by using a Monte Carlo model of the diode coupled to a multi-tone harmonic balance technique, which describes accurately the electron transport in the diode at the THz band.

Science and technologies based on terahertz frequency electromagnetic radiation (100 GHz-30 THz) have developed rapidly over the last 30 years. For most of the 20th Century, terahertz radiation, then referred to as sub-millimeter wave or far-infrared radiation, was mainly utilized by astronomers and some spectroscopists. Following the development of laser based terahertz time-domain spectroscopy in the 1980s and 1990s the field of THz science and technology expanded rapidly, to the extent that it now touches many areas from fundamental science to 'real world' applications. For example THz radiation is being used to optimize materials for new solar cells, and may also be a key technology for the next generation of airport security scanners. While the field was emerging it was possible to keep track of all new developments, however now the field has grown so much that it is increasingly difficult to follow the diverse range of new discoveries and applications that are appearing. At this point in time, when the field of THz science and technology is moving from an emerging to a more established and interdisciplinary field, it is apt to present a roadmap to help identify the breadth and future directions of the field. The aim of this roadmap is to present a snapshot of the present state of THz science and technology in 2017, and provide an opinion on the challenges and opportunities that the future holds. To be able to achieve this aim, we have invited a group of international experts to write 18 sections that cover most of the key areas of THz science and technology. We hope that The 2017 Roadmap on THz science and technology will prove to be a useful resource by providing a wide ranging introduction to the capabilities of THz radiation for those outside or just entering the field as well as providing perspective and breadth for those who are well established. We also feel that this review should serve as a useful guide for government and funding agencies. OA hybrid

The LOCUS atmospheric sounder is a satellite-borne THz radiometer concept, for studying molecular species in the mesosphere and lower thermosphere. We report waveguide-integrated THz quantum-cascade lasers for use as 3.5 THz local oscillators. A waveguide-integration scheme, using an integrated diagonal feedhorn significantly improves power outcoupling. 1.3 mW THz emission is demonstrated in a space-qualified Stirling cryocooler at 57 K, with ∼15° beam divergence.

The Far Infrared Spectroscopic Explorer (FIRSPEX) is a novel European-led astronomy mission concept developed to enable large area ultra high spectroscopic resolution surveys in the THz regime. FIRSPEX opens up a relatively unexplored spectral and spatial parameter space that will produce an enormously significant scientific legacy by focusing on the properties of the multi-phase ISM, the assembly of molecular clouds in our Galaxy and the onset of star formation; topics which are fundamental to our understanding of galaxy evolution. The mission uses a heterodyne instrument and a ~1.2 m primary antenna to scan large areas of the sky in a number of discreet spectroscopic channels from L2. The FIRSPEX bands centered at [CI] 809 GHz, [NII]1460 GHz, [CII]1900 GHz and [OI]4700 GHz have been carefully selected to target key atomic and ionic fine structure transitions difficult or impossible to access from the ground but fundamental to the study of the multi-phase ISM in the Universe. The need for state-of-the-art sensitivity dictates the use of superconducting mixers configured either as tunnel junctions or hot electron bolometers. This technology requires cooling to low temperatures, approaching 4K, in order to operate. The receivers will operate in double sideband configuration providing a total of 7 pixels on the sky. FIRSPEX will operate from L2 in both survey and pointed mode enabling velocity resolved spectroscopy of large areas of sky as well as targeted observations.

TeraSCREEN is an EU FP7 Security project aimed at developing a combined active, with frequency channel centered at 360 GHz, and passive, with frequency channels centered at 94, 220 and 360 GHz, imaging system for border controls in airport and commercial ferry ports. The system will include automatic threat detection and classification and has been designed with a strong focus on the ethical, legal and practical aspects of operating in these environments and with the potential threats in mind. Furthermore, both the passive and active systems are based on array receivers with the active system consisting of a 16 element MIMO FMCW radar centered at 360 GHz with a bandwidth of 30 GHz utilizing a custom made direct digital synthesizer. The 16 element passive receiver system at 360 GHz uses commercial Gunn diode oscillators at 90 GHz followed by custom made 90 to 180 GHz frequency doublers supplying the local oscillator for 360 GHz sub-harmonic mixers. This paper describes the development of the passive antenna module, local oscillator chain, frequency mixers and detectors used in the passive receiver array of this system. The complete passive receiver chain is characterized in this paper.

Since their first demonstration in 2002, the performance of terahertz frequency quantum cascade lasers has developed extremely rapidly. We consider the potential use of terahertz frequency quantum cascade lasers as local oscillators in satellite-borne instrumentation for future Earth observation and planetary science missions. A specific focus will be on the development of compact, waveguide-integrated, heterodyne detection systems for the supra-terahertz range.

The sub-millimetre/microwave range of the spectrum has been exploited in the field of Earth observation by many instruments over the years and has provided a plethora of information on atmospheric chemistry and dynamicshowever, this spectral range has not been fully explored in planetary science, having been exclusively employed to carry out ground-based measurements. To this end, a sub-millimetre instrument, the Orbiter Terahertz Infrared Spectrometer (ORTIS), is studied by the University of Oxford and the Rutherford Appleton Laboratory, to meet the requirements of the European Space Agency's Cosmic Visions 2015-2025 programme-in particular, the Europa Jupiter System Mission (EJSM), which has the European Space Agency and the National Aeronautics and Space Administration as partners. ORTIS is designed to measure atmospheric temperature, the abundance of stratospheric water vapour and other jovian gases, and is intended to be capable of retrieving vertical profiles of horizontal windspeed in the stratosphere for the first time, from Doppler-shifted emission lines measured at high spectral resolution. In this work, a preliminary study and implementation of the estimation of windspeed profiles on simulated spectra representative of Jupiter is presented, detailing the development of the retrieval algorithm, showing that a sub-millimetre instrument such as ORTIS should be able to retrieve windspeed profiles to an accuracy of about 15 m/s between 70 and 200 km/0.1-10 mb using a single near-limb measurement, for expected noise amplitudes. © 2010 Elsevier B.V.. All rights reserved.

Fiber-ring-based optical frequency comb generators are analyzed to understand their behavior and limitations. A numerical frequency-domain model is described for studying dispersion and other phase mismatch causing effects in the fiber ring cavity, as well as for predicting the spectral and temporal evolutions of the comb in time. The results from this analysis are verified with experimental measurements. A flat optical comb, with a terahertz span within a 6-dB power envelope and containing 100 comb lines, with a suppressed central comb line, is demonstrated. The comb shows an excellent coherence dependent on the phase noise from the radio frequency synthesizer that drives the comb generator. Improvement in the error correction loop also enables the comb spacing to be set at precise 12.5-MHz intervals without having to adjust the system. Fast frequency switching of the comb line spacing is demonstrated for the first time. The comb line spacing,can be switched to any operation frequency with a resolution of 12.5 MHz between 6 and 12.5 GHz, as limited only by the microwave circuit used. The switching time is less than 1 s, and the spectral profile of the comb is maintained.

We demonstrate high-performance THz QCLs lasing at 2.2, 2.53, 3.5 and 4.7 THz, which target absorption lines of water, methane, hydroxyl and atomic oxygen respectively. Reliable single-mode targeting of gas species is obtained through the use of a photonic lattice design. A highly reproducible micro-machined waveguide block yields narrow beam-divergence and enables future integration of a complete THz heterodyne system including local-oscillator, mixer, and feed-horn.

This paper reports on the design and evaluation of a high-throughput assay system for investigating the biological effects of microwave/millimeter wave radiation. The approach presented here differs significantly from others with the deployment of a continuous culture device and sample handling technology that allows the rapid presentation of test samples in a uniform physiological state. Segmented flow of the sample passes through a fundamental mode millimeter-wave exposure cell that is, in this prototype, well characterized between 26 and 40 GHz. A sensitive bioluminescence-based reporter monitored the temporal evolution in each assay segment using optical detector arrays mounted at three observation points, i.e., before the sample entered the cell and during millimeter-wave exposure and postexposure. Using this approach, the continuous monitoring of bioluminescence may be used on different time scales to measure cell energy metabolism, gene expression and growth. Exposed and control samples were monitored and any combination of stimulus parameters, namely, radiation frequency, intensity, and polarization (or any modulation of these) that induced a response that exceeded a noise threshold of the system could be further investigated automatically in real time, for example, with increased frequency resolution and at different power levels. Environmental parameters such as the temperature and magnetic and electric fields were carefully controlled. This sensitive “active” search system may have application in industry, biomedical research, and environmental health.

We describe a new Nb SIS junction heterodyne waveguide receiver working in the frequency range between 800–810 GHz. The receiver is light and compact and is designed to fit most infrared and optical telescopes with the minimum of reconfiguration. We present a description of the receiver and its associated RF performance.

Ice desorption is the main mechanism causing the release of complex molecules from the solid to the gas phase in star forming regions. A Schottky diode millimetre-wave heterodyne radiometer, developed by the Millimetre-Wave Technology Group, RAL Space, UK, has been used for laboratory-based analysis of interstellar ice analogues. The sensitivity and stability of the instrument has been characterised to determine its suitability for use with a future unique desorption facility under development at the Open University, UK. We describe our method of radiometer calibration and the results obtained, and provide example emission spectra acquired by using the radiometer with a gas cell.

Ultra-sensitive superconducting tunnel junction heterodyne receivers used for astronomy research require relatively low levels of local oscillator (LO) power. When configured as an imaging array, however, the LO power required substantially increases and the provision and distribution of a harmonically generated LO signal to multiple pixel elements becomes a technically challenging task. Furthermore, the difficulty of generating LO power is compounded as the operational frequency is increased into the supra-THz region (

EMCOR is a heterodyne receiver for the frequency range of 201 to 210 GHz. It has been designed for ground-based measurements of various minor constituents of the stratosphere involved in ozone chemistry. Since the aim was the detection of faint spectral lines, a superconducting tunnel junction has been chosen as mixer element and special care has been taken in developing the calibration unit of the system. The front-end is completed by a quasi-optical system, a solid state local oscillator with electronic tuning and a HEMT pre-amplifier. In the back-end an acousto-optical spectrometer is employed to analyse the signal. A PC controls the whole system. The instrument has been installed at a high mountain site in the Swiss Alps.

We report on a low-cost technique for the photonic generation of wideband continuously tunable millimeter-wave local oscillator signals. It is based on sideband filtering using an optical phase modulator and thin-film dense wavelength- division-multiplexing filters. The generated millimeter-wave signal exhibits low phase noise, and its frequency covers the W - and F -bands, from 75 to 140 GHz.

The successful European Space Agency (ESA) Planck mission has mapped the Cosmic Microwave Background (CMB) temperature anisotropy with unprecedented accuracy. However, Planck was not designed to detect the polarised components of the CMB with comparable precision. The BICEP2 collaboration has recently reported the first detection of the B-mode polarisation. ESA is funding the development of critical enabling technologies associated with B-mode polarisation detection, one of these being large diameter half-wave plates. We compare different polarisation modulators and discuss their respective trade-offs in terms of manufacturing, RF performance and thermo-mechanical properties. We then select the most appropriate solution for future satellite missions, optimized for the detection of B-modes., Comment: 16 pages

A Fourier transform spectrometer, based on a Martin-Puplett polarisation rotation interferometer and using broad-band blackbody noise sources, has been used to study the sideband response i.e., conversion gain, of a room temperature Schottky diode sub-harmonic mixer operating at 300 GHz. The technique enables the response of the mixer to be characterised and preferentially tuned to one sideband, thereby improving the rejection of unwanted spectral components which can be present in the mixer image sideband.

We present the results of an instrument concept study for a low cost terahertz sounder of the mesosphere and lower thermosphere (MLT). Recent advances in the development of Quantum Cascade Laser (QCL) technology to be used for Local Oscillators (LOs) mean that it has now become viable for the first time to build compact, low weight heterodyne receivers in the terahertz (THz) frequency range [28]. Some of the most important atmospheric constituents of the MLT region, e.g. atomic oxygen (O) and the hydroxyl radical (OH), can only realistically be measured at THz frequencies. The technical challenges of THz remote sensing result in a large uncertainly of the global distribution of these species. Recent research indicates that the MLT region exhibits links to processes associated with climate change. From this follows a strong need to measure the composition and dynamic of the MLT region more accurately and more comprehensively.

The Atacama Large Millimeter Array is a radio telescope array of 66 antennas designed for high performance scientific imaging, covering a frequency range of 27-950 GHz. Each antenna has a front end with ten receiving bands, and each band has a local oscillator which is synchronized between all antennas. We describe a high precision tunable millimeter-wave photonic local oscillator reference system, which is used as the synchronizing reference for all ten bands on each receiver.

The quasi-optical modulation of linear polarization at millimeter and sub-millimeter wavelengths can be achieved by using rotating half-wave plates (HWPs) in front of polarization-sensitive detectors. Large operational bandwidths are required when the same device is meant to work simultaneously across different frequency bands. Previous realizations of half-wave plates, ranging from birefringent multi-plates to mesh-based devices, have achieved bandwidths of the order of 100%. Here we present the design and experimental characterization of a reflective HWP able to work across bandwidths of the order of 150%. The working principle of the novel device is completely different from any previous realization, and it is based on the different phase-shift experienced by two orthogonal polarizations reflecting, respectively, off an electric conductor and an artificial magnetic conductor.

A model for the structure of the massive molecular cloud G 34.3 + 0.2 is derived from extensive observations of carbon monoxide lines, including J = 1-0, J = 2-1, J = 3-2 and J = 6-5 (CO)-C-13, and J = 2-1 (CO)-O-18. The model consists of a slowly collapsing turbulent cloud with density constant out to a radius of 0.15 pc, but falling off as similar to R(-2) out to 3.25 pc. The CO abundance is constant. Within the framework of the basic cloud structure it is possible to predict line shapes for the 492-GHz P-3(1)-P-3(0) CI line, for different assumed radial behaviour of the CI abundance. These are compared with observations. The best fit is found with CI proportional to r(0.9) from the perimeter of the cloud into r = 0.24 pc where A(v)= 55. The column-density-averaged [CI]/[CO] ratio for G 34.3 + 0.2 is very similar to that for the rho Ophiuchi molecular cloud, notwithstanding the very different structures of the two clouds. Models for producing high carbon abundances deep in clouds that may have difficulty in explaining the results include those with external ultraviolet penetration around clumps, or dynamic mixing, or in which the initial [C]/[O] ratio is greater than unity.

A 70-GHz bandwidth commercial photodiode has been coupled to W-band waveguide and used as a photomixing source from 75 to 170 GHz. Maximum power conversion efficiency of 1.8% was obtained at 75 GHz, where an optical input of +10 dBm yielded a nonsaturated millimeter-wave (mm-wave) power of -7.5 dBm. Optimizing the photomixer backshort tuning at individual frequencies showed that the mm-wave power decreased with frequency to a level of -30 dBm at 170 GHz. Fixed tuning allowed the generation of power across the full waveguide band from 75 to 110 GHz, with a variation within 5 dB across the majority of the band.

Most parts of the electromagnetic spectrum are well understood and exploited, but the terahertz region between the microwave and infrared is still relatively under developed. Potential receiver applications are wide-ranging and cross-disciplinary, spanning the physical, biological, and medical sciences. In this spectral region, Schottky diode technology is uniquely important. InP MMIC amplifiers are generally limited to frequencies less than ∼200 GHz, above which their noise performance rapidly deteriorates. Superconducting circuits, which require cooling, may not always be practical. Either as varistor diodes (heterodyne mixing), or varactor diodes (sub-millimetre power generation), Schottky technology underpins terahertz receiver development. Two important developments have occurred in recent years. First, the underpinning technology base has demonstrably matured. Planar Schottky diode technology has been shown to be practical at frequencies as high as 2,500 GHz, and frequency multipliers have been shown to be capable of generating 100s of mW at frequencies around W-band. Secondly, circuit designs can now be optimised theoretically with CAD electromagnetic structure simulators and non-linear analysis programs. New high-speed computer controlled mills, improved lithographic capabilities and micro-machining techniques also offer exciting new options for cavity and circuit manufacture. This paper describes the Schottky diode technology currently being developed at the Rutherford Appleton Laboratory. Discrete diode components are described as well as integrated diode/filter circuits. Frequency multiplier diode structures are reported which include novel substrate transfer techniques to reduce the effects of dielectric loading and self-heating.

We describe the design and characterisation of a cryogenic millimetre/sub-millimetre wave calibration load, cooled by use of a closed cycle refrigerator that is used to test the performance of the ALMA receiver front-end system. Use of the refrigerator removes the need for liquid cryogen (nitrogen) cooling and allows for long duration, and unattended operation independent of orientation angle. Key requirements of the load include provision of a well-characterised and constant brightness temperature over a wide frequency range (from ~100 GHz to ~1 THz) polarisation insensitivity, high emissivity and mechanical stability. Test and verification of the load performance characteristics is achieved by using several measurement techniques; these are presented and compared with measurements made using a liquid cryogen load (cooled reference).

TELIS (TErahertz and submm LImb Sounder) is a three-channel balloon-borne heterodyne spectrometer for atmospheric research. The observational techniques of TELIS can be compared to the presently flying MLS instrument on board NASA's EOS-Aura satellite, but TELIS is built with a new generation of cryogenic heterodyne detectors and novel compact systems suitable for integration into the confined space of a balloon borne cryostat. TELIS will fly on the MIPAS-B2 gondola. The two instruments together will yield the most complete set of stratospheric constituents, measured so far. TELIS is a cooperation between the European institutes DLR (PI-institute), RAL and SRON. First flight foreseen in the spring of 2008 from Teresina, Brasil. The three TELIS receivers provide simultaneous vertical profile measurement of a range of molecules. The 500 GHz channel is developed by RAL and will produce vertical profiles of BrO, ClO, O 3 and N 2 O. The 1.8 THz channel is developed by DLR and will mainly target the OH radical, and will also measure HO 2 , HCl, NO, NO 2 , O 3 , H 2 O, O 2 and HOCl. Finally the 480 - 650 GHz channel is developed by SRON and IREE and will measure profiles of ClO, BrO, O 3 , HCl, HOCl, H 2 O and its 3 isotopomers, H 2 O 2 , NO, N 2 O, HNO 3 , CH 3 Cl and HCN. In this paper, the science and technology of TELIS will be discussed with emphasis on the channel developed by SRON. It contains a Superconducting Integrated Receiver (SIR), which combines on a 4x4 mm 2 chip the low-noise SIS mixer and its quasioptical antenna, a superconducting phase-locked Flux Flow Oscillator (FFO) acting as Local Oscillator (LO) and a SIS harmonic mixer (HM) for FFO phase locking. The latest results from the pre-flight test and integration campaigns will be presented.

A single photomixer generated the local oscillator signal for a four element focal plane array of niobium superconducting heterodyne receivers working at 4 K. The photomixer incorporated an InGaAsP photodiode, driven by two 1.55 mum laser beams, that operated at an ambient temperature of 77 K. Its output signal propagated through stainless steel waveguide to the 4 K cryostat stage. Distribution of LO power to each superconducting junction was accomplished via broadwall waveguide couplers. Double sideband noise temperatures of 40plusmn5 K , integrated over the 3.85 to 7.85 GHz IF band, were measured at LO frequencies from 132 to 175 GHz.

TELIS (TErahertz and sub millimeter LImb Sounder) is a three-channel balloon-borne cryogenic heterodyne receiver on which a number of novel features are tested. It flies together with the MIPAS-B2 instrument and the combination can yield the most complete map of atmospheric species ever measured from one platform. The three channels are selected at 500 GHz, tunable between 500 and 650 GHz and at 1.8 THz, and employ a SIS mixer, a Superconducting Integrated Receiver and a HEBM. The first flight is foreseen in 2006.

A low-noise 810 GHz heterodyne receiver for astronomy is described. The mixer consists of a NbTiN SIS junction, mounted in a half-height rectangular waveguide mixer block. The receiver has been successfully tested on the UK Infrared Telescope, Hawaii, with a DSB system noise temperature of 280 K at 808 GHz. Observations of several astronomical sources have verified the sensitivity of the instrument.

A compact 500 GHz SIS heterodyne radiometer is currently being developed at the Rutherford Appleton Laboratory (RAL), UK. The radiometer can be integrated with a new state-of-the-art balloon borne terahertz and sub-millimetre wave limb sounder (TELIS) and performs remote sounding of key molecular constituents within the Earth's atmosphere. The paper presents the design concept of the receiver and current status of development.