When we look into space with existing infrared, radio and microwave sensors, we see less than half the light in our galaxy. Most of this "missing" light lies in the terahertz (THz) or far-infrared part of the spectrum (1-10 THz, 30-300 micron wavelength). Indeed, the "invisible" gases in the Earth's atmosphere and the "dark" dust and gas clouds between stars all glow with distinctive THz fingerprints, providing a wealth of hidden information urgently needed by atmospheric and space scientists. Despite this great potential, existing THz sensor systems are too large, fragile and complex for most applications outside the laboratory and lack the sensitivity needed for studying reactive gases. Furthermore, this lack of technological readiness limits the prospects for THz systems being deployed in space. A short time-window is available for the UK to invest in real-world demonstrations of key THz components and sensing techniques and secure a place in forthcoming space missions. Without this, the potential for a UK researcher to lead the world in this emerging area will be lost. In this fellowship, I will overcome the previous limitations of THz gas sensors by developing high-sensitivity systems based on quantum-cascade lasers (QCLs) - QCLs are highly compact sources of THz radiation, which yield >1000 times the power of any similar-sized device. Unlike previous THz-QCL-based gas-sensing schemes, I will use high-precision analytical chemistry techniques. This will include the use of custom-made reflective cavities and multi-pass gas cells in which THz radiation passes repeatedly through the gas under study, yielding an estimated 100x improvement in sensitivity. I will also work with my project partners in RAL Space to control the frequency of my THz sensors precisely, giving 10x improvement in the resolution of spectral "fingerprints" compared with free-running QCLs. Together these advancements will allow the first laboratory observation of key atmospheric reactions and underpin future deployment in satellite applications. Specifically, I will develop the first high-precision gas sensors operating in the THz band, with the robustness, compact size and sensitivity needed for installation on a satellite or the International Space Station. The UK and European Space Research communities have taken great interest in THz gas sensing, with at least two in-flight systems currently under consideration. These include "TARDiS" - a UK-led THz astrophysics system for the International Space Station, and "LOCUS" - an Earth-observing THz satellite for studying the climate and space-weather effects in the upper atmosphere. The same THz instrumentation would allow ultra-precise monitoring of chemical processes in industry or in research laboratories. Trace gases will be revealed, together with the composition of highly-reactive gas species, which cannot be distinguished reliably using existing techniques. Key examples include allowing manufacturers to ensure that vehicles meet strict new emission targets (Euro 6) or allowing atmospheric scientists to understand chemical processes that occur in the upper atmosphere, providing critical missing pieces of information needed to model the Earth's changing climate. I shall also bring these high-sensitivity techniques into relevant application environments for the first time, through the development of a compact and portable "industrial evaluator" system, containing the complete optical, electronic and cooling systems needed for high-precision THz gas sensing. I will undertake a series of industrial placements with my project partners in the STFC RAL Space division to prove key applications within long-path atmospheric gas sensing. Furthermore, I will build a wider network of academic and industrial beneficiaries to establish and sustain THz gas-sensing as a UK strength within analytical chemistry. Through this, I will initiate the first commercialisation of this new technology.

Since the discovery of the carbonised papyri at Herculaneum in the 18th century, there has been a great deal of interest in accessing the content contained in the scrolls preserved by the intense heat from the eruption of Mount Vesuvius in 79 CE. The first attempts to open these scrolls were made by hand using a knife, but this caused them to break into fragmented chunks. Subsequently in 1756 a machine was invented to create a safer method of unrolling, which was more successfully applied to numerous scrolls. However, in many cases it was impossible to keep the different layers of papyrus from sticking to each other, and so substantial portions of text remained hidden in even successfully opened scrolls, while hundreds of scrolls remained too firmly carbonised to unroll at all. The content of these fully intact scrolls, together with that of text under the stuck-on layers remains a mystery. New technology offers a solution. In the early 21st century the application of non-invasive CT scanning, a concept already proved by project members, reveals new possibilities. The structure of a scroll can be rendered digitally in three dimensions, revealing the layers of the papyrus in the scroll's circumference. Computational methods for algorithmically separating, unrolling, and flattening these layers have been developed by project members over the past decade. The virtual unrolling method has been successfully applied to P. Herc. 375 and 495. Nevertheless, despite such an achievement, the ink does not appear with any significant clarity. And while faint traces of a handful of Greek letters have been transcribed, there is currently no means to verify and replicate such results. This project aims to address the problem of detecting ink in this non-invasive imaging and thus definitively solve the long-standing problem posed by the Herculaneum papyri. In 2016 project members successfully applied the virtual unrolling method to a carbonised Hebrew scroll from the site of Ein Gedi in Israel. The ink was immediately visible, but this was due to the fact that it was contaminated with heavy trace elements and thus naturally appeared in CT scanning. The carbon-based ink used in Herculaneum papyri cannot be visualised in the same way. However, we now know that the ink is weakly contaminated with lead. We thus propose a new method called Dark Field X-ray Imaging. This reveals ink by isolating and capturing trace elements, such as lead, in its composition. To enhance the resulting ink signal further we introduce a new neural network called Reference-Amplified Computed Tomography (RACT) to amplify both the ink's presence and the shapes of the Greek characters for improved legibility. This method will definitively solve the problem of reading the text hidden in the Herculaneum papyri. To add value, the project will make the data generated by this process accessible to researchers and the curators responsible for these artefacts, by developing a new digital platform, the Augmented Language Interface for Cultural Engagement (ALICE), ensuring that the data produced by the Dark Field X-ray Imaging and RACT processes is accessible, can be properly curated, and that the extracted text can be digitally edited. Moreover, ALICE includes the functionality for integrating 3D models of the original artefact and for recording the metadata that explains both how the text was created and from where in the object's geometry the text originates in the model generated along with its digital edition. This is necessary for scientifically verifying and replicating any subsequent analysis or publication of the data. Significantly, for other cultural heritage artefacts that contain hidden text, our new imaging techniques and digital platform will be built using open architecture standards; the source code will be easily adaptable for non-invasive reading of writing inside other intractable artefacts, such as burnt books, book-bindings, and mummy cartonnage.

The terahertz (THz) region of the electromagnetic spectrum spans the frequency range between microwaves and the mid-infrared. Historically, this is the most illusive and least-explored region of the spectrum, predominantly owing to the lack of suitable laboratory sources of THz frequency radiation, particularly high-power, compact, room-temperature solid-state devices. Nevertheless, over the past decade, THz frequency radiation has attracted much interest for the development of new imaging and spectroscopy technologies, owing to its ability to discriminate samples chemically, to identify changes in crystalline structure, and to penetrate dry materials enabling sub-surface or concealed sample investigation. One of the most significant recent developments within the field of THz photonics has been the THz quantum cascade laser (TQCL). These high-power compact semiconductor sources have opened up a host of new opportunities in the field of THz photonics and have attracted significant research interest world-wide. However, there is the need to develop techniques for measurement of the phase of the radiation field emitted from TQCLs, thereby providing a complementary technology to currently established incoherent detection schemes. Furthermore, there is a need to explore fully the advances that can be made through control and manipulation of the phase of the THz field emitted by TQCLs. My vision is to initiate a range of research programmes with the aim of probing, manipulating and utilising the coherent nature of TQCL radiation. This will lay the foundations for a wealth of research opportunities in THz photonics, as well as facilitating the exploitation of THz technology for fundamental science and also for real-world applications. I will develop both optical and electronic techniques for coherent detection/measurement of the field emitted by TQCLs. One means of achieving optical coherent detection is through the up-conversion of the phase and amplitude of the THz field into the near-infrared band with an electro-optic (EO) crystal. This approach will also allow the large field amplitudes and narrow line-widths of TQCLs to be exploited, enabling QCL radiation to be sampled using a broad-area EO crystal and a standard optical CCD. This will open up a significant range of opportunities for exploiting well developed visible/near infrared detector and CCD technologies within THz science. In parallel, I will develop coherent detection techniques by down-conversion of the THz field to radio frequencies. I will accomplish this through heterodyne phase-locking the fields from two TQCLs using a Schottky diode. I will investigate coherent detection using self-mixing in TQCLs. This method relies on sensing junction voltage perturbations induced by feedback of the radiation field into the TQCL cavity, enabling coherent detection of the field using a single TQCL device as both source and detector. Using this approach, linewidth narrowing in TQCLs will be investigated, as well as techniques for three-dimensional 'detector-less' imaging and tomography. I will also establish a programme concentrating on the radio-frequency control and manipulation of the THz field through the use of dynamic and static gratings, generated and controlled via the interaction of surface acoustic waves (SAWs) with TQCL devices. This approach will be used to provide a non-contact means to apply a potential modulation to TQCL devices, thereby providing a distributed feedback mechanism for the THz wave. As part of this I will develop TQCLs with reduced active regions thicknesses and TQCL mesa structures. The combination of all these technologies will be combined to demonstrate the first 2D phase-sensitive THz tomography system using QCLs, the first full-field imaging system combining TQCLs and commercial CCD technology, and high-resolution THz gas spectroscopy.

With this Future Leaders Fellowship, I will lead a research team that will build the first self-consistent data assimilation (DA) and continuous verification scheme for space weather forecasting, which is critically needed to improve our physical understanding of, and preparedness for, hazardous space weather. Space weather is a global natural hazard which can severely impact society, industry, and be a risk-to-life. It is a known risk to energy security, communications, aviation, and satellite services. Severe space weather is driven by Coronal Mass Ejections (CMEs), which are violent eruptions of magnetised plasma from the Sun's atmosphere. Cost-effective mitigation of space weather therefore relies on forecasting the arrival and properties of CMEs at Earth. Due to the potential seriousness of space weather, it is included in the UK's National Risk Register, and is planned for in the UK's Severe Space Weather Preparedness Strategy. Thus there is a crucial need to both better understand the physics of CMEs and to improve space weather forecasting capability. However, CME prediction has failed to improve in a decade of intense research, due to both knowledge gaps and observational limitations. Sophisticated computer models are used to simulate CMEs flowing through the solar wind to Earth. However, although these models are grounded in the relevant physics, they struggle to accurately represent observed CMEs. There are two key reasons for this; firstly, the starting conditions of these models are very uncertain due to observational limitations; secondly, the representation and balance of processes in the models is incorrect - indicative of our limited knowledge of physics controlling CMEs. Heliospheric Imagers (HI), such as those developed by UKRI's STFC for NASA's STEREO mission, provide the only consistent observations of CMEs and the solar wind flowing over the whole domain from the top of the solar atmosphere to Earth. These observations show CMEs being both distorted and eroded as they flow through the highly structured solar wind, but they are under-exploited in space weather research and forecasting. DA is the process of combining models and observations, accounting for the uncertainty in each, to provide a best estimate of a system's state. By assimilating a wide range of meteorological observations, DA has revolutionised the accuracy of terrestrial weather prediction, but more importantly improved physical understanding of atmospheric processes. With DARES, we will develop a HI-based DA scheme that will revolutionise our understanding of CME physics and improve space weather forecasting skill. Working at the University of Reading, my team will collaborate with colleagues at the UK Met Office Space Weather Operations Center, UKRI's Rutherford Appleton Laboratory and KU Leuven. By comparing our HI DA constrained CME simulations against observations of CMEs flowing past Earth, and state-of-the-art spacecraft observatories such as ESA's Solar Orbiter and NASA's Parker Solar Probe, we will discover the physics crucial for understanding CME evolution. In doing so, DARES will provide the critically needed knowledge and tools required improve space weather forecasting skill. DARES is timely as it will help maximise the UK's return-on-investment from Vigil, the European Space Agency (ESA) space weather monitor to be launched in 2029. DARES also directly aligns with the UK National Space Strategy to "protect and defend our national interests in" space and "lead pioneering scientific discovery" as well as Pillar 1 of the UK Severe Space Weather Preparedness Strategy.

Stratocumulus clouds are common over S England occurring about 25% of the time. The clouds are rather thin with clear sky above, so if they break up then an overcast day is suddenly transformed into a clear sunny day; alternatively they can rapidly develop and spread over the sky changing a sunny day into a gloomy overcast one. Their formation, persistence and dispersion are surprisingly difficult to forecast. On a global scale they form widespread cloud sheets over the cold ocean water, for example, off the coast of California, Peru and Namibia. These cloud sheets have an important effect on the climate of the earth, as they reflect the sunlight straight back to space rather, and so their presence has an overall cooling effect. It is important to represent any changes in the future extent of these clouds sheets correctly if we are to have accurate predictions of future global warming. The models used for weather forecasting and for predicting future climate change split the atmosphere into large grid boxes which are typically up to 500m deep and tens or hundreds of km across, with only two numbers used to describe the cloud in each box (e.g the amount of cloud and the mass of cloud water). Stratocumulus clouds are difficult to forecast because they are often only 100m deep and can block out the sun, but are not deep enough to fill a model grid box. In a layer about 1km deep close to the surface of the earth the air is being continuously mixed and stirred in the vertical and the stratocumulus clouds form at the top of this mixed 'boundary' layer. The existence of the clouds is governed by a delicate balance between the moisture from the surface of the ground or the ocean which feeds the clouds, and the mixing of dry air above the boundary layer tending to disperse them. Another important mechanism is the formation of drizzle in the clouds which tends to remove the moisture and so disperse the clouds. The cloud droplets themselves are formed on small dust particles, so the properties of the clouds are dependent upon the level of dust or pollution in the air. The purpose of this proposal is to make detailed observations of the vertical structure of stratocumulus clouds over a period of several years with lidars and radars on the ground. A radar sends out short pulses of radio waves; the cloud scatters some of these waves back to the radar, and by timing how long the echo takes to be returned and by measuring its strength we can calculate the height of the cloud and how much water it contains and identify if the cloud is producing drizzle. The vertical velocity of the cloud and drizzle drops can be inferred from 'Doppler shift', the change in radio frequency of the reflected wave. A lidar works on the same principle but uses light; we have lidars that sense the light reflected of dust particles in the air so we see how many of these particles are present, and then sense the vertical movement of the air from the Doppler shift of the lidar echoes. Other lidars can detect how much moisture is in the air and so by combining these observations we can measure the vertical movement of moisture into the clouds, the drizzle falling out of the clouds, the mixing of dry air at cloud top and see how these relate to the evolution of the cloud and its persistence or break up. Once we understand these processes we can try to improve the forecasts. To this end we are collaborating with the Meteorological Office and the European Centre for Medium Range Weather Forecasting, so we can test out improved means of representing these stratocumulus clouds in their operational forecast models. The aim is to produce models which provide better weather forecasts of whether a day is to be cloudy or sunny. In addition, a better representation of the extensive regions of stratocumulus over the cold oceans will increase our confidence in the accuracy of the predictions of global warnings.