High speed data communication underpins many important facets of modern life. High speed internet, online learning, high-definition video streaming, cloud computing, video conferencing, online gaming, artificial intelligence and the inner works of computers themselves all rely on the ability to transfer vast quantities of data at high speed. With the continuous introduction of increasing advanced and data hungry applications, the demand for bandwidth has grown relentlessly and is set to continue into the future. As data transmission rates increase so does power consumption and in some cases can dominate the power usage of the entire computing system. The growth of power use in the ICT industry is a major global concern with predictions showing that it could account for 20% of electricity usage worldwide and emit up to 5.5% of the world's carbon emissions by 2025. To support the growth in data demands it is of paramount importance that data communication technology is able to keep pace whilst minimising power use. Communication links are continuously being converted from working in the electrical domain to the optical domain since much more data can be transmitted optically with lower power consumption. The conversion is moving to progressively shorter links as the available technology becomes more cost effective and electrical bandiwdth limits are reached. Silicon photonic technology has been a key enabler of the conversion particularly in the data centre application space since the technology allows production of integrated photonics transceiver chips in a reliable, low-cost CMOS (Complementary metal-oxide-semiconductor) manner. Within the optical transmission link the process of converting electrical data into an optical format typically dominates power consumption. It is performed either by using an external optical modulator or by directly modulating the laser and for shorter optical links to be viable, their power usage must be reduced. For example, after almost two decades of research the performance of the silicon based optical modulator has manged to reach 100Gbaud with 1 pj/bit power consumption (including drive and tuning power), but power requirements for off chip and on chip links are ~100fJ/bit and 10fJ/bit respectively. For silicon photonic data transmission technology to meet these stringent energy and bandwidth requirements the hybrid integration of materials with stronger electro-optic effects onto silicon is essential as this will allow vast reductions in power consumption. In this proposal, we apply an advanced geometrically defined crystal growth process, tunnel epitaxy, to grow III-V semiconductors onto silicon photonic chips for use in a new-generation of optical modulation technology. The approach offers unique advantages in terms of footprint, yield, material quality and the ability to laterally grade doping levels during the growth process allowing precise optimisation of the trade-off between device bandwidth and optical loss. Combining the strength of the silicon photonics expertise at Southampton and the III-V on Si manufacturing at Cardiff, we will design and fabricate both external optical modulators and directly modulated light sources with state-of-the-art performance, targeting both short and ultra-short data reach applications. We will produce devices with 100Gbaud transmission with order of magnitude improvement in drive power which will enable the next generation of ultra short links to be viable. The developed technology will positively impact a large range of applications providing wide reaching societal and economic benefits.

Silicon Photonics is currently transforming data communications, and beginning to impact longer reach applications. However, Silicon Photonics is now maturing and current commercially available transceivers mainly utilise modulators operating at 25Gb/s. Laboratory demonstrators for next generation systems either use multiple parallel lanes of 25Gb/s devices, or perhaps more complex modulation techniques to achieve higher aggregate data rates. Even the fastest research modulators, when integrated with drivers, operate up to approximately 50Gb/s OOK (or corresponding PAM4 modulation to reach a net aggregate speed of 100Gb/s). Researchers worldwide are trying to improve such modulators to squeeze the last few percentage points of improved performance out of these devices, or are turning to integration of other materials, which increases fabrication complexity and cost, and potentially reduces yield. In this work we have invented a way to improve the modulator/driver combination not by a few percent, but by 100%, which will lead to dramatic improvements in data rate, power consumption, and cost of implementation. We will demonstrate 100Gb/s OOK and 200Gb/s PAM4, as well as a novel Optical Time Division Multiplexing (OTDM) system. In effect, we have found a way to transfer functions that were traditionally done in the electronic domain, to the optical domain, saving cost and energy and dramatically improving performance. The proposal provides detailed simulations of the proposed work as preparatory demonstration of the viability of the new techniques. This includes typical characteristics of modulators previously fabricated at the Southampton, which will now be operated in a different mode. Consequently, we are confident that the chances of success are very high. Electronic drivers will be designed at Southampton and subcontracted to TSMC in Taiwan for fabrication. All optical devices will be fabricated at Southampton using the Silicon Photonics Foundry service called CORNERSTONE. The new approach has already led to 2 patent applications, and we suspect others will follow as we progress with the research. Both investigators and the research investigator are inventors, so the team is ideally placed to carry out the work.

From an Information and Communication Technology (ICT) perspective, the 21st century is characterized by an explosion of requests for communication capabilities, high-performance computing, and cloud storage. Over the last few years, global Internet traffic has been growing exponentially. In this picture, transporting such an amount of data with existing electrical- interconnects and switching technologies will soon reach the "bottleneck" in terms of thermal loading, capacity, latency and power consumption. Optical- interconnects and switch fabrics combined with photonic integrated circuits (PICs) are seen as one of the most promising routes to push such limits. Silicon (Si) photonics is now considered as a reliable photonic integration platform. The beauty of Si Photonics stems from its ability to integrate microelectronics and photonics on a single Si chip utilizing standard CMOS IC technology. An important subset of this area is hetero-integration of III-Vs on Si, where the aim is the make use of III-V materials, with superior optical properties, to provide an efficient optical gain medium to circumvent the fundamental physical limitation of Si, i.e. Si cannot efficiently emit light, yet keeping the capability of light-routing, modulating, detecting and cost advantages of Si. In a breakthrough development, the investigators' group in UCL have shown that it is possible to grow epitaxially high-performance quantum dot (QD) lasers directly on Si substrates, opening up the possibility to monolithically integrate various types of III-V optoelectronic devices on Si. The pace of research on monolithic III-V/Si integration has then been dramatically accelerated and an increasing number of prestigious research groups including Bowers' group at UCSB and Arakawa's group at Tokyo University, and major Si chip companies, i.e. Intel, are currently devoting considerable programmes in this area. In addition to III-V/Si lasers, monolithic III-V/Si semiconductor optical amplifiers (SOAs) are also attracting significant interest as the key components for next-generation photonic integrated optical- interconnects and switching fabrics, as the application of SOAs is not limited only to compensate for loss and maintain signal levels as the signal propagates throughout a large number of optical components within the PICs, it is also used as a mature gating element for optical switches and has the advantages of ease of control, smaller footprint, low operating voltage, high ON/OFF extinction ratio, and fast transition times of the order of nanoseconds. However, such a III-V/Si SOA has not been developed to date. Building on the established expertise in monolithic III-V/Si QD lasers at UCL, this project proposal aims to develop the world's first monolithic III-V QD SOA on CMOS-compatible on-axis Si (001) substrates. In contrast to conventional native substrate based SOAs or III-V/Si SOAs using either flip-chip bonding or wafer bonding, the proposed method is fundamentally different, since the III-V SOAs will be integrated on Si by direct epitaxial methods, offering the possibility to achieve high-yield, low-cost and large-scale Si-based PICs, which is expected to be the technology platform to address next-generation optical- interconnect and switching solutions. With further development in Si photonics, i.e., providing the microelectronics world with the ultra-large-scale integration of photonic components, there will be scope to target applications in important areas such as consumer electronics, high-performance computing, medical and sensor solutions, and defence. This project will benefit from guidance from and joint work with both industrial as well as academic partners and will leverage major UK-based industrial and academic strengths in materials (e.g., CSC, EPSRC NEF) device processing (e.g., EPSRC CSHub, Glasgow) and photonics (e.g., Rockley, Lumentum), who are also well positioned to exploit this research.

Silicon Photonics, the technology of electronic-photonic circuits on silicon chips, is transforming communications technology, particularly data centre communications, and bringing photonics to mass markets, utilising technology in the wavelength range 1.2 micrometres - 1.6 micrometres. Our vision is to extend the technical capability of Silicon Photonics to Mid -Infrared (MIR) wavelengths (3-15 micrometres), to bring the benefits of low cost manufacturing, technology miniaturisation and integration to a plethora of new applications, transforming the daily lives of mass populations. To do this we propose to develop low-cost, high performance, silicon photonics chip-scale sensors operating in the MIR wavelength region. This will change the way that healthcare, and environmental monitoring are managed. The main appeal of the MIR is that it contains strong absorption fingerprints for multiple molecules and substances that enable sensitive and specific detection (e.g. CO2, CH4, H2S, alcohols, proteins, lipids, explosives etc.) and therefore MIR sensors can address challenges in healthcare (e.g. cancer, poisoning, infections), and environmental monitoring (trace gas analysis, climate induced changes, water pollution), as well as other applications such as industrial process control (emission of greenhouse gases), security (detection of explosives and drugs at airports and train stations), or food quality (oils, fruit storage), to name but a few. However, MIR devices are currently realised in bulk optics and integrated MIR photonics is in its infancy, and many MIR components and circuits have either not yet been developed or their performance is inferior to their visible/near-IR counterparts. Research leaders from the Universities of Southampton, Sheffield and York, the University Hospital Southampton and the National Oceanography Centre will utilise their world leading expertise in photonics, electronics, sensing and packaging to unleash the full potential of integrated MIR photonics. We will realise low cost, mass manufacturable devices and circuits for biomedical and environmental sensing, and subsequently improve performance by on-chip integration with sources, detectors, microfluidic channels, and readout circuits and build demonstrators to highlight the versatility of the technology in important application areas. We will initially focus on the following applications, which have been chosen by consulting end users of the technology (the NHS and our industrial partners): 1) Therapeutic drug monitoring (e.g. vancomycin, rifampicin and phenytoin); 2) Liquid biopsy (rapid cancer diagnostics from blood samples); 3) Ocean monitoring (CO2, CH4, N2O detection).

"Semiconductors" are synonymous with "Silicon Chips". After all Silicon supported computing technologies in the 20th century. But Silicon is reaching fundamental limits and already many of the technologies we now take for granted are only possible because of Compound Semiconductors (CS). These include The Internet, Smart Phones, GPS and Energy efficient LED lighting! CSs are also at the heart of most of the new technologies expected in the next few years including 6G wireless, ultra-high speed optical fibre connectivity, LIDAR for autonomous vehicles, high voltage switching for electric vehicles, the IoT and high capacity data storage. CSs also offer huge opportunities for energy efficiency and net zero. CSs are often made in small quantities and using bespoke techniques and manufacturers have had to put together functions by assembling discrete devices. But this is expensive and for many of the new applications scale-up and integration, along the lines of the Silicon Chip, are needed CDT research will involve the science of large scale CS manufacturing, manufacturing integrated CS on Silicon and applying the manufacturing approaches of Silicon to CS; it will generate novel integrated functionality and all with an emphasis on finding environmentally sustainable manufacturing methods. CIVIC PRIORITY: This CDT is a fundamental part of the strategic development of the CS Cluster centred in South Wales, and in linking it to activity across the UK. It is part of a wider training strategy including apprenticeships, MScs and CPD, to train and upskill the entire workforce. The latest skills requirements have been identified by partner companies and through working with Welsh Government, CSconnected and the CS Applications Catapult The partners support the CDT financially and with their time. This is because the limiting factor to rapid cluster growth is skilled people. The expected PhD level jobs increase for the existing cluster companies alone would mop up all the students trained by this CDT. We provide a £2k stipend top-up to maximise recruitment from all backgrounds. However, the CDT does more - clusters are about cross-fertilisation of people and ideas and the CDT combines academics from 4 universities with leading and complementary expertise in CS. We form teams of two academics from different universities, one industry supervisor and the PhD student to create and carry out each PhD. The CDT also ensures the whole cohort regularly works together to exchange new knowledge and ideas and maintain breadth for each student. The UK and Welsh administrations see CS as an opportunity to boost the economy with high technology jobs and the UK government uses the CDT as part of its pitch to overseas companies to locate here. APPROACH and OUTCOMES: a 1+3 program where Year 1 (Y1) is based in Cardiff, with provision via taught lectures and transferable skills training, hands on and in-depth practical training and workshops led by University and Industry Partner staff. Following requests from Y2-4 students the industry workshops are presented in hybrid format so all Y2-4 students can further benefit from this program and where we now cycle presenters, companies and specific topics over 3 years. A dedicated training clean room allows rapid practical progress in a supportive environment, learning from doing, experts and the rest of the cohort and then an industry facing cleanroom, co-located with industry staff and manufacturing scale equipment, where students learn the future CS manufacturing skills. This maximises exchange of ideas, techniques and approach and the potential for exploitation. Both students and industry partners have praised the practical skills this produces. Y2-Y4 consist of an in depth PhD project, co-created with industry and hosted at one of the 4 universities, and specialised whole cohort training and events, including energy audit, research ethics and innovative outreach