This proposal seeks funding to create a Centre for Doctoral Training (CDT) in Connected Electronic and Photonic Systems (CEPS). Photonics has moved from a niche industry to being embedded in the majority of deployed systems, ranging from sensing, biophotonics and advanced manufacturing, through communications from the chip-to-chip to transcontinental scale, to display technologies, bringing higher resolution, lower power operation and enabling new ways of human-machine interaction. These advances have set the scene for a major change in commercialisation activity where electronics photonics and wireless converge in a wide range of information, sensing, communications, manufacturing and personal healthcare systems. Currently manufactured systems are realised by combining separately developed photonics, electronic and wireless components. This approach is labour intensive and requires many electrical interconnects as well as optical alignment on the micron scale. Devices are optimised separately and then brought together to meet systems specifications. Such an approach, although it has delivered remarkable results, not least the communications systems upon which the internet depends, limits the benefits that could come from systems-led design and the development of technologies for seamless integration of electronic photonics and wireless systems. To realise such connected systems requires researchers who have not only deep understanding of their specialist area, but also an excellent understanding across the fields of electronic photonics and wireless hardware and software. This proposal seeks to meet this important need, building upon the uniqueness and extent of the UCL and Cambridge research, where research activities are already focussing on higher levels of electronic, photonic and wireless integration; the convergence of wireless and optical communication systems; combined quantum and classical communication systems; the application of THz and optical low-latency connections in data centres; techniques for the low-cost roll-out of optical fibre to replace the copper network; the substitution of many conventional lighting products with photonic light sources and extensive application of photonics in medical diagnostics and personalised medicine. Many of these activities will increasingly rely on more advanced systems integration, and so the proposed CDT includes experts in electronic circuits, wireless systems and software. By drawing these complementary activities together, and building upon initial work towards this goal carried out within our previously funded CDT in Integrated Photonic and Electronic Systems, it is proposed to develop an advanced training programme to equip the next generation of very high calibre doctoral students with the required technical expertise, responsible innovation (RI), commercial and business skills to enable the £90 billion annual turnover UK electronics and photonics industry to create the closely integrated systems of the future. The CEPS CDT will provide a wide range of methods for learning for research students, well beyond that conventionally available, so that they can gain the required skills. In addition to conventional lectures and seminars, for example, there will be bespoke experimental coursework activities, reading clubs, roadmapping activities, responsible innovation (RI) studies, secondments to companies and other research laboratories and business planning courses. Connecting electronic and photonic systems is likely to expand the range of applications into which these technologies are deployed in other key sectors of the economy, such as industrial manufacturing, consumer electronics, data processing, defence, energy, engineering, security and medicine. As a result, a key feature of the CDT will be a developed awareness in its student cohorts of the breadth of opportunity available and the confidence that they can make strong impact thereon.

The THz part of the electromagnetic spectrum has a number of potential applications which include oncology (skin cancer imaging), security imaging, THz bandwidth photonics, production monitoring and astronomy. The U.K. has been one of the pioneering countries in THz research but also in the exploitation of the technology with a number of companies including TeraView, QMC Instruments and Thruvision. At present most commercial imaging and spectroscopy systems use expensive femtosecond lasers with photoconductive antenna which fundamentally limits the power output to the microWatt level. Virtually all the applications referenced above require room temperature sources with over 10 mW of output power if parallel, fast, high performance imaging and/or spectroscopy systems are to be developed.While interband recombination of electrons and holes in Si and Ge are inefficient due to the indirect bandgap of the semiconductors, intersubband transitions provide an alternative path to a laser for low energy radiation such as THz frequencies. Intersubband unipolar lasers in the form of quantum cascade lasers have been demonstrated using III-V materials. Powers up to 248 mW at 10 K have been demonstrated at THz frequencies but due to polar optical phonon scattering and the associated reduction in intersubband lifetimes as the temperature is increased, such devices only operate at cryogenic temperatures. Previous work has been undertaken on p-type Si/SiGe quantum cascade lasers but due to large non-parabolicity and large effective mass (0.3 to 0.4 m_0) in the valence band, significant gain above 10 cm^-1 is difficult to engineer.In this proposal, we propose to use pure Ge quantum well designs and L-valley electrons for the first experimental demonstration of a n-type Si-based quantum cascade laser grown on top of a Si substrate. We demonstrate that the low effective of 0.118 m_0 and long non-polar lifetimes in the Ge/SiGe system potentially provide gain close to values demonstrated in GaAs THz quantum cascade lasers at 4 K and also potentially allow 300 K operation. Further the cheap and mature available Si process technology will allow at least a x100 reduction in the cost of THz quantum cascade lasers compared to GaAs devices. Such devices could be further developed into vertical cavity emitters (i.e. VCSELs) for parallel imaging applications or integrated with Si photonics to allow THz bandwidth telecoms. Finally we propose optically pumped structures which have the potential for broadband tunability, higher output powers and higher operating temperatures than THz quantum cascade lasers.This programme has brought together the modelling and design toolsets at Leeds University with the CVD growth expertise at Warwick University combined with the fabrication and measurement expertise of SiGe devices at Glasgow University to deliver internationally leading research. We have a number of industrial partners (AdvanceSis, Kelvin Nanotechnology and TeraView) who provide direct exploitation paths for the research. Successful room temperature quantum cascade lasers are an enabling technology for many new markets for THz applications including oncology (skin cancer imaging), security imaging, production monitoring, proteomics, drug discovery and astronomy.

The terahertz (THz) frequency region within the electromagnetic spectrum, covers a frequency range of about one hundred times that currently occupied by all radio, television, cellular radio, Wi-Fi, radar and other users and has proven and potential applications ranging from molecular spectroscopy through to communications, high resolution imaging (e.g. in the medical and pharmaceutical sectors) and security screening. Yet, the underpinning technology for the generation and detection of radiation in this spectral range remains severely limited, being based principally on Ti:sapphire (femtosecond) pulsed laser and photoconductive detector technology, the THz equivalent of the spark transmitter and coherer receiver for radio signals. The THz frequency range therefore does not benefit from the coherent techniques routinely used at microwave/optical frequencies. Our programme grant will address this. We have recently demonstrated optical communications technology-based techniques for the generation of high spectral purity continuous wave THz signals at UCL, together with state-of-the-art THz quantum cascade laser (QCL) technology at Cambridge/Leeds. We will bring together these internationally-leading researchers to create coherent systems across the entire THz spectrum. These will be exploited both for fundamental science (e.g. the study of nanostructured and mesoscopic electron systems) and for applications including short-range high-data-rate wireless communications, information processing, materials detection and high resolution imaging in three dimensions.

Visible light is only a very small part of the whole electromagnetic spectrum. The radio spectrum is also very familiar to most people, but less well known is the range of wavelengths in between. In this project we are particularly interested in a part of the spectrum that has come to be known as the terahertz band, so called because the frequency is around 1 THz. Light in the terahertz band can pass through materials that are opaque to visible light, but yet, the wavelength is still small enough to resolve features smaller than 1 mm. Because of this terahertz has attracted a great deal of interest for applications where we need to see through materials, but also take good sharp pictures. Applications include medical and security imaging, particularly because terahertz is non-ionising so can be safely used with humans.Unfortunately terahertz technology suffers from some significant difficulties that requires research to overcome. Bright terahertz sources are difficult to make, so considerable effort is needed to improve what we have at the moment. Terahertz is energetically similar to ambient radiated heat, so sensors have to be both sensitive and highly descriminating. In a complete terhertz imaging system all aspects of the technology and its components are important in determining the overall performance. This project is therefore dedicated to improving sensor performance.There are a number of attributes that we would like for a good sensor. It should be small, consume little power, be very sensitive, and ideally, if it it to be used in an camera, fast enough to allow video rate imaging. We propose to use the optical properties of semiconducting materials and carefully designed metallic structures to capture terahertz radiation. We will demonstrate that these structures can be used to make an array of sensors, just as you would find in a normal camera, and that the sensors are sensitive and selective to terahertz. In the same way that mainstream photography has benefited from microelectronics to make digital cameras possible, we will also be able to make use of integrated circuit technology so that many sensors can be cheaply and efficiently put on to a single chip.Our project has attracted support from leading UK companies including Teraview and Selex-Galileo that have immediate routes to market for successful technology. Our aim is to complete the research that will demonstrate new technologies to the point where further investment will enable the creation of new products that can be used by scientists, clinicians and the security services in the not to distant future.

The unique way that light interacts with magnetic/non-magnetic metal ultra-thin films with thicknesses less than 1/5000th the width of a human hair has recently been shown to offer a route to producing novel sources of radiation with wavelengths that cover a wide range stretching from the mid- to far- infrared. This emission covers the THz region that lies between the microwave and the infra-red wavelengths of the electromagnetic spectrum; a wavelength range that remains difficult to cover, but has an enormous potential for a diverse range of applications. For example, THz radiation is particularly useful for security screening of people at airports due to its non-ionising properties, as well as for looking at the spectral fingerprints of materials including explosives, drugs and dust particles. The atomic properties of interfaces are well known to be critical to the functionality of many technologically important devices, examples include spin-torque transfer magnetic random-access memory (STT-MRAM), the sensors and media used in hard disk drives and new, artificial multiferroics. This project is focused on developing much needed understanding of how the emission process from ultra-thin magnetic structures depends on the material properties. By gaining understanding of how the underlying mechanisms are responsible for the emission process we will be able to demonstrate commercially-viable emitters. More specifically, the first emitters will be realised that operate without the need for an external magnetic field, overcoming the limitation this requirement currently imposes on the active emitting area and output energy. THz radiation also provides a currently untapped approach to investigating spin-based devices. The knowledge gained in understanding the relationship between material properties and THz emission will prove invaluable in the design of spintronic devices being developed for the next generation of data storage devices. The overall goal is the development of sources of THz radiation that will have impact in a number of future application areas, in particular when looking at the spectral fingerprints of materials for detecting dangerous gases and dust particles which present serious health and safety concerns in areas such as the mining industry. Hence, the development of well-understood spin-based emitters would have a direct impact on UK economic success by enabling the development of new applications of THz radiation and spin-based devices that will add to the technological advancement of society.