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 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.

The generation of ultrafast and intense light pulses is an underpinning technology across the electromagnetic spectrum enabling time-resolved measurements, nonlinear photonics, coherent control of matter, and frequency comb synthesis for high-precision metrology and spectroscopy. Yet in the terahertz (THz) region of the electromagnetic spectrum (~0.5-5THz), which spans the frequency range between microwaves and the mid-infrared, a compact semiconductor-based technology platform for intense and ultrafast pulse generation has yet to be realised. Established pulse generation schemes, based on excitation of photoconductive emitters or nonlinear crystals using bulky and expensive near-infrared lasers systems, offer only low frequency modulation, or broadband emission with little control of the spectral bandwidth and pulse width. These limitations are significantly hindering the development of the THz field not only in the UK but internationally, with adverse consequences for both fundamental scientific research and the development of future applications in metrology, materials analysis and molecular spectroscopy, and ultra-high speed THz communications. One promising solution to closing this technological gap is the THz frequency quantum cascade laser (QCL) - a compact and high-power semiconductor laser based on a quantum-engineered semiconductor superlattice. However, modelocking these sources is inherently difficult to achieve due to the very fast gain recovery time in these structures. Indeed, active modelocking approaches adopted to date have succeeded only in achieving pulse widths down to ~4ps, and only low output powers are possible. In this programme we will explore a radically new approach to pulse generation in lasers, based on the phenomenon of self-induced transparency in which pulses of the correct energy and pulse duration propagate without loss in the laser cavity whilst the growth of continuous waves is supressed. Although this concept has been discussed since the 1960s, the observation of this effect in semiconductor devices has remained elusive owing to the typically short coherence times of inter-band laser transitions. QCLs, however, are the ideal tool to realize SIT-modelocking owing to their large dipole moments, relatively long inter-subband coherence times, and, importantly, the possibility of combining resonant gain and absorbing periods with engineered dipole moments. We will explore the coherent interaction of intense, ultrafast THz pulses with intersubband semiconductor heterostructures and THz QCL devices for the first time. Although these measurements are of fundamental interest in their own right, the investigation of such systems will lead to the development of the first modelocked semiconductor laser exploiting self-induced transparency. Through this approach, we will bring about a step change in QCL modelocked technology and develop THz QCLs into a foundational, compact semiconductor technology for generating intense and ultrafast THz pulses, with inherent advantages of high powers, broad spectral coverage and the ability to electrically-control the emission properties. This will pave the way for the application of modelocked THz QCLs across a wide range of areas of academic and industrial relevance, including non-linear THz science, quantum optics, ultra-high-speed THz communications, and high-precision metrology and molecular spectroscopy. But that is not all. We will also demonstrate proof-of-principle applications of these new QCL sources for molecular spectroscopy, leading to a compact, all-solid-state and electrically-controlled multi-heterodyne THz spectrometer offering >500 GHz spectral coverage and sub-millisecond acquisition times. Through this goal we will translate to the THz region the unequalled combination of broad spectral coverage, high detection sensitivity, narrow spectral resolution and fast acquisition enabled by laser frequency combs at mid- and near-infrared frequencies.

Many applications of THz radiation require sources that are compact, low-cost, and operate at room temperature. In this project, a low-noise optically-controlled THz array antenna system will be developed, addressing a significant barrier in the adoption of THz technology. We will demonstrate a novel 'system on a chip', integrating a thin film antenna array, photodiode array, semiconductor optical amplifier (SOA) array and optical beam forming network. The SOA array enhances the pump power and ensures all array elements are evenly pumped. The beam former is used to control the phase difference between the THz radiation from different THz antennas, and thus scanning of THz beam can be realized. A THz repetition frequency mode-locked laser will be used as the light source to lock the phase of optical signals in the chip, greatly reducing the linewidth of the THz emission. The advantages of this THz emitter system include a high peak intensity due to radiation from the antennas combining coherently, room temperature operation, continuous-wave operation, compact form factor, and a narrow steerable beam. The sources will be assessed for use in systems for high-bandwidth wireless communications and for medical imaging.