Today's communication networks need to be supported by an ultra-broadband optical backbone in order to respond to the enormous demand for data exchange. Without the use of photonic components, the "magic" of being 24/7connected on a global scale just by using our portable devices would be impossible. Recently, a new branch of science, called plasmonics, has gained great momentum in the scientific community, since it brings the promise to be complementary to photonics. For instance, in the realm of plasmonics, devices can function on a nanometric scale (1 nanometer [nm] = a billionth of a meter), with consequent advantages in terms of versatility, scalability, and reduced power consumption. The proposed project "Tunable plasmonics for Ultrafast Switching at Telecom Wavelengths" is focused on novel materials for plasmonic applications (namely titanium nitride -TiN; and aluminum doped zinc oxide - AZO). Besides solving fundamental issues typical of plasmonic devices such as poor transparency and low damage threshold, these two materials unable the possibility to engineer the light-matter interaction at will. This can be achieved either by changing the fabrication procedure or in a more dynamic fashion by means of an external excitation such as a laser beam or an applied voltage. The core active material at the center of this project is a new kind of AZO developed inside the collaborative effort between Heriot-Watt University in UK, and the Birck Nanotechnology Center in USA. This "special" AZO is grown by unconventional methods and it exhibits ultrafast optical response (i.e. after the material properties are altered by an optical pulse, it restores its original behavior on a time scale shorter than 1 ps = 1/1000000000000 sec). One fundamental goal of this project is gaining a deep knowledge of the physical mechanism behind the ultra-fast behavior of AZO (still not fully understood) and use this knowledge to further optimize the material for application in ultra-fast photonics. In addition to this, in order to properly evaluate the potentials of both AZO and TiN in the real world, this project includes the fabrication and testing of an optical modulator prototype (the modulator being the most fundamental building block for encoding information). This device will be interfaced with the external world with input/output TiN-based plasmonic waveguides and will exploit AZO as active core material for performing the ultra-fast signal encoding. Numerical simulations foresee outstanding performances in terms of compactness, reduced power consumption, and ultra-fast operational speed.

Some of the most exciting experiments planned in the UK and internationally - from studying extreme light-matter interactions, to the exploitation of quantum technologies - are demanding unpreceded performance in mirror coating technology. Optical thin film coatings appear ubiquitously in the technology around us, however current available performances will not meet the requirements for, and will thus limit the exploitation from, many of these experiments. For example, emerging extreme light-matter experiments are now handling power densities an order of magnitude higher than those previously achieved. This includes major UK infrastructures, including the Central Laser Facility (CLF) and the Scottish Centre for the Application of Plasma-based Accelerators (SCAPA), in addition to partnership initiatives in Europe including the 850MEuro European funded Extreme Light Infrastructure (ELI). All these experiments will soon require laser damage threshold (LDT) performance in the highly reflective mirrors at a level not currently available. This proposal, for the first time, will seek to exploit advanced optical coating technologies, developed with the field of gravitational wave astronomy, for use in intense-light matter experiments. Moreover, the capabilities developed will significantly support existing activities within the Quantum Technologies for Fundamental Physics (QTFP) and the UK's continued effort in gravitational wave astronomy.

Atomically thin materials offer a new paradigm for control of electronic excitations in the extreme two-dimensional (2D) limit in condensed matter. Recently this concept has been developed further when artificial potentials for electrons were created in heterostructures consisting of stacked 2D layers held together by van der Waals forces, and light was used to access and manipulate electronic spin and valley degrees of freedom in atomically-thin semiconducting transition metal dichalcogenides (TMDCs). A significant world-wide effort in the last 5 years has resulted in intense studies of optical properties of TMDC atomic layers in the linear regime. Here, we propose to use this new class of (2D) semiconducting crystals to demonstrate unexplored approaches to exploiting nonlinear optical phenomena on the nano-scale in regimes unattainable by other ultra-fast photonic materials. To achieve this, we will exploit robust excitonic complexes observable up to room T, which will be generated and controlled in artificially created vertical stacks of 2D atomic layers. Giant nonlinearities enabling ultra-fast control of light with light of low intensity will be realised and explored in such van der Waals heterostructures placed in optical microcavities, operating in the strong light-matter coupling regime that we demonstrated recently. In this regime part-light-part-matter polaritons are formed, with the exciton part responsible for the strong nonlinearity and the photon part providing efficient coupling to light. This work will open a new route to development of highly nonlinear nano-photonic devices such as miniature ultra-fast modulators and switches, with high potential to impact on a new generation of signal processing and quantum technology hardware.

The isolation of single-atomic layer graphene has led to a surge of interest in other layered crystals with strong in-plane bonds and weak, van der Waals-like, interlayer coupling. A variety of two-dimensional (2D) crystals have been investigated, including large band gap insulators and semiconductors with smaller band gaps such as transition metal dichalcogenides. Interest in these systems is motivated partly by the need to combine them with graphene to create field effect transistors with high on-off switching ratios. More importantly, heterostructures made by stacking different 2D crystals on top of each other provide a platform for creating new artificial crystals with potential for discoveries and applications. The possibility of making van der Waals heterostructures has been demonstrated experimentally only for a few 2D crystals. However, some of the currently available 2D layers are unstable under ambient conditions, and those that are stable offer only limited functionalities, i.e. low carrier mobility, weak optical emission/absorption, band gaps that cannot be tuned, etc. In a recent series of pilot experiments, we have demonstrated that nanoflakes of the III-VI layer compound, InSe, with thickness between 5 and 20 nanometers, have a "thickness-tuneable" direct energy gap and a sufficiently high chemical stability to allow us to combine them with graphene and related layer compounds to make heterostructures with novel electrical and optical properties. The main goal of this project is to develop graphene-hybrid heterostructures based on this novel class of two-dimensional (2D) III-VI van der Waals crystals. This group of semiconductors will enrich the current "library" of 2D crystals by overcoming limitations of currently available 2D layers and by offering a versatile range of electronic and optical properties. From the growth and fabrication of new systems to the demonstration of prototype devices, including vertical tunnel transistors and optical-enhanced-microcavity LEDs, our project will provide a platform for scientific investigations and will contribute to the technology push required to create new routes to device miniaturization, fast-electronics, sensing and photonics. There is great potential for further growth of all these sectors as the fabrication of 2D systems improves and as new properties are discovered and implemented in functional devices.

Single-photon counting - the ability to faithfully capture the single quantum of light - is a critical capability for a wide range of new low-light sensing applications and a host of emerging photonic quantum technologies. This proposed Programme Grant aims to significantly expand the operational region of single-photon detectors well beyond silicon's 1000nm wavelength limit into the short-wave infrared (SWIR) region of wavelengths between 1400nm to 3000nm, and part of the mid-wave infrared (MWIR) region between 3000nm and 5000nm. By scaling up SWIR and MWIR semiconductor and superconductor single-photon detectors to large area focal plane arrays, we will produce revolutionary new cameras with picosecond timing resolution which can be used, for example, to see though fog in automotive lidar scenarios, as well as allowing imaging and sensing in new applications in environmental monitoring, healthcare, and security and defence. The project will involve the design and fabrication of innovative new detector platforms of Ge-on-Si and III-V semiconductor detectors. The detectors are capable of single-photon sensitivity in the SWIR and MWIR regions, and will be fabricated in detector array format. We will also examine superconducting nanowires to expand their operation into the MWIR regions and fabricate arrayed detector configurations. A key part of the project is to integrate these arrayed detector technologies with read-out circuitry capable of rapid, low latency delivery of single-photon data. In addition, we will utilise micro-optic technology to optimise detection efficiency and demonstrate multiple wavelength filtering. The cameras will be designed for use in a range of applications areas, including lidar, where the time-of-flight of the return photons can be used for the measurement of distance. In arrayed detector format, we will make cameras from which we will demonstrate three-dimensional imaging at long distance, where the sensitivity and time-resolution will enhance imaging through dense fog and other obscurants. We will demonstrate our detector technologies in quantum cryptography applications, where encryption keys can be shared between two users. By sending data encoded in single-photons it is possible for the sender and receiver to share a secure, random key known only to them. The most critical component in this form of quantum communication is the single-photon detector - we will demonstrate the use of our detectors both in optical fibre and free-space quantum key distribution scenarios. Other emerging applications in spectroscopy and biophotonics will be demonstrated.