Below are the first tranche of projects for the 2020 intake, which will give you an idea of the types of PhD projects that might be available in the Centre for Doctoral Training. Because circumstances can change between now and the final catalogue publication in November 2020, not all of these projects may be available. Students indicate their preferences in January and projects are confirmed in February. These are preliminary titles and descriptions, which may change as the project is co-developed by the student and supervisor(s) during the spring semester.
1.55 μm lasers epitaxially grown on silicon for optical interconnects
Dr Qiang Li, Cardiff University
Achieving high performance, long lifetime, and electrical injection silicon-based lasers will deliver the last missing building block in silicon photonics, not only enabling optical interconnects, but also serving other growth markets such as cloud computing, light detection and ranging (LIDAR) and chip-scale wearable sensors. This project aims to develop high-performance 1.55 µm C-band quantum dot and nano lasers to benefit from the lowest fibre attenuations. Advanced heteroepitaxy techniques will be studied to yield highly strained nanostructures with superior optical qualities and low dislocation density III-V materials on silicon. The material and device characteristics and possible integration scheme with silicon photonics will be explored.
III-V quantum dot lasers grown on patterned silicon and SOI substrates by MBE
Professor Huiyun Liu, University College London
There is a strong demand for integrating of III-V materials and devices on silicon platform for high-efficient photonics light sources and high mobility electronics devices. However, there are significant challenges for direct epitaxial growth, including anti-phase boundaries (APBs), threading dislocations (TDs) and micro-cracks. In last few years, significant progress has been made in reducing these three types of defects with a few micrometre III-V buffer layers. In addition, the total thickness of III-V layers is limited to a few micrometres (< 6µm) by the formation of micro-cracks due to a mismatch in thermal expansion between the III-V epilayers and a Si substrate.
In this project, we will exploit the potential of patterned silicon to suppress the defect formation within the patterned area on Si or SOI substrates. Most importantly, we would like to grow nano-size holes within the patterned areas by grow group-IV materials, such as Si and Ge by using twin-MBE system at UCL to relax the strain between III-V layers and silicon substrates, and hence to solve the problem due to thermal expansion mismatch between III-V layers and silicon substrates.
Applying the Resonant-State Expansion to VCSELs and non-uniform waveguides
Dr Egor Muljarov, Cardiff University
The resonant-state expansion (RSE) is a novel theoretical method in electrodynamics which was recently invented in Cardiff [DOI:10.1209/0295-5075/92/50010]. It provides a new paradigm in physics, presenting a disruptive technology outperforming known computational methods, as demonstrated in [DOI:10.1103/PhysRevA.90.013834], and offering an intuitive understanding of the properties of a physical system due to an explicit use of its resonances. The key idea of the RSE is that it uses the eigenmodes of a basis system, calculated analytically or numerically, for finding the eigenmodes of a target system, in a rigorous and efficient way.
This project on computational electromagnetics will be focused on developing an efficient computational tool based on the RSE to describe light propagation through a layered optical system. This tool will allow us to calculate the electromagnetic fields and the eigenmodes of a system consisting of a large number of layers, where each layer is described by its own dielectric constant and may have additionally an in-plane inhomogeneity. One of the main applications of the developed computational tool will be an efficient calculation of the light emission and optical modes of vertical-cavity surface-emitting lasers (VCSELs), for their study and optimisation. Other applications may include non-uniform waveguides and photonic-crystal fibres.
Assymetrical Spacer Layer Tunnel (ASPAT) Diodes Integrated Circuitsode
Professor Mohamed Missous, University of Manchester
The ASPAT is a novel high frequency which was devloped at Manchester and Cambridge. The ASPAT diode is an excellent microwave detector combining wide dynamic range with low excess noise and a high temperature stability. This project aims to make the world’s first Monolithic Microwave Integrated Circuits (MMIC) based on quantum mechanical tunneling . This project will be conducted in collaboration with linwave ltd where a range of circuits will be made and tested for a variety of high frequency aplications ( up to 100GHz).
Broadband power amplifiers in GaN technology
Dr Roberto Quaglia, Cardiff University
Power amplifiers able to cover multi-octave bandwidths while maintain good efficiency figures are necessary in many radar and countermeasure systems. GaN transistor technology, thanks to its unique characteristics, is becoming the best choice for these applications, but more work on the design aspects must be pursued. Based on previous work on configurable PA architectures, this project aims at exploring further the characteristics of GaN, using initially technology from third foundry and potentially transferring the designs to Cardiff technology. In particular, having control of technology will lead to advantages in terms of device layout design and passive circuit requirements.
Developing high performance GaN based avalanche photodiodes by using a novel growth method
Professor Tao Wang and Professor John David, University of Sheffield
There is an increasing demand for developing GaN based photodetectors, which can find a wide range of applications in harsh environment gamma ray sensing, flame sensors, atmospheric ozone detection, communications, bio-photonics. Given the wide bandgap and the high breakdown voltage of GaN in theory, GaN has been accepted to be an ideal candidate for the fabrication of high performance avalanche photodiodes (APD) for the applications in both short wavelength and visible spectral regions. It is better than SiC as the bandgap can be continuously tuned over a wide wavelength range. Although high brightness blue LEDs have been commercialised, the crystal quality of GaN is still a great challenge for some particular optoelectronics which need to work under extreme conditions, for example, GaN based APDs which need to operate under a very high bias to amplify the photocurrent and detect weak signals. So far there have been only a few papers which show any signs of reproducible impaction ionization and avalanche multiplication. A fundamental challenge is that the critical electric field reported on GaN is far from its theoretical value (3 MV/cm) due to the material issues. Very recently, the Sheffield team has developed a novel two-dimensional growth mode benefiting from our high temperature AlN buffer technology which is different from the classic two-step growth approach. With this new growth approach, our GaN demonstrates an extremely high breakdown field of 2.5 MV/cm approaching the theoretical limit of GaN.1 This project aims to achieve the 1st true GaN APD worldwide.
1. S. Jiang, T. Wang et al, ACS Appl. Mater. Interfaces, 12, 12949−12954(2020);
Development III-nitrides HEMTs for microLED display applications
Professor Tao Wang, University of Sheffield
There is a significantly increasing demand on III-nitride based power electronics due to their intrinsic properties of high breakdown voltage and high saturation electron velocity. So far, AlGaN/GaN-based high electron mobility transistors (HEMTs) are developed mainly based on c-plane GaN where a large sheet carrier density is produced as a result of spontaneous and piezoelectric polarisations. As a consequence, such polarisations generally lead to a depletion-mode transistor, while it is ideal to have enhancement-mode GaN devices in practical applications due to safety requirements. Generally speaking, like GaAs based HEMTs the growth of an AlGaN/GaN heterostructure with a modulation doping along a non-polar direction provides a simple but promising solution, where the polarisations are eliminated and thus the sheet carrier density can be tuned simply through optimising the doping level. This HEMTs will be epitaxially integrated with LEDs or microdisk laser, aiming at ultra-fast response Li-Fi application.
Effects of plasma etching and plasma deposition on the performance of VCSELs
Professor Peter Snowton, Cardiff University
Vertical Cavity Surface Emitting Lasers (VCSELs) are commercially important with an increasing range of applications and are required over an increasing range of wavelengths. The plasma etching of the cylindrical VCSEL geometry and, following steam oxidation of the current and optical apertures, passivation of the exposed sidewalls using plasma deposition are key steps in producing high efficiency devices. The project will focus on developing efficient processes for new materials such as dilute nitrides and bismides for long wavelength operation. The student will also research how the etch process effects the subsequent steam oxidation process and how leakage current and subsequent degradation are affected by etch and passivation processes. We envisage new devices designs where the surface passivation layer also creates localised strain in the surface region affecting near surface recombination and refractive index. We also envisage developing processes to provide the greatest uniformity and yield over large area substrates. Students will utilise modelling packages such as nextnano and photon design for device design and understanding device results.
Energy Harvesting for Autonomous Systems
Professor Peter Smowton & Professor Gao Min, Cardiff University
A key part of the vision of the Internet of Things is the large number of autonomous sensors relaying information back through the web. To be truly autonomous a sensor must be able to derive energy from the environment to carry out the sensing action and to relay the information back to the connected system. Here we focus on the mechanisms by which communications based on light emission by LEDs or lasers can be powered by energy harvested from the environment. The PhD project will involve understanding compound semiconductor heterostructure devices and the absorption and emission of radiation, the change and exchange of energy and entropy in such a system, designing a laser or LED powered by the heat and light available in the environment, fabricating such device and demonstrating communication of information at some level.
Engineering interaction effects in semiconductor nanostructures
Dr Sanjeev Kumar, University College London
Recently discovered fractional quantisation of conductance in the absence of quantising magnetic field in one-dimensional semiconductors using GaAs/AlGaAs heterostructure has allowed many new quantum phenomena to be envisaged which were inaccessible before. This PhD project will involve investigating interaction effects in coupled and uncoupled bilayer electron gases. A bilayer system provides an outstanding platform to investigate quantum transport as the separation between electrons can be as close as the Bohr radius therefore the quasi-particles which give rise to a fractional quantisation in conductance could be investigated in a variety of interacting regimes. The project will involve answering a number of original questions as how nature allows electrons to self-organise and give rise to fractional quantisation and the spin and charge phases of quasi-particles including entanglement. The project is experimental as well as theoretical and will involve training in the cleanroom for high-quality device fabrication and low-noise measurements at extremely low temperatures and high magnetic field.
Enhanced mm-wave load-pull for characterisation of CS technology
Dr Roberto Quaglia, Cardiff University
CS technology will be winning its race with Si-based solutions if it will keep proving itself as the best choice in terms of performance for transmitters. At mm-wave, the intrinsic better properties of CS technology can translate in better output power and efficiency only if the design of power amplifiers exploits at maximum these properties. In order to a) guarantee that the devices have the potential to be outperforming competition, and b) provide designers with the needed information to exploit the outstanding characteristics of CS transistors, it is necessary to support technology with high quality transistor characterisation capabilities. EPSRC has recently funded £1.4m for a mm-wave large signal characterisation setup which provides a world-unique platform to perform these characterisation campaigns. This project aims at boosting the capabilities of the system by introducing i) hybrid active/passive capabilities, ii) modulated signal capabilities, and iii) further frequency extension. In collaboration with the company which built the original measurement system and an end-user of the technology, which provides clear steer on what is required from the system.
Experimental Investigations into Localization and New Concepts for Qubits
Professor Sir Michael Pepper, University College London
The key element here is the role of the interaction between electrons allied to disorder which results in a new form of electron localization in which their distribution becomes frozen. The unusual consequences of this state include a finite temperature zero conductance state never previously observed, one of the consequences is the localization of energy such that heat does not diffuse. The stability of the electron states in this regime may result in the slow and measurable spread of entanglement and a decoherence free qubit structure. In a similar manner to the localization of light in certain Cold Atom structures we will be able to modify the localization of energy with the speed of its transmission leading to a study of this process in a way not previously possible. Although there is a large amount of new physics to be investigated there are also a number of aspects which point to new methods of storing and manipulating quantum information. The different configurations of electrons allow themselves to be used as elements for storage of information in a way not previously considered which can be investigated. In this project, experimental samples based on III-V semiconductors (GaAs, InGaAs, InAs, etc) will be processed in the cleanroom and measurements will be performed at UCL. Advanced 111-V device structures will form an important part of this project which will seek to develop new device concepts for quantum information.
Exploring the nanoscale optoelectronic properties of low-dimensional materials via terahertz spectroscopy and near-field terahertz microscopy
Dr Jessica Boland, University of Manchester
Low-dimensional semiconductor materials are extremely attractive in the field of nanotechnology owing to their potential as building blocks for ultrafast optoelectronic devices, including solar cells and photodetectors. Dirac materials, in particular, have emerged as promising candidates for more energy-efficient devices, owing to their perfectly-conducting surface states and doping tuneability. However, to develop functional optoelectronic devices, an in-depth understanding of carrier transport in these materials is essential. Terahertz spectroscopy provides a perfect, non-contact, non-destructive tool for examining the electrical conductivity of a material and extracting key transport parameters, such as mobility, carrier lifetime and extrinsic carrier concentration. Recent advances have also pushed the spatial resolution of this technique down to the nanoscale. By combining terahertz spectroscopy with scattering-type near-field optical microscopy (SNOM), electrical conductivity and ultrafast carrier transport in these materials can be mapped in 3D with <1ps temporal resolution and <30nm spatial resolution. This project will exploit this technique to conduct the first investigation of nanoscale carrier transport in III-V nanowires and topological insulator nanowires. It will employ surface-sensitive measurements to examine the exotic surface conductivity response in Dirac materials independently from the bulk for the first time. This will provide a unique insight into the underlying physical mechanisms governing transport in these materials that will directly feed into development of next-generation devices (namely terahertz photodetectors).
GaN based mm-wave Integrated Receiver for Communications and Radar applications
Professor Khaled Elgaid & Dr Roberto Quaglia, Cardiff University
This project concerns the development of GaN based millimetre-wave arrays receiver technology that utilise pixels composed of antenna-coupled Schottky detectors for security, radar and communications applications. The aims and objectives of this research are to design, fabricate, experimentally validate, and integrate GaN based Schottky barrier diodes, with matching circuits and antennas into an array configuration. The project will investigate key active/passive devices and integrated circuit design and technology optimisations for performance. This includes: Schottky barrier diodes intrinsic and extrinsic parameters, transmission media, integrated power dividers combiners and integrated antenna. The project will be linked with industry needs and include student interacting with an industry partner in the form of devices/circuits design, characterisation and verifications.
Hardware-in-the-Loop GaN Power Amplifier Design, Realisation and Optimisation for 5G Systems
Dr James Bell & Professor Johannes Benedikt, Cardiff University
High-data-rate, high-efficiency front-end systems are becoming key components in realising highly efficiency, long-range telecommunications systems for 5G and beyond. This project will implement a novel hardware-in-the-loop Gallium Nitride power amplifier development procedure, that will automate designs for performance and predict output metrics, allowing for optimisation prior to amplifier manufacturing and testing. The project will utilise Cardiff University’s broadband characterisation capabilities, with advanced design software and LabVIEW used to control of the characterisation steps. Amplifier designs will be fabricated using compound semiconductor technologies and tested to verify the novel design techniques and design algorithms work. Optimisation of approaches will allow for rapid load-pull characterisation – a key element volume production using emerging device technologies for future applications.
Highly-efficient SiC-based Power Converter for Electric Vehicles
Dr Wenlong Ming, Cardiff University
In this PhD project, a comprehensive comparison will be made between Silicon Carbide (SiC)-based voltage-link inverters and current-link inverters for the application of Electric Vehicles. Two 50kW 600V SiC MOSFET motor drives will be built to demonstrate the comparison, i.e., one with a traditional dc link and output EMI filter and the other with a current dc link and combined EMC and voltage harmonic filter. The study will include analysis of the cost, size and EMC performance of both methods. Key innovations of this project come from the design of the filters and control scheme for voltage/current-link inverters and measurement of the whole system efficiency.
Hybrid inorganic/organic semiconductor hetero structures for high-performance LEDs
Dr Leszek Majewski, University of Manchester
An excellent progress has recently been made in the development and commercialization of semiconductor light-emitting devices that operate in the visible part of the electromagnetic spectrum, mainly due to the rapidly growing demand for displays, solid-state lighting, and related applications. Of particular significance are the developments in gallium nitride and organic semiconductor technologies. GaN-based inorganic semiconductor structures have found application as efficient, robust, short-wavelength-visible- and UV-light sources. Organic semiconductor light-emitting diodes (LEDs) in turn can span the entire visible spectrum and have low-cost potential due to high-throughput manufacturing, opening attractive prospects for displays and lighting. These two semiconductor families have their own especial advantages and disadvantages. Most notably, GaN structures offer excellent electrical properties but are inherently prone to the complex and expensive fabrication techniques that characterize inorganic semiconductors. Their spectral coverage is also limited, although down-conversion with phosphors allows access to other colours and white-light emission. On the other hand, organic semiconductors offer excellent luminescence properties, with a greater variety of emission wavelengths and significantly higher photoluminescence efficiencies, but exhibit less good electrical behaviour. Combining the best attributes of each family would clearly be an attractive proposition. The aim of this project is to explore novel combinations of inorganic/organic semiconductor hetero structures. This will involve characterisation of the employed semiconductor materials but also evaluation of the fabricated nanostructures and light-emitting devices.
Integrated Colloidal Quantum Dot (QD) smart optoelectronic devices for the Internet of Things (IoT)
Dr Bo Hou, Cardiff University
QD and related optoelectronic devices are promising candidates for IoT applications. Benefit from their customisable bandgap, solution processability, strong light-matter interaction and robust photo/electrical stability; QDs have shown their great promise in various optoelectronic devices. According to Touch Display, Research and IDTex recently estimated that the overall QD market would reach $10.8 billion by 2026. However, it is highly challenging to obtain a comprehensive and fundamental understanding of underlying physics before they can be well integrated into IoT technology. To develop QD IoTs with minimum power consumption and sustainable energy supply, three key challenges will explore: i) Low power consumption Cd-free QD LEDs; ii)High-detectivity QD Phototransistors; iii) the Cardiff University, Physics Department and Institute for Compound Semiconductors, have extensive experience in PV and integrated electronics/photonics which can be complementary by introducing high-efficiency QD indoor solar cells technologies to demonstrate an entire solution-processed integrated IoT module.
Low Energy Electron Microscopy studies of GaAs growth on Silicon. Search for alternative integration strategies based on direct observation of GaAs growth dynamics.
Dr Juan Pereiro Viterbo, Cardiff University
III-As semiconductors’ properties make them ideal candidates for the development of photonic circuits. The integration of photonics and current electronics requires epitaxial growth of III-As on Silicon. Up to now challenges associated to the different lattice parameters and atomic structures of these materials have hindered technological development.
Cardiff University hosts a unique Low Energy Electron Microscope combined with a III-As Molecular Beam Epitaxy reactor. This unique experimental system enables us to record videos with atomic resolution in the z direction and 5-10 nm resolution in the x/y plane of the III-As samples. The understanding of dynamic mechanisms associated to the growth of III-As on Silicon will provide an advantage to achieve full integration and understand the limits of epitaxial approaches.
This project consists of the study of growth of III-As thin films and nanostructures on Silicon substrates and will explore innovative alternatives to integration via epitaxial growth. During the PhD project the student will become an expert in Ultra High Vacuum systems, Molecular beam epitaxy and thin film growth and electron microscopy, as well as work on the modelling and simulation of growth processes. The project will suit more experimentally minded students.
MOVPE Growth & Characterisation of Sb-containing and InP-related III-V Compound Semiconductors
Dr Qiang Li, Cardiff University
IQE is interested in supporting a student in the following typical areas of interest where it believes Cardiff University has a strong background for underpinning research into Antimony-containing materials and also those involving Indium Phosphide (InP). These materials are prevalent in strategic Short-Wave and Mid-Wave Infrared detector and laser devices used in many sensing, imaging and spectroscopy technologies. Such technologies are prevalent in any defence, security and environmental applications, to name but a few. The project will involve supporting a student strongly associated with the MOVPE tool at IQE-Si, using a technique complementary to the existing MBE process already established within the university. We anticipate the research will target areas, similar to these: InP processes for Quantum Technology/SWIR-related products, QCLs in the MIR; Sb-based mixed alloy systems Al(Ga)AsSb on InP/GaSb for e.g. detectors, SPADs; or any Sb-related MOVPE development/improvement technology.
MBE-grown 1550-nm III-V quantum-dot materials and devices on Si substrate for Si photonic systems
Professor Huiyun Liu, University College London
Silicon microelectronics has been the engine of the modern information for almost 50 years. In our everlasting quest to process more and more data at faster speeds, while using the smallest components the $100 billion silicon industry has successfully overcome many critical issues. The next critical problem in the evolution of modern information system is to overcome the limitations of metal interconnectors. Recently high performance electrically pumped 1300-nm quantum-dot lasers have been successfully demonstrated on Si substrate between Cardiff and UCL. Pushing the lasing wavelength of III-V/Si quantum-dot laser to most desirable telecom wavelength of 1550 nm will be next challenge for epitaxial growth. 1550-nm quantum-dot laser will be exploited in this project based on our techniques on Si-based and Ge-based 1300-nm InAs/GaAs quantum-dot lasers, GaAs-based GaAsSb metamorphic 1550-nm quantum dots [Appl. Phys. Lett. 92, 111906 (2008)] and conventional InP-based 1550-nm quantum dots. Once practical 1550-nm III-V/Si quantum-dot device having been demonstrated, the integration of photonic component with Si waveguides will be studied for Si Photonic System, hence further integration with Silicon microelectronics. The methodology will focus on exploiting novel growth techniques by using new epitaxial facility, Molecular Beam Epitaxy on InGaAsP/InP, InAlGaAs/InP, and GaAsSb/GaAs quantum-dot system at UCL EEE department, device-processing facilities at LCN, and study the optical and structural properties by ATM and STM at UCL.
New Quantum Phenomena with Applications in Quantum Computation
Professor Sir Michael Pepper, University College London
This project is based around new physical effects which we have discovered in the past 18 months. Using semiconductor nanostructures we have made two discoveries with important implications for quantum information processing and the development of new quantum phenomena. The first is our demonstration that using quantum wires in which electrons are confined by gates the spins line up as in a magnet. This will allow the creation of new spintronic functions and also studies of integrated quantum circuits in which both spin and charge can be controlled. The second is our discovery of fractional quantisation of conductance in the absence of a magnetic field. There have been numerous theoretical suggestions that this might occur, more or less since the discovery of the magnetic field induced Fractional Quantum Hall Effect, but the surprising feature is that we found it in a system never considered before, the relaxation of one-dimensional electrons in a second dimension. The strong repulsion causes a line of electrons to split into two or more and then form a particular configuration which we are studying both experimentally and theoretically. This new configuration corresponds to the creation of a new charge entity comprising a number of electrons which behave as if it was a new particle with an effective fractional charge. This opens up a new area with applications in quantum computation, which may resist the decoherence which scrambles the information. In this project, experimental samples based on III-V semiconductors (GaAs, InGaAs, InAs, etc) will be processed in the cleanroom and measurements will be performed at UCL.
Optical Sources for Atomic Sensors
Dr Samuel Shutts, Cardiff University
This project will focus on developing optical sources required for alkali-vapour based chip-scale atomic sensors, including clocks and magnetometers.
The precise measurement of time is fundamental to the effective functioning of services such as transport links, mobile communications, financial transactions, security and reliable energy sources. Currently, Global Navigation Satellite Systems (GNSS) provide the timing signal for these critical infrastructure services but is vulnerable to disruption. As such, there is a need for wide-spread miniaturised atomic clocks to provide a timing reference in the event of an outage.
Measurement of magnetic fields, to pico-tesla precision is required for applications such as geophysical mapping, underwater ordinance detection and monitoring electrical pulses generated within the human body. Current magnetometer are bulky and dissipate substantial power in operation, due largely to the gas discharge lamp. Compound semiconductor light sources could reduce this power consumption significantly, whilst reducing the size and weight.
Next generation chip-scale atomic clocks and magnetometers, based on an effect know as coherent population trapping, require a reliable optical source with specific output characteristics. Vertical Cavity Surface Emitting Lasers (VCSELs) are the ideal candidates to meet the required specification and have the advantage of being compact and energy efficient. The student will have the opportunity to take part in this wide-reaching activity and develop novel ways to ensure that target specifications of VCSELs can be achieved. This will involve understanding the physics of atomic sensors, designing, implementing and testing methods to produce lasers with required characteristics, for example, single-mode emission, reduced spectral linewidth and polarisation stability.
Photoelectrochemical CO2 reduction on light-absorbing semiconductors
Professor Julia Weinstein, University of Sheffield
We seek to develop a photoelectrochemical cell for light-driven reduction of CO2. We have recently developed cheap electro-catalysts for CO2 reduction based on Earth-abundant Mn instead of Nobel-metals. The major disadvantage of these catalysts is that they are photo-unstable under light irradiation of <500 nm, and only work as electrocatalysts. To use light to activate the catalysts, we must use a photosensitiser, or a light-absorbing semiconductor electrode, which absorbs light and transfers electrons to the immobilised catalyst. Synthesis of the catalysts, including bespoke ligand design for versatile modes of anchoring to surfaces, will be done under the guidance of Simon Pope (Cardiff). TiO2 and Ta2O5 will be used to develop the methodology; then moving onto N-doped Ta2O5 and finally to visible light-absorbing semiconductors through wider collaborations within the CDT. Surface characterisation, photo-, electro-, and photelectrocatalytic studies, and advanced ultrafast spectroscopic characterisation of the new semiconductor-catalyst hybrids will be undertaken in the new LaserLab (Sheffield). Outcome could be a new system for photoelectrochemical CO2 reduction.
Photonic-crystal cavities and systems with 2D optical grating treated by the resonant-state expansion
Dr Egor Muljarov, Cardiff University
The resonant state expansion (RSE) is a novel powerful theoretical method that has been recently developed in electrodynamics [DOI:10.1209/0295-5075/92/50010], for calculating the eigenmodes of an optical system. The RSE has been verified and tested on optical systems of different material, shape and dimensionality. Furthermore, its performance has been compared with commercially available solvers, such as those using FDTD (Lumerical) and FEM (ComSol), showing on 2D and 3D examples that the RSE can be several orders of magnitude more computationally efficient [DOI:10.1103/PhysRevA.90.013834]. Recently, the RSE has been applied to photonic crystal systems with 1D periodicity and normal incidence [arXiv:1908.06916].
In this project on computational electrodynamics, the application of the RSE will be extended to systems with 2D grating and non-normal incidence of light. The high efficiency of the RSE will allow us to optimise the structural and material parameters of these systems for achieving desired optical properties and further manufacturing of grating systems by the ICS or the project partners. Another aim of the project will be applying the RSE to calculation of the high-quality resonant modes of photonic-crystal cavities. It is expected that a precise and efficient tool capable to supersede commercially available software will be developed during this project.
Photon-numbEr REsolving AvalanChe PhotodetecTORs for Free-Space Optics and LiDAR (PERCEPTOR)
Professor Diana Huffaker, Cardiff University
LiDAR (laser-based radar) systems are a major part of many real-world systems for three-dimension imaging [i.e. self-driving cars], topographic measurements from air and space along with atmospheric sensing. A crucial feature is “remote range-finding” needed to determine distance and movement of individual obstacles or targets. Standard single-photon detectors are ineffective due to inherent after-pulsing associated with Geiger-mode operation. However, an ultra-sensitive detector operating in linear (sub-Geiger) mode is capable of photon-number resolution (PNR) with few-photon sensitivity. This technique avoids photon-counting saturation and is proving important for long distance LIDAR and 3D imaging at a few-photon level.
This project (PERCEPTOR) seeks to demonstrate PNR functionality in the eye-safe regime (1300-1650nm) enabled by a new solid-state material, AlAsSb lattice-matched to the industry-standard InP platform. This work builds on our recent demonstration that AlAs0.56Sb0.44, has an extremely large ionisation coefficient ratio capable of very high-performance linear avalanche photodiodes (APDs) that surpass Si in noise characteristics and sensitivity. PERCEPTOR enables a PNR architecture with low cost, size, weight and power delivering a premium product for the emerging LiDAR ($4.1B by 2023).
Quantum dot lasers for next generation of datacenters and hard drives
Dr Nicolás Abadía, Cardiff University
Seagate has recently demonstrated a groundbreaking hard drive that can store up to 20TB and is ready for commercialization in late 2020. Behind this achievement lies the Heat Assisted Magnetic Recording (HAMR) technology . By temporarily heating the magnetic material at the time of recording, the areal density capacity of the hard disk drive can be significantly increased up to 2 Tbpsi . To heat the recording material, a laser is used inside the read/write head of HAMR drives. The goal of the project is to develop quantum dot lasers and coupling mechanism to excite the Near Field Transducers for HAMR drives. The PhD student will design and characterize a quantum dot laser and the coupling mechanism suitable for HAMR applications. Based on our previous research [1,2], the work will be carried out in close collaboration with the world-leading American company Seagate and University College London.  C. Zhong, N. Abadía, et al. ‘Effective Heat Dissipation in an Adiabatic Near-Field Transducer for HAMR’. Optics Express (2018). DOI:10.1364/OE.26.018842
 N. Abadía, C. Zhong, et al. ‘Optical and Thermal Analysis of the Light-Heat Conversion Process Employing an Antenna-Based Hybrid Plasmonic Waveguide for HAMR’. Optics Express (2018). DOI:10.1364/OE.26.001752
Quantum Integrated Circuits and Electrons Spins in Semiconductor Nanostructures
Professor Sir Michael Pepper, University College London
Proposed schemes of quantum information and computation utilise the spin of the electron either singly or in the form of double/triple quantum dots with more complex spin textures. It is proposed to investigate nanostructures based on GaAs, InGaAs and InAs which integrate a zero-dimensional quantum dot with a quantum wire which can act either as a spin polarizer or as a spin detector depending on the configuration. Single, Double and Triple Quantum Dots can act as qubits in quantum information schemes and the resistance of the quantum wire is sensitive to electron distribution nearby and so can act as a readout mechanism of the spin configuration such as singlet, triplet and a more general spin state. Simulations will be performed on the spin states of different quantum dot configurations and the accuracy of reading out the spin states with a quantum wire detector and possible applications as qubits in a quantum information scheme. In order to accomplish this the operating limits such as temperature, geometry and factors affecting the spin polarization will be investigated and calculated. For example, as the temperature is raised, or the carrier concentration in the wire is lowered, the spin direction becomes increasingly ill-defined and the incoherent regime is reached. At this stage the nature of transmission through the dot will be investigated as it is an open question as to how the electrons will be transmitted if the spin is not a good quantum parameter. The project will involve compound device fabrication and measurements at UCL.
Surface Activated Wafer Bonding for the next generation of hybrid devices
Prof Oliver A Williams, Cardiff University
Compound semiconductor devices are often thermally limited under high power conditions due to their modest thermal conductivity. Some piezoelectric compounds have relatively low acoustic wave velocities that limit the frequencies they can operate at, precluding their exploitation in 5G networks. Many of these problems can be alleviated by a hybrid materials approach, that is by combining materials with complementary properties. For example, diamond has been grown on GaN to yield three times the power handling capabilities or power transistors. Diamond layers integrated with Aluminium Nitride have produced the highest frequency performance of Surface Acoustic Wave filters which are currently be investigate for 5G base station filters. Growing one material on another is always limited by thermal expansion coefficients, as materials are generally grown at elevated temperatures. One solution to this is to grow the two materials independently and bond them together. In this project we will develop a Surface Activated Wafer Bond (SAWB) that will in principal allow any two materials to be bonded together. This process is carried out by polishing both materials atomically flat, irradiating them with an atom beam to create dangling bonds and then fusing them together under high vacuum at room temperature. We will focus on bonding diamond (grown and polished in house), Ga2O3, GaAs, LiNbO3 and GaN. Once the process is working it can be easily applied to different materials.
Theoretical modelling of optical nonlinearity in semiconductor quantum dot devices
Dr Sang Soon Oh, Cardiff University
Photoluminescent semiconductor quantum dots (QD) have been commercially used for the light emitting devices and also used as single photon emitting sources in quantum optic devices. However, there are still many issues to be resolved such as balanced and efficient carrier injection and long-term stability. Together with the optimization of electrical properties, the optical design of devices is essential to achieve required or better light extraction efficiency. In this project, the PhD student will study optical nonlinear processes in QDs of different materials in particular optical down conversion and dynamic excitation of carriers. To predict the optical properties of semiconductor QD devices, both numerical modelling on optical modes and analytical modelling for carrier and photon dynamics will be carried out. The developed models will be applied to improve the design of commercial on-chip quantum dot LEDs and analyse the optical properties of QD embedded single photon emitters.
Uncooled mode-locked quantum-dot laser monolithically grown on CMOS-compatible silicon substrate with Tbit/s transmission capacity
Dr Siming Chen, University College London
The high demand for large bandwidth and low power consumption optical networks between and within data centers lead to intensive efforts in the development of photonic interconnects employing WDM towards the realization of greater than 1 Tbit/s transmission capacity. Quantum dot (QD) semiconductor mode-locked lasers (MLLs) are a promising candidate for next-generation highly efficient and realizable WDM source due to their advantages such as ultrashort optical pulse trains at high repetition rate, wide coherent spectrum with a fixed channel spacing, compact size, low power consumption, and the ability for monolithic integration with silicon. This project aims to jointly investigate advanced epitaxy, QD technology and high-end nano-fabrication techniques to develop uncooled QD MLLs monolithically grown on CMOS-compatible silicon with multi-Tbit/s transmission capacity. This research supports the technological advance of the MLLs, manifesting itself as a compelling on-chip WDM source for optical interconnects in future low-cost, large-scale silicon PICs.