High Power Uni-Travelling Carrier Photodiodes for THz Wireless Communications

To enable ultra-high speed and capacity wireless data systems (providing data rates of more than 10 Gbit/s), we need to look to higher carrier frequencies, in particular at currently unallocated regions of the electromagnetic spectrum above 275 GHz (USA) or 300 GHz (Europe), the Terahertz frequency range. One of the most promising techniques for compact, high power, broad band, room temperature THz sources is optical heterodyne generation (or photomixing) in ultra-fast photodetector, uni-travelling-carrier photodiodes (UTC-PD).

The current achieved highest THz output power from UTC-PD at 300GHz and 450GHz are: Two identical UTC-PDs and a T-junction are monolithically integrated in a single chip exhibiting peak output power of 1.2mW at 300 GHz; Travelling-wave uni-travelling-carrier photodiodes (TW-UTC-PD) delivers narrow-band output power of 0.15mW at 450 GHz. These reported advanced designs and technologies can cover wireless communication ranges of up to 10 metres, with data rates of 10~20Gbit/s. To cover wider ranges (eg: double the transmission distance) at higher data transfer speed, the THz transmitter's output power needs to be increased to at least twice the current level.

The objective of this project is to model and develop UTC-PD structures capable of higher output powers, to achieve 2.5mW at 300GHz and 0.3mW at 450GHz.

UTC-PD output power is dependent upon input optical power absorption distance, O/E (optical to electric) conversion efficiency and power loss in the device. To enhance optical power absorption distance and coupling efficiency, the absorption layer shape and structure, for example, planar multimode waveguide can be optimised. To increase the O/E conversion efficiency, high performance spot size converters can be developed. To control the device heating from optical carrier generation, epitaxial growth on relatively high thermal conductance silicon substrates can be considered. The two key concepts to be investigated in this project are: a) growing UTC-PD structures epitaxially on silicon substrates to improve heat-sinking; b) creating arrays of UTC-PDs on silicon to achieve spatial power combining.

Relevance to EPSRC thematic areas: Engineering (sub-theme: RF and microwave devices), Information and communication technology (sub-theme: optical communications), Physical sciences (sub-theme: optoelectronic devices and circuits).

The impact of the CDT in Connected Electronic and Photonic Systems is expected to be wide ranging and include both scientific research and industry outcomes. In terms of academia, it is envisaged that there will be a growing range of research activity in this converged field in coming years, and so the research students should not only have opportunities to continue their work as research fellows, but also to increasingly find posts as academics and indeed in policy advice and consulting.

The main area of impact, however, is expected to be industrial manufacturing and service industries. Relevant industries will include those involved in all areas of Information and Communication Technologies (ICT), together with printing, consumer electronics, construction, infrastructure, defence, energy, engineering, security, medicine and indeed systems companies providing information systems, for example for the financial, retail and medical sectors. Such industries will be at the heart of the digital economy, energy, healthcare, security and manufacturing fields. These industries have huge markets, for example the global consumer electronics market is expected to reach $2.97 trillion in 2020. The photonics sector itself represents a huge enterprise. The global photonics market was $510B in 2013 and is expected to grow to $766 billion in 2020. The UK has the fifth largest manufacturing base in electronics in the world, with annual turnover of £78 billion and employing 800,000 people (TechUK 2016). The UK photonics industry is also world leading with annual turnover of over £10.5 billion, employing 70,000 people and showing sustained growth of 6% to 8% per year over the last three decades (Hansard, 25 January 2017 Col. 122WH). As well as involving large companies, such as Airbus, Leonardo and ARM, there are over 10,000 UK SMEs in the electronics and photonics manufacturing sector, according to Innovate UK. Evidence of the entrepreneurial culture that exists and the potential for benefit to the UK economy from establishing the CDT includes the founding of companies such as Smart Holograms, PervasID, Light Blue Optics, Zinwave, Eight19 and Photon Design by staff and our former PhD students. Indeed, over 20 companies have been spun out in the last 10 years from the groups proposing this CDT.

The success of these industries has depended upon the availability of highly skilled researchers to drive innovation and competitive edge. 70% of survey respondents in the Hennik Annual Manufacturing Report 2017 reported difficulty in recruiting suitably skilled workers. Contributing to meeting this acute need will be the primary impact of the CEPS CDT.

Centre research activities will contribute very strongly to research impact in the ICT area (Internet of Things (IoT), data centre interconnects, next generation access technologies, 5G+ network backhaul, converged photonic/electronic integration, quantum information processing etc), underpinning the Information and Communications Technologies (ICT) and Digital Economy themes and contributing strongly to the themes of Energy (low energy lighting, low energy large area photonic/electronics for e-posters and window shading, photovoltaics, energy efficient displays), Manufacturing the Future (integrated photonic and electronic circuits, smart materials processing with photonics, embedded intelligence and interconnects for Industry 4.0), Quantum Technologies (device and systems integration for quantum communications and information processing) Healthcare Technologies (optical coherence tomography, discrete and real time biosensing, personalised healthcare), Global Uncertainties and Living with Environmental Change (resilient converged communications, advanced sensing systems incorporating electronics with photonics).