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Postgraduate Research on Photonics as an Enabling Technology

Final Report Summary - PROPHET (Postgraduate Research on Photonics as an Enabling Technology)

[PDF version also attached]

The photonics industry in Europe accounted for revenues of €49 billion in 2006, a 12% growth on the previous year, prompting expectations that photonics production will soon exceed that of microelectronics in Europe. Photonics is an important enabling technology, with photonic devices such as semiconductor lasers and solar cells underpinning advances in areas as diverse as communications, energy, environment and life sciences. The rapid growth in the field is forecast to result in relevant qualified workforce shortages in the near future, a fact highlighted by the European Technology Platform, Photonics21, in its Strategic Research Agenda. The agenda identifies as key challenges for the next few years the training of qualified photonics researchers and the improvement of research coordination and cooperation across Europe.
This Marie Curie Initial Training Network, “Postgraduate Research on Photonics as an Enabling Technology” (PROPHET), aims to address these issues by training a cohort of 14 early stage researchers (ESRs) and 5 young experienced researchers (ERs) in the full gamut of skills required to undertake a career in photonics. The consortium, consisting of 9 academic partners, 4 full industry partners and 2 associate partners (one of whom is industry), are ideally positioned to impart these skills, with world-class expertise ranging from growth and fabrication, through structural and device characterisation and theory, up to instrument assembly and industrial applications. The strong industry involvement in the consortium ensures an application-specific, end-user target for the researchers’ training, and gives each network fellow experience of the industry workplace. The core skills of photonics can be applied across a wide range of applications, and this will be demonstrated by leveraging the same fundamental skills for specific applications in each of the four diverse areas of communications, energy, environment and life science. Each Work Package comprises a joint project involving 5 E(S)Rs, with industry-specific objectives.

The overall objectives of the Network are:
• To produce a cohort of well-qualified young photonics researchers with an industry-focussed and multi-disciplinary skill set
• To develop new mode-locked lasers for communications applications
• To advance the state-of-the-art in photovoltaic cells for energy applications
• To develop mid-infrared laser sources for gas sensing in environmental applications
• To improve the capabilities of optical coherence tomography for life sciences applications

The PROPHET network organised a total of 4 workshops, a summer school and a conference over the course of 4 years. The workshops addressed the topics of “Growth and Fabrication of Photonic Materials and Devices” (Lancaster, UK, October 2011), “Theory and Modelling in Photonics” (Pavia, Italy, April 2012), “Industry Applications of Photonics” (Paris, France, March 2013), and “Characterisation of Photonic Materials and Devices” (Berlin, Germany, March 2014). Attracting average attendances of ~40 people, they included tutorial-style talks from high-profile speakers, as well as soft skills activities for the Fellows, including presentation and outreach training and mock job interview exercises. The PROPHET Summer School was organised in collaboration with the iNOW consortium, and held over 2 weeks in Cargèse, Corsica in August 2013. The school had over 70 participants and 44 invited speakers, including Nobel Laureate Prof. Zhores I. Alferov, as well as the two PROPHET Visiting Researchers, Prof. Yasuhiko Arakawa and Prof. David Sampson. In June 2014, the final network conference was held in Cork, Ireland, with over 50 attendees, and a public outreach event. 9 of the 14 ESRs in PROPHET have already successfully defended their theses, with the remaining 5 expected to do so in early 2016. The network has published over 80 journal articles and 3 book chapters. The project website has received 12,400 visits from 6,900 users since its launch in Feb 2011.

WP1 Photonics Enabling Communications Applications
This workpackage aimed at exploiting mode locked lasers emitting in the O-and C-band for very high capacity data transmission. It involved fellows at CNRS LPN, TUB, III-V Lab, Tyndall, U2T (now FINISAR) and FVB-WIAS. Optimized growth of strain engineered QDot and QDash-based active media allowed the demonstration of high performance mode locked lasers. Hybrid mode locking and dual tone injection were investigated both theoretically and experimentally for InAs/GaAs QD-based mode locked lasers. Optimized devices were packaged and allowed in particular 160 Gb/s RZ DQPSK error-free data transmission. Frequency combs generated using InAs/InP QDash based mode locked lasers showed a high spectral bandwidth, leading to 160 transmission channels for devices with 10 GHz repetition rates. SSB-OFDM type data transmission showed a record aggregate capacity of 1.344 Tb/s per chip. Devices with DBR-based mirrors were further developed for their integration in Photonic Integrated Circuits.

WP2 Photonics Enabling Energy Applications
This work package explored novel solar cell geometries and materials for improved efficiency, and involved Fellows at USTAN/YORK, ULANC, IMM-UNICT, UPAV and ST. Highlights of the work include the world-first demonstration of a novel light-trapping scheme that affords patterning of both sides of the solar cell (based on an original idea developed by the ESR Fellow at USTAN), and an extended photoresponse beyond 1 μm wavelength achieved in GaAs based solar cells containing stacked layers of GaSb quantum dots in the n or p region. The first quantitative comparison of different light trapping strategies was also performed; we were able to show clearly where dielectric scatterers and plasmonic scatterers have relative advantages, and how to separate the desired absorption losses in silicon from unwanted losses in the metal. A particular highlight of the simulation work at Pavia was the insight that the highest performance solar cells could be achieved with thin (≈50µm thick) silicon because of improved electrical properties and an improved robustness against imperfections.

WP3 Photonics Enabling Environment Applications
This work package dealt with the development of novel diode lasers for high sensitivity optical gas sensing applications. The WP involved Fellows at ULANC, Tyndall, LPN, III-V Lab and PROCAL. Two different technologies were explored: InP based quantum dash lasers for gases with fingerprint absorption wavelengths up to 2 µm, and type II Sb-based quantum dot lasers for gases at wavelengths beyond 2 µm. A novel process was developed, based on high duty cycle semiconductor gratings, for the fabrication of laterally-coupled DFB lasers. This approach was implemented on optimized epi-wafers and produced single mode lasers with high spectral purity emitting at 1.986 µm. The lasers exhibited output power up to 4.5 mW per facet and low threshold current down to 65 mA, suitable for detection of NH3 gas, which is important for industrial applications. Similarly, a hybrid device structure was developed to fabricate long wavelength InSb quantum dot lasers by combining p- InAsSbP and n+ InAs plasmon cladding layers, which resulted in an improvement of the maximum operating temperature from 60 K to 120 K in pulsed mode. Type II InSb/InAs quantum dot diode lasers emitting at 3.1 μm were demonstrated and characterized for use in gas sensing.

WP4 Photonics Enabling Life Science Applications
This work package focused on the medical imaging technology called Optical Coherence Tomography (OCT), specifically aiming to realise new tunable laser sources for OCT at 850nm and 1060nm wavelengths. The WP involved Fellows at NCU, Tyndall-UCC, TUB, USTAN/YORK and Superlum. Multilayered quantum dot broadband Semiconductor Optical Amplifiers (SOA) at 1060nm have been developed, demonstrating a peak chip gain of more than 25 dB at the target wavelength of 1064 nm. The group at Tyndall-UCC introduced an interferometric technique utilizing a 3x3 coupler and square-law detection to resolve the phase and intensity dynamics of high speed swept sources. Clear mode-locking behaviour was recorded on the forward sweep associated with a narrow line-width, and high line-width chaotic pulsing was observed in the backward sweep. Superlum has developed a prototype tunable semiconductor laser for 1025-1095 nm. The group at NCU designed and constructed an OCT set-up using this light source. The device was applied to image and characterize immersed contact lenses, achieving an imaging range of more than 5mm. Measured OCT signal roll-off was 6 dB at 3 mm depth. These parameters enabled us to visualize soft contact lenses with high precision.

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