Nanoscale Manufacturing Via Stretching Process

The underlying technology, a process called cold-drawing, has been used for almost a century in the production of synthetic fibres such as polyester and nylon, and has become a rather mature technique for producing fibres of high tensile strength and flexibility. Dr. Wei Lei and his collaborators, however, have found that under the right conditions, some materials embedded within synthetic polymer fibres naturally break apart into pieces of uniform length. The cause of this surprising phenomenon is a kind of wave that sweeps along the polymer fibre during the drawing process. This wave is known as a “neck,” since it propagates in the form of a depression with moving “shoulders” that continuously extend in opposite directions until they span the whole fibre length. As the shoulders propagate down the fibre, they pinch off any brittle threads embedded in the fibre, which break apart into short, uniformly sized segments, as shown in Figure 1. The result can be a bundle of short rods kept in place within the polymer fibre, or the polymer can be dissolved away to leave a collection of separated rods of precisely matched size and shape, which can be of a nanoscale size that would be difficult to manufacture by other methods. This represents a novel, scalable route to production of nano- and micro-particles of almost arbitrary cross-section. Remarkably, the cold-drawing process is highly robust and can even be done - literally - by hand! 

One potential application of this work is controlling the optical properties of macroscopic composite structures through dynamical and thermoreversible nano-scale mechanical processes. Another application which should be realizable in the near future is scalable fabrication of micro- and nanoparticles with arbitrary cross sections - at a level far beyond what is achievable with current chemical synthesis methods. Indeed, the work rests at the cross-section of many disciplines ranging from polymer mechanics to optics to nanotechnology, leading to an important path to scale up the production of larger quantities of nanoparticles, nanorods, and nanowires of a very wide variety of compositions.

                         Figure 1. Fragmentation via a cold-drawing-induced mechanical-geometric instability.​

Fibre Fabrication

Featured Research Capability

Finally, a Singapore home for optical fibre R&D

NTU is home to the only optical fibre fabrication facility in Singapore, and has strong ties with the University of Southampton’s Optoelectronics Research Centre (ORC), which is renowned for its expertise in fibre technology and understanding of photonics.

Little wonder then, that despite having only been officially launched two years ago, the Centre for Optical Fibre Technology (COFT) is well-placed to capitalise on a growing demand for fibre-optics sensors and lasers.

That is where COFT – the newest of five centres under The Photonics Institute (TPI) – comes in. A joint endeavour between DSO National Laboratories, the Agency for Science, Technology and Research (A*STAR) and Nanyang Technological University (NTU), it was founded to develop core capabilities and technologies for specialty optical fibre fabrication and characterisation.

Says COFT’s Director, Prof Shum Ping: “The development of such optical fibres is vital to advancing fibre-based devices, as off-the-shelf commercial fibres often cannot provide the necessary requirements for advanced applications.”

“COFT is uniquely positioned to provide these specialised fibres through working with individual researchers, groups or companies to understand their requirements, as well as designing and fabricating the fibres to suit their applications with our state-of-the art optical fibre fabrication and characterisation equipment.”

Developing a pool of talent for the Singapore photonics scene

Set up with the help of the ORC, COFT benefits from its close collaboration with the University of Southampton-based centre.

To date, eight COFT research staff have each spent at least a year at the ORC, where they received training that was invaluable in helping them to launch COFT when they returned home. One of them is Mr Daryl Ho, who joined COFT as a researcher straight after graduating from NTU.

He shares: “The opportunity to acquire new knowledge in a field different from my area of study and be a pioneer in starting a new initiative at NTU were strong motivations. However, the overriding factor was the offer of training in the United Kingdom from a renowned institution on the fibre fabrication process.”

Dr Sidharthan Raghuraman is another to have benefited from the ORC stint. He says: “I was fortunate enough to be part of the group sent to ORC, who are pioneers in the field of optical fibres.”

“We received extensive training in all aspects of fibre fabrication and characterisation from the experts. The knowledge and confidence we gained from our stint at ORC enabled us to hit the ground running on our return to Singapore.”

Dr Sidharthan is proud that, in a short space of time, the COFT team was able to “fabricate fibres which can rival the best that the industry can offer in terms of design, loss and performance”.

Today, the Centre is capable of fabricating a full range of optical fibres - from passive and active silica fibre, to soft-glass and polymer fibres.

A hub for optical fibre fabrication and fibre-based devices research

COFT’s achievements to date include local fabrication of the following specialty fibres:

· Air-core erbium-doped fibre, which demonstrates, for the first time, broadband operation of an optical orbital angular momentum (OAM) erbium doped fibre amplifier (EDFA) for lower-order OAM modes;

· Highly nonlinear Ge-doped fibre, demonstrating broadband supercontinuum generation with record high power;

· Hollow core fibre with split cladding design for anti-resonant effect;

· Rectangular core fibre with a large mode area that is induced to bend in only one direction.

These projects were spearheaded by principal investigator Asst Prof Yoo Seongwoo, a faculty member who is actively involved in the operations of the facility. Says Asst Prof Yoo: “It is essential that capabilities are developed quickly to fabricate a wide range of novel, high quality fibres. This would generate confidence among potential collaborators in COFT’s ability to deliver fibres that meet their requirements.”

Says Mr Ho, who assisted with the latter project in his role as a specialist in preform post-processing: “It was hugely gratifying to watch the shape of the fibre core being moulded into a rectangular one after several rounds of post-processing.”

“The properties of the rectangular core fibre make it useful for lasing purposes. High power fibre lasers have several real-world applications at present, including laser manufacturing, cutting, marking and engraving.”

Looking ahead, Prof Shum wants to push on and shape COFT into a hub for optical fibre fabrication and fibre-based devices research, saying: “Our Centre gathers fibre-based technology and applications in Singapore under one roof and has partnered overseas universities and research groups to develop ways to manufacture special optical fibres and related technologies.”

He believes that being under the TPI umbrella will aid this ambition, as COFT now receives more attention from members of the global academic and industry community. It can also “leverage on the critical mass of photonics research in NTU and foster interdisciplinary collaboration across research groups”.

Mr Ho agrees, adding that COFT will also benefit from TPI’s close links to the LUX Photonics Consortium, a joint initiative by NTU and the National University of Singapore, and supported by the National Research Foundation, Prime Minister’s Office, Singapore.

“Such connections can only be beneficial for COFT as they create potential opportunities for us to work closely with industry partners to deliver fibres of greater purpose and impact to society. At the same time, companies within the industry are made aware of the technologies and capabilities available at COFT.”

He is excited about the potential for growth in Singapore’s photonics scene, enthusing: “Light has the potential to be the next frontier of engineering and may even replace existing electronic technologies in several ways, such as in circuitry and components.”

“With efforts being made to encourage closer cooperation between the industry and research community, a bright future lies ahead for the field of photonics.”​