Playing with light: Scientific curiosity leads to breakthroughs

When a ray of light shines at a particular angle from water to air, as it hits the surface of water—which is denser than air—the ray is reflected back into the water instead of entering the air. This physical phenomenon, known as total internal reflection, forms the basis of the transmission of light in optical fibre. In a glass fibre, which consists of a denser core and less-dense cladding, light moves forward from one end of the fibre to the other as a result of total internal reflection.

The development of optical fibre communications has made possible many developments in the Information Age. For decades, optical engineers have been working to reduce the signal loss in fibre transmission. In 1991, Prof. Philip RUSSELL proposed the development of a kind of hollow-core fibre, which is now known “photonic crystal fibre” (PCF), and which offered the potential to reduce the loss even further. Unlike standard optical fibre, PCF has a periodic array of microscopic hollow channels running along its length. This microstructure enables properties that are unachievable in standard optical fibre. Three decades later, PCF is being studied in laboratories all over the world, and it has found applications in telecommunications, structural and environmental sensing, spectroscopy, biomedicine, and endoscopy, to name just a few.

In this issue, PAIR talks to Prof. Russell, PAIR Senior Fellow, on his three-decade-long journey of “riding the PCF wave”. Prof. Russell is now the Scientific Director of the Russell Centre for Advanced Lightwave Science (RCALS) in Hangzhou, China, having retired as the Director of the Max Planck Institute for the Science of Light in Erlangen, Germany. When asked what drove his venture into PCF, the scientist explained that it was simple curiosity. “Give academics freedom,” Prof. Russell said. He shared with PAIR his views on freedom in university research, the lack of which is, in his words, “a perennial problem”.

Light travels in a new way: Photonic crystal fibre

The unique structure of PCF enables light to be guided in a way that is unachievable in conventional optical fibre. How is this made possible?

The refractive index determines how fast light travels. If the refractive index is higher, light travels more slowly. Guiding light requires two different refractive indices. In conventional fibre, the difference between the two indices is just a few percent, and this small difference restricts the extent to which light can be controlled.

In PCF, the index difference is much greater, approaching 50%. As a result, when light hits the interface between silica and vacuum, it reflects more, and thus light scattering is stronger compared to conventional fibre. This greatly enhances our ability to control the flow of light. As a result, PCF has made possible all kinds of novel experiments on light-matter interactions at a fundamental level, mostly outside the purview of optical telecommunications.

PCF represents a revolutionary step forward in optical science, in which two different materials are put together in a novel way, resulting in a new substance with entirely new optical properties not observable in the materials alone.

Depictions of photonic crystal fibres guiding white light. Left: PCF with solid glass core. Right: Hollow-core PCF used in chemical sensing (Max Planck Institute for the Science of Light).

PCF finds application in many areas. How is PCF used in real-world applications?

The applications of PCF are many and varied. A key example is the use of solid-core PCF to realise a light source some 10,000 times brighter than the sun, containing all the colours of the rainbow—and more besides. This “white light laser” is known as a supercontinuum light source.

In solid core PCF, the large index contrast enables better control of the propagation of light guided in the core. When invisible ultrashort pulses of near-infrared light are launched into the core, nonlinear optical phenomena in the glass create new colours. A useful analogy is an audio system in which the volume is turned up to the point at which the sound becomes distorted and very unpleasant. This distortion is caused by the creation of new audio frequencies, as a result of the nonlinear response of the amplifier. At the current time, commercial supercontinuum sources based on solid core PCF are in use in almost every laboratory in the world, across many fields, for example, in medicine, microscopy and spectroscopy.

In conventional fibre, light travels entirely in glass, which causes Rayleigh scattering and absorption, resulting in signal loss and impairing the performance of telecommunications systems. Hollow core PCF, in contrast, operates in an “engineering sweet spot” where light spends almost no time in the glass, greatly reducing material-related loss. This is highly desirable in fibre communications, as it opens up the possibility of using shorter wavelengths, because Rayleigh scattering (which increases with the fourth power of the wavelength) is greatly reduced in the hollow core.

Another application of hollow-core PCF is extreme temporal compression of short pulses. Light is a wave, and one pulse normally contains many oscillations. In gas-filled hollow core PCF, pulses can be temporally compressed to a single cycle, with a duration of about 1 fs (a thousand-million-millionth of a second). By launching a short pulse of light into an argon-filled hollow PCF and controlling the pressure, we were able to adjust the chromatic dispersion so that all the colours travelled at the same group velocity. Not only could we compress pulses to very short durations, but we were able to generate pulses of very bright deep and vacuum-ultraviolet light. The ability to pressure-tune the dispersion has turned out to be a game-changer in the field of nonlinear fibre optics. These breakthroughs were the result of curiosity-driven exploration of nonlinear phenomena in gas-filled hollow-core PCF, and not a carefully planned research project.

One of the first experiments on supercontinuum generation in small-core PCF (inset), carried out at the University of Bath.

What are some of the challenges or future directions for PCF development?

Since the emergence of hollow PCF, there has been quite remarkable progress in reducing transmission loss. As a result, telecom engineers, who were initially highly sceptical, are now increasingly interested in using hollow-core PCF, given its advantages over conventional fibre. It is possible that, with more research, even lower transmission losses might be achieved in the future, making hollow-core PCF very attractive for reducing the complexity and cost of long-haul telecom systems.

Optical micrographs of the light guided in the first PCFs by photonic band gaps. The experiments were carried out at the University of Bath.

The wall between conservatism and innovation

A start-up you founded was acquired by ASML, the world’s largest supplier of lithography systems for the semiconductor industry. Lithography systems incorporate light sources based on hollow core PCF to build electronic chips. From your experience, what are some important factors in achieving successful scientific applications?

Most of my research over the years has been curiosity-driven. By bringing together disparate, apparently unconnected ideas, we dream up novel experiments, and then set about testing them in the laboratory. Sometimes they lead to potential commercial applications, and if they are sufficiently promising, we explore further by talking to a patent attorney.

Transferring ideas from basic research to commercial application is challenging. We might have what we think is a wonderful idea for a novel application, but it is often difficult to convince the university patent office that it is worth pursuing. This can be very frustrating. In contrast, in a company the barriers to filing a patent are much lower, and funds are available to defend it. Perhaps governments should consider setting up some form of national fund to support research institutions, pooling money from entrepreneurs, angel investors and individuals, to support patenting and related activities.

More universities are including scientific commercialisation (e.g., establishing start-ups of spin-offs) in their criteria for evaluating research performance. What are your views on this research management model?

Forcing creative people to think commercially can be a very bad idea, and can stifle innovation and result in an over-emphasis on incremental short-term research. Although some researchers are natural entrepreneurs, many are altruistically focused on extending scientific knowledge, not making money, and they need freedom to think, be creative and use their imagination. By indulging in “management consultancy” and focusing too tightly on commercialisation, universities risk killing off exploratory curiosity-driven research: the science of the future.

When scientists face pressure

There is growing expectation placed on university researchers to conduct research and deliver practical solutions. How can universities better support scholars in managing their teaching, research and administrative duties and pressure?

Give them as much freedom as possible, and try to balance teaching, research and administration according to each individual’s skills and interests.

Give freedom, step out and disrupt the status quo: Scientific collaboration and international mobility

Collaboration is one way to spur innovation in research. Universities encourage academics to pursue various forms of collaboration, be it interdisciplinary, intersectoral or interinstitutional. What are your thoughts on this trend?

Collaborations between research institutions can be really beneficial, provided that there is genuine complementarity among the cooperating groups, and that the benefits outweigh the costs of all the additional travelling and report writing. If these conditions are not met, collaborations can be more trouble than they are worth.

The best kind of collaboration is often spontaneous, bottom-up. One might meet someone at a conference, start talking, and together come up with an idea for a novel scientific experiment or a ground-breaking application. This underscores the importance of networking, conference travel and face-to-face discussions in research. Although online interactions are better than nothing, they cannot replace in-person meetings.

After completing my PhD, I was able to work at universities and research centres in the UK, France, Germany and the USA, expanding my horizons and my scientific knowledge, gaining fresh experience, and building a network of friends and colleagues. Sadly, in the current international climate, it is much less easy for many to travel freely abroad due to confrontational politics and the related visa restrictions. This is a great pity, as science is one of the few things that brings the world-wide community together, since it is one of the few things we can all agree on.

What are your recommendations to young researchers?

Aim to work on something else after your PhD, preferably in a different group; make the most of the opportunities as a researcher to travel the world; and never stop expanding your experience and your knowledge. Furthermore, work on building a network of fellow scientists by participating in conferences, workshops and summer schools.