A shining light in photonic research

A project in Sweden is bringing several breakthroughs in non-linear optics research into play together and the potential for high-speed, high-bandwidth capabilities for telecommunications and the internet is extremely promising. Projects talks to Fredrik Laurell and Katia Gallo at the Royal Institute of Technology in Stockholm about this groundbreaking work.

For many people living in the lap of luxury would entail being able to download and watch any movie on high-definition TV (HDTV) any time they wanted – or to download any music and shop from the Web without enduring the bandwidth and time constraints of their PCs.

These are just some of the applications that could be enhanced as a result of pioneering research being carried out by physicists in Sweden into photonic crystals (attractive optical materials for controlling and manipulating the flow of light).

Their research has significant implications for all-optical signal processing: nonlinear optical chips afford signal processing capabilities which could remove speed and transparency bottlenecks in future ultra-high capacity telecommunications systems. This would ensure that the internet can cope with ever-increasing data traffic and bandwidth-hungry applications that could also include banking and e-medical care.

A project run by Fredrik Laurell and Katia Gallo at KTH, the Royal Institute of Technology, in Stockholm explores a new class of photonic crystals, based on structured nonlinearities, by bringing several breakthroughs in optics – nonlinearity, photonic crystals and integrated optics – into play together. Ultimately this will enable novel all-optical chips for applications ranging from telecommunications and quantum computing to biology and sensing.

Gallo talks about the advantages of using non-linear optics: “It enables us to control light by light. This is why it plays a key role in photonics – the science and technology to generate, transfer and process information by means of photons. Optical nonlinearities can allow photonics to do what electronics does on electronic signals by means of diodes and transistors in a chip.”

Gallo is an associate professor at KTH who has doctorates in electronic engineering and physics. She has worked in Italy, France, the US and at Southampton University in the UK, from where she was delighted to link up with Laurell, a professor in physics at KTH who is also Chairman of the Swedish Optical Society and on the board of the European Optical Society. He has been a pioneer in the field of non-linear optics since his PhD in 1990.

“The quadratic optical nonlinearities used in this project are traditionally employed for frequency conversion, ie, to generate new wavelengths (colours) from a laser beam,” Gallo says. “But they can also in principle enable all-optical devices, which control and reshape an optical signal by means of another (change its path, modulate its intensity, switch it on and off, etc).

As for the range of applications possible as a result of the research, it depends on the evolution of technology, but Laurell and Gallo can point to optical fibre telecommunication systems and laser physics, in particular all-optical signal processing in high-capacity networks (a field on which Gallo worked at the Optoelectronics Research Centre in Southampton, with which she still actively collaborates from KTH) and quantum optics. “The nonlinear photonic crystals we are developing can enable engineering new sources of multi-entangled photons in compact and fibre-compatible  devices formats [entanglement is the key ingredient for quantum cryptography and quantum communications].

“In the long term,” Gallo adds, “we want also to explore the possibilities offered by our technology towards optical lab-on-a-chip systems for applications in the bio-sciences, a field of excellence of Swedish research, with particular reference to the interdisciplinary research pole being built across KTH and the Karolinska Institute in Stockholm.”

Gallo says she knew of Laurell’s work and that it was a great opportunity to come to Sweden. “I could not have come here by any other means,” she says. “It has been refreshing to learn about new things, and from a research and development point of view it was a great thing for me.”

Laurell says: “Katia was a perfect match in terms of what we were doing. She now has a prestigious position here. My role is as a supervisor, though I would say I am more of a mentor here. Research is successful if you have the will, the talent and the curiosity.

“If you are hard-working, have a bit of luck, and good collaborators, you can go far. It is great to follow your interests, but you always have to be on your toes – always try to do better rather than fall back on your work.”

Gallo has funding for six more years, courtesy of her research fellowship from the Swedish Research Council (Vetenskapsrådet). “We were lucky to get these funds – it has been very useful for our group,” she says.

Within the context of the history of the study of photonic crystals since 1887, what is the significance of their work? “We are exploring a brand new approach to photonic crystals,” says Gallo. “A photonic crystal is a material patterned with periodic dielectric nanostructures. Its simplest, uni-dimensional, form [the Bragg mirror] was indeed studied as far back as in 1887, but the real ‘revolution’ came 100 years later when Eli Yablonovitch and Sajeev John extended this idea to 2D and 3D periodic structures.

“More recently the research on photonic crystals has been rapidly expanding to include nonlinear optical phenomena,” she continues. “To put it simply, the photonic crystal structure allows us to engineer the desired optical response of a device and the nonlinearity makes the latter tunable – i.e., you can change the response with an optical control.

“Thus far, the main approach to making nonlinear photonic crystals has essentially focused on periodic structuring of the linear properties [refractive index] in nonlinear media. With the Marie Curie IEF at KTH I have been looking into a completely different approach instead, i.e., making ‘purely nonlinear’ photonic crystals, created by periodic structuring of the nonlinear properties (without affecting the linear properties).

“This is a new field for research on fundamental physics of nonlinear systems, which could be engineered to mould light in time and space not through (linear) diffraction and interference (as in conventional photonic crystals), but directly by means of the coherent and local nature of parametric interactions mediated by the nonlinear lattice.”

How have they drawn on the study of biometrics to further their understanding of photonic crystals? Nature gives us some superb examples, e.g., the ideal, diamond-like structure of a photonic crystal found in the iridescent green scales of a beetle from Brazil in 2006.

“That is an interesting question,” says Gallo. “‘Traditional’ photonic crystals (i.e., linear structuring) were first made following Yablonovitch’s and John’s ideas, and it was later realised that similar structures were already present in nature (beetles, opals, butterfly wings, etc).

“Maybe, who knows, that will happen also for our purely nonlinear photonic structures,” she explains. “With a similar thought in mind, I have been monitoring the scientific literature in the last years, looking for some biological analogies of our artificial nonlinear crystal, yet so far have found no evidence of them. Maybe there’s no specific evolutionary advantage in such structures, or maybe one just needs to get better insights into nature and that will come up some time in the near future.”

Gallo says that she and Laurell can gain wider recognition for their work and obtain more funding by performing fundamental research to understand these structures and further refining their technological tools towards higher resolutions which will enable higher efficiencies and ever more sophisticated device designs. “This is what I shall do in the coming years.”

They will be looking for the best groups in the field to work with, as now they have secured long-term funding for their work – up to ten years, through the recently established Linnaeus centre for Advanced Optics and Photonics (ADOPT) in Stockholm, says Laurell.

Gallo sums up their project in a nutshell: “The basic support to address fundamental questions on materials and nonlinear optics would provide the necessary scientific base for gaining further recognition through the development of devices and applications, with national and international collaboration.”

Click here to access the project website.

Published: Monday, 24th October 2011