Leaders in Light
In 2016, the Royal Society’s Hamilton Memorial Prize for Early Career Research Excellence was awarded to DWC Associate Investigator Miro Erkintalo. Miro came from Finland to New Zealand in 2012 and received the prize for his contribution to nonlinear optics. The prize celebrated new theoretical approaches that have led to a series of international breakthroughs in the development of optical frequency combs.
Optical frequency combs are the most precise measuring devices in existence. By placing an unknown light source against a ‘comb’ of thousands of evenly spaced frequencies of light they can measure the unknown frequency to one part in several billion. Until recently, only well-funded labs could afford them as they required a table top of expensive lasers. But now a new technology has emerged to enable the development of cheap compact frequency combs. The secret is a tiny loop made out of a dielectric medium (such as glass), called a microresonator. When laser light with a single well-defined frequency (or colour) is shone on this loop, it creates hundreds, sometimes thousands of evenly spaced frequencies of light, perfect for an optical frequency comb. When microresonators were first discovered many top international research groups raced to develop them. But for years no one could make them work in a way that could benefit applications. The contributions of Miro and his colleagues Stéphane Coen and Stuart Murdoch (both DWC Principal Investigators) has enabled the whole field to shift forward.
"In the future, these microresonators could be incorporated into handheld portable devices, used to send data around the internet and look for planets outside our solar system."
When Miro first developed the theory that won him the Hamilton Prize, he wasn’t thinking of frequency combs. He was immersed in another field, that of optical fibres. It was only at a conference in 2013 that he came across microresonators. “There was a lot of hype and a lot of people working on them,” he said. “but the frequency combs they were developing were unstable, chaotic and noisy. What became apparent was that no one knew how to model them.”
Miro knew that there are two ways of thinking about optical experiments; one is to focus on the frequencies of light present; the other is to consider a short pulse of light that is formed by the superposition of all the different frequencies. Just a year before, Miro had developed a new theoretical approach that proved that these two points of view would lead to the same result. Together with Stuart, he ran experiments in optical fibres and proved this was true.
The microresonator researchers had been approaching the problem from the frequency perspective and were ending up with thousands of equations to solve and very little insights. Miro saw that using the short pulse perspective could be the answer. He rushed back to Auckland and started work on the problem with Stéphane, who had already obtained encouraging preliminary results. Focusing on short pulses made the whole experiment appear suddenly simple and clear. In 2013 they published their theory and it was a smash hit. Now all the international groups use their theoretical models.
That was the start of an exciting new episode of DWC research. The new theory revealed that, although thousands of times smaller, microresonators should behave very similarly to large scale fibre loops that had already been studied by Stéphane, Stuart, and Miro. The fact that such large fibre loops are much easier to control and study gave the team a unique opportunity to systematically explore the dynamics relevant to microresonator frequency combs. By sharing their expertise, Stéphane, Stuart, and Miro have been able to guide the top international microresonator groups to overcome the roadblocks and develop stable optical frequency combs.
In 2013, the group started making their own microresonators and running direct frequency comb experiments to complement their work. They are proud to now have the ability to create their own microresonator frequency combs in the lab.
“I think we’re the only people in the world that have simultaneous experiments in microresonators and fibre cavities, which gives us a unique advantage,” Miro said. “And we are very well received in both communities for our research.”