The weight of light

COBRA is doing great. They’ve landed a twenty-million-euro Gravitation Grant, thanks to which the future of the research school is safeguarded for the next ten years. That time span will allow the club of Eindhoven electrical engineers and physicists to take on the issue of our ever-growing, energy-sucking digital data traffic - with photonics, the technology of light.

The Gravitation Grant is meant for consortiums of the very best research groups in a specific area of expertise - it’s only the best three percent that’s eligible for the grant. The grant proves that COBRA has built an impressive reputation over the past two decades. And the people involved feel that way, too. “Internationally, we hold an authoritative position in our field”, says main applicant Meint Smit. “In the world of optical communication technology, everyone knows COBRA.” His colleague and scientific director of COBRA prof.dr. Harm Dorren concurs: “We’ve become a brand”.

The status of COBRA is also corroborated by an independent body. Several years ago, an evaluation committee stated that COBRA doesn’t face any competition from other academic research groups in Europe. It’s only commercial laboratories and “one or two US universities” that are a match for COBRA.

In 1998, COBRA was acknowledged as one of six national Top Research Schools and has since operated in large part on funding from the Depth Strategy support program by NWO, as National Research Center Photonics. That program ended last January 1, says Dorren. “This Gravitation program, the Research Center for Integrated Nanophotonics, is really the successor of NRC Photonics.”

The most important difference is the incorporation of Physics of Nanostructures (FNA) of prof.dr. Bert Koopmans, and Plasma and Materials Processing (P&MP) of Erwin Kessels. Both professors are from the Department of Applied Physics. “Now the share of Electrical Engineering and Applied Physics within COBRA is more well-balanced”, says Smit. “And our scope has widened, too.”

“The COBRA groups continue each other’s work almost organically”

FNA and P&MP bring expertise to the COBRA table in the areas of spintronics and atomic layer deposition. Originally, COBRA knows a vertical structure. Of the three initial core groups, Photonics and Semiconductor Nanophysics (supervised by prof.dr. Paul Koenraad, who’s also the director of education at COBRA) develops new materials, which are subsequently processed into optic components by the group headed by Meint Smit (Photonic Integration, PhI). The third core group, Electro-Optical Communications (ECO, run by Ton Koonen and Harm Dorren), researches how those components might fit into full systems like data centers and networks that are the final link to the end user. “We continue each other’s work almost organically”, as Dorren puts it. And that’s exactly what the parties involved consider the basis for the success of COBRA.

With the Gravitation Grant, the TU/e researchers want to find a solution to a serious problem that’s quickly surfacing. According to Moore’s Law, the capacity of our computers grows by a factor of approximately one hundred every ten years. However, data traffic increases thousandfold in the same time. To prevent everything from crashing, the infrastructure is currently being expanded like mad: mega data centers spring up everywhere. Still, that process has to end at some point, if only for the fast-growing electricity consumption of those behemoths. Smit: “In the United States, data centers are already responsible for two percent of the total energy consumption. Taking the current technology into account, that percentage will rise to more than twenty within the next ten years. And that means trouble.”

Closer to home, inside even, limitations are tangible, says Ton Koonen. The last meters to the end user, through fiber-to-the-home and wireless networks, that’s where telecom networks consume most of their energy, he says. “And here, too, optical techniques can reach a higher capacity and save energy.”

If the current trend continues, either consumers will be paying much more for their digital services within the foreseeable future, or they’ll have to settle for last-century (download) speeds. Unless there’s a way to market a faster and more energy-efficient technology. And optical communication through light signals instead of electrical currents is just that. So the strategy researchers have come up with is to make all data communication between and within computers optical, down to the PCBs and chips. Eventually, only the processors will be electronic, and the entire data stream will consist of light.

That development is already in full swing: today, long-distance data transport is already done by means of optical fiber exclusively. In the next decade, COBRA wants to make important contributions to the integration of photonics in computers, data centers, and the last meters to the end user.

And it’s not just NWO that has contributed to the new Research Center for Integrated Nanophotonics; TU/e will be paying for the operating costs of the cleanroom (NanoLab@TU/e). The university has also granted COBRA the appointment of four female researchers from the WISE tenure program. Smit: “That will bring our percentage of female staff members to 25, which is quite an impressive score in our line of work”.


COBRA: Research Center for Integrated Nanophotonics

Goal of the new COBRA program is to improve the energy-efficiency and speed of digital systems by making as much of the data transport as possible optical (as light, with the use of new photonic technology). In line with the vertical setup of COBRA, the new technology is implemented at several levels, ranging from the manipulation of a few light particles to complete optical networks. A short overview of future plans.

One of the important challenges is the development of optical memories in which the data stream can be stored temporarily. Right now, there isn’t a good method to store information as light yet, says Paul Koenraad. “But it’s an interesting challenge to try to do so with spintronics (through the interaction between light and the magnetic properties of atoms, TJ), which is why Bert Koopmans’ group is involved in this project.”

“Forcing light into a much smaller space than it wants to be in”

We also have to bear in mind that the optical components have to be fitted onto ever-smaller electronics. That’s not easy either, Koenraad explains: “Electronic transistors only measure a few dozen nanometers, which is much smaller than the wavelength of light. We therefore have to find ways to force the light into a much tinier space than it actually wants to be in. Doing so requires new semiconducting materials, and structures with numerous thin layers of isolators and metals.” The latter issue will hopefully be resolved by the expertise of that other newcomer, Erwin Kessels. And then there’s Kessels’ colleague dr. Ageeth Bol, whose work on graphene may offer interesting possibilities for optical technology.

The ultimate step towards miniaturization is using only a few light particles (photons) to work with. After all, less light generally means a more economical system. Prof.dr. Andrea Fiore of PSN has been working on such systems. It’s unsure whether or not they will ever be implemented on a large scale, because they have to be cooled to a few degrees above absolute zero. Meint Smit doesn’t see a problem there, though: “Truly, Andrea is working on the verge of what’s possible, but he may well learn things we can use in other aspects of the program.”

On the other end of the spectrum we find the input of ECO, where Harm Dorren works on optical networks and optical switches in data centers (see also the article on doctoral candidate Stefano Di Lucente in Cursor 8). Within that same group, Ton Koonen is responsible for the final meters to the consumer. Those can be made optical too, wireless even, with the use of infrared light.

The level in between – the chips – is program leader Meint Smit’s area. “In the time of NRC Photonics we’ve learnt how to integrate large numbers of optic components on a single chip”, he says. Among other things, it’s led to an optic switch matrix including over 450 components, which has been developed in the group of prof.dr. Kevin Williams. “It’s one of the most complex chips that’s ever been created”, according to Smit. “Now we want to merge optical and electronic switches on one chip. We hope to realize that by developing a layer with an entirely optical network that goes on top of the electronic chip.” Every second, that layer will transfer hundreds of billions of bits to and from the processors (cores) on that chip. Part of this process is the development of small, fast, and energy-efficient light sources on chips: nanolasers, of which each chip will need thousands. That challenge will be tackled with the help of the group of Andrea Fiore.