AUSTRALIAN FRONTIERS OF SCIENCE, 2005
Walter and Eliza Hall Institute of Medical Research, Melbourne, 12-13 April
All-optical technologies: Towards optical logic!
Professor Ben Eggleton, Research Director, Centre for Ultrahigh bandwidth Devices for Optical Systems (CUDOS)
Given that this is a session with a theme, this first presentation will start off at the very high-level big picture to provide some very general context for the research that our Centre is involved in, followed by Professor Gu, who will focus on one aspect of the research at CUDOS.
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What is CUDOS doing? To start off, as I said, with the 100,000-foot or probably the 20,000-kilometre view: we all know the world is already networked very effectively via optical fibre communications systems, and this has really taken off in the last 10 to 20 years. We are all benefiting from the invention of the optical fibre, the invention of the laser, which allows us to transmit vast amounts of information – internet, telephony, wireless – through optical fibres that are, effectively, light-pipes. That works very, very effectively.
[SLIDE: What is CUDOS doing?] (second slide)
However, if one takes a closer look one can see that these optical communications networks are still being held together by what an optical physicist would say are clumsy hybrid opto-electronic solutions to functions, like routing and switching.
I show here an example of an actual router that is really the key to these communications systems and enables the distribution of information throughout a network. The heart of this router is electronic switching. And electronic switching, which is so effective for logic in computers, simply can’t keep up with the speeds that our communications networks are growing at.
Furthermore, although the electronics in principle are very small, switching generates enormous amounts of heat and you suffer a number of practical problems – in particular, air conditioning and so forth become real practical issues.
[SLIDE: What is CUDOS doing?] (third slide: Currently: micro electronic chips)
Now, if we could shrink down that router to something the size of a thumbnail, we would find that the electronic switching really starts to bottleneck as we approach the bandwidths and the speeds that we anticipate over the next decade. What CUDOS is doing is working on an alternative solution. We call this the ‘micro photonic chip’. Let’s look at those words briefly.
‘Micro’ implies very small. Specifically, as a micron it is a thousandth of a millimetre. Our research is focusing on very small-scale photonic structures.
‘Photonic’ implies that we are maintaining information in the optical domain. Rather than transferring the signal from optical to electronic, performing the switching/signal processing electronically, if we maintain the signal in the optical domain at all times we take advantage of ultra-fast optical switching that can be thousands of times faster than the switching underpinning the electronic routers.
[SLIDE: What is CUDOS doing?] (fourth slide)
This microphonic chip that we are working towards can be microscale, and so the research we are doing is studying the physics of light propagation in microscale structures, microfabrication, and theoretical studies of light propagation in these systems.
You can see here, schematically illustrated at a very crude level, that the bottleneck we saw in the electronic router is not occurring in this optical system.
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It is worth actually drawing an analogy with electronics. Since the Second World War we have seen a revolution in electronics following the invention of the transistor at Bell Labs. We have seen a progression from initially very clumsy valves that looked nothing like the electronic solutions we see in current systems. We saw a very rapid evolution towards integrating multiple functions into a single microelectronic circuit, and microchips getting smaller and more compact every year. In fact, as we now speak, palm pilots – which are my favourite electronic toy – are really starting to improve our efficencies for data storage and even communications.
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Standing back: we have seen the evolution from large-scale computer systems. One of the earliest computers, many decades ago, took up more space than my laboratory in the School of Physics at the University of Sydney, and offered very, very low-power computations. We see now laptop computers and so forth that have incredible speeds, incredible hard-drive memories. We have seen an incredible progress over the last half a century.
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Why can’t we see the same evolution in photonics, where we can achieve multiple functionalities on a single chip but replace the electronic switching, which is inherently limited by the physics of the semiconductors that underlie the switching in electronic systems, by ultra-fast optical switching? In a sense, this slide shows an optical chip cartoon/diagram. It doesn’t represent a real structure but gives you an idea of the photonic chip that we talk about.
These are optical fibres, these specific fibres are microstructured fibres, a unique class of fibre, and on this chip you can see a range of different structures fabricated by a number of different lithography technologies that CUDOS researchers are performing. You see here a demultiplexer, an optical switch and optical logic and so forth.
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Why can’t we tap in to the enormous success of microelectronics and the actual fabrication paradigm, if you like, that was developed for the microelectronics industry? Here is an example of a clean room. There are thousands of clean rooms like this around the world that use state-of-the-art electron beam lithography, photolithography, etching and so forth to fabricate microelectronic circuits. Can we use the same platform to fabricate photonic circuits and photonic chips? The answer is, in fact, yes, we can. And that indeed is what we are doing.
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Now, standing back a little bit and thinking about the benefits to society of communications systems and more bandwidth: my first comment is to say that it is interesting, looking back in history, that it has always been very difficult to anticipate and predict the benefits of better communications infrastructure. Through history there has always been a debate. The conservatives will say, ‘How can we possibly benefit from more bandwidth than we have at this present day? I can’t imagine new services, I can’t imagine anything out there beyond what we have now.’ On the other hand, there is always this desire for more bandwidth. We have seen continued growth in bandwidth over a century. Services do appear and they are emerging. And I want to talk very briefly about some of the directions that we are seeing.
Having said that, I find it fascinating to go back a few centuries and look at some great quotes. This is one that I stumbled across:
Why has no serious trial yet been made of the qualifications of so diligent a courier? And if he should be proved competent to the task, why should not our kings hold councils at Brighton with their ministers in London? Why should not our government govern at Portsmouth almost as promptly as at Downing Street? Let us have electric conversazione offices, communicating with each other all over the kingdom, if we can.
So this is almost 200 years ago, someone anticipating the benefits of high-speed communications systems.
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One of the benefits that we are starting to see right now – and this is a benefit or an application that I am looking forward to very much, and this does exist, although right now it is an expensive solution, not a solution that many people can access – is in the form of e-meetings, relying on the high-speed communications systems to allow people to get together without necessarily having to travel.
Now, you can’t replace the networking that is provided by actually getting together and meeting people, but I am really looking forward to a world where we can benefit from a high-speed communications system that provides us with this type of capabilities.
I show here a fellow who is, I guess, having what is a very effective conversation with his colleagues – someone out in the field, someone in an art gallery, and someone on the top of a high building in the city, presumably an architect or an engineer. I’m not sure what they’re actually talking about, but you get the idea that communications systems are breaking down barriers and allowing people to work more effectively, and we can look forward to more of this, going forward.
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The idea of true telecommuting, one of the big benefits of communications systems, is emerging. And we are starting to see this used somewhat ubiquitously in industry.
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E-teaching is another good example, where high-speed communications systems allow students, in particular, to communicate with each other – they provide teachers with an ability to teach remotely, and provide students with an ability to communicate with each other, both within their classrooms, across the country and internationally. I think that is a very important, tangible benefit of communications systems.
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Finally, here is an example that I think is particularly relevant to Australia. This is another example that is being talked about all the time, and I am sure that in this audience you can talk about this more intelligently than I can. It is the notion of e-health, taking advantage of very, very high-speed communications systems to provide remote surgery and so forth. This is an example of an application that is emerging and that will benefit as we continue to advance the communications systems of the future.
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The Centre of Excellence, CUDOS, is a collaboration of five university groups across Australia. It is a big team of people working collaboratively on a number of key projects that are focused on building the next generation devices that will replace these electronic switches, enabling us to make microscale optical solutions that will enable the continued growth in bandwidth for these next generation communications systems.
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We have a broad foundation, shown on the left-hand side of this slide, that relies on our skills and experience in lasers, materials, lithography – lithography is simply printing structures, we use the same lithography that is used in the microelectronics industry to print microscale and nanoscale structures. Photonic crystals are a class of metamaterials that have analogous properties to semiconductors. Properties that semiconductors have for electrons, these photonic crystals can have for photons. They allow us to control light on a very small scale.
We have access to some of Australia’s state-of-the-art microfabrication facilities, and we are using these facilities to fabricate devices. We are focusing on these applications for these next generation communications systems. And the ultimate vision of a photonic chip can be realised if we can pull together this team, fabricate a device that in practice will look something like the one on this slide – obviously, it won’t have the colours that I have added. These devices will have bandwidths that are hundreds if not thousands of times faster or higher than the bandwidths provided by the electronic switches and routers of existing systems.
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Just to look at some of the research that this entails: obviously, at this scale the propagation of light is still classical but requires very sophisticated solutions of the governing equations, Maxwell’s equations. We have a very strong program in electromagnetics of light in these systems.
Here is a photonic crystal, the metamaterial I talked about. It comprises holes that are drilled into a substrate. The period of that structure is about a micron, so it is a pretty small structure. You can see that the ability of light to bend very sharply around a corner is one of the very fascinating properties of these structures.
We are developing very novel fabrication techniques so that we can fabricate structures on a scale of microns. An example could be holography. Holography allows you to create resonant periodic structures, where the period is determined by the interference angle in that interferometer you can see.
We are using very novel optical fibres; we are taking advantage of a new class of optical fibre called holey fibres, or microstructured fibres. They are essentially like standard fibres that we use in communications systems but, as you can see, there are holes that run along the length of the fibre. These holes provide some unique guidance properties, and we are taking advantage of these fibres. As one application we are using these fibres to actually interconnect from the standard fibre that is used in the backbone onto these chips.
One of the fascinating areas of research that represent great potential for this idea of microphotonic circuits is using these photonic crystals, these metamaterials, that are periodic structures. This an SEM of a photonic crystal structure fabricated, using lithography, into a silicon wafer. The period of this structure is about half a micron. It resembles some of the semiconductors that we are familiar with in the context of solid-state physics. The only difference here is that the lattice constant, the period, is about 1000 times longer, roughly the wavelength of light.
This line defect here represents the circuit path to the light, and this is a junction where the light is split into two different directions. You can start to see an analogy with electronic circuits. We are actually controlling light in much the same way that electronic circuits control electrons.
We are using novel laser fabrication processes to micromachine different structures into different materials. Here is an example of a hole drilled into a metal. The structure period here is about 10 microns, but we can fabricate structures with periods of around half a micron, and these structures are very well suited to this photonic circuitry. Professor Gu is going to talk about that in the next presentation so I won’t focus on that.
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Finally, let me talk about this photonic crystal metamaterial that I talked about. This is really one of the central themes of our research. These are materials that are periodic – that is, there is a lattice with a period roughly the wavelength of light, that is, about half a micron, and just as X-rays can undergo scattering in solid-state materials, light will undergo sprag scattering in these materials.
We are not so much interested in the scattering properties but rather the confinement properties of light in these systems. Here is an example of a beam propagating through a structure along a defect. By simply removing one rod we are able to turn that beam around a corner at 90°. This is believed to be one of the unique properties of photonic crystals.
This is a simulation solving Maxwell’s equations, where you can see a very sharp bend. The scale of this system is about 10 microns by 10 microns. It is worth drawing a comparison here with a single mode fibre. I don’t have an example, but single mode fibre is like a fishing line. This is what we transport data with. If I wrap that around my finger, the loss is enormous. The bend loss of a single mode fibre is a few centimetres. Here we are bending light around a very sharp transition of a few microns, so this is taking advantage of this metamaterial that I have discussed.
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These materials can possess very strong dispersion. Dispersion is a property of a material that can be used to spread pulses of light. The pulses of light are really the bits of information that we propagate through our communications network, so these metamaterials can manipulate, reshape short pulses of light. This allows us to do some basic signal processing applications in the communications systems.
We can even think of constructing very, very small-scale dispersive elements. Here is a prism, much the same type of structure that you learnt about in high school physics, maybe undergraduate physics. A prism will disperse a white light beam into its different colours, the colours of the rainbow. This prism is made out of this metamaterial. The metamaterial has very strong anisotropy, it has very strong dispersion – it has a dispersive power several orders of magnitude larger than the dispersive power of a classical prism – and it can be made on the scale of 10 microns. This is a very important building block for microphotonics.
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The final theme – and this is something that it is difficult to go into in this presentation – is that we are taking advantage of very strong optical nonlinearities in these materials. We are placing optical nonlinearities inside these metamaterials; these nonlinearities have been enhanced by many orders of magnitude. This is what enables us to do this very fast optical switching which is the key to the high bandwidth for these solutions of the future.
[SLIDE: [series of photos, beginning with two people in lab]]
Finally, we are tapping in to state-of-the-art fabrication in Australia to fabricate devices using lithography. We are using this lithography to construct structures on a scale of microns. Here is an example of an Opera House – it is not a photonic structure, it is not an optical router or an optical switch, but it is a good example of what we can fabricate using this lithography, using this microfabrication. And if we can fabricate something on this scale (I think you can see Nicole Kidman walking up the stairs there) you can certainly think about fabricating devices that manipulate light in these photonic chips.
We are fabricating metamaterials, using focused ion-beam lithography. You see here a material with a period, again, of about half a micron.
We are modelling light propagation in these structures and we are discovering some very fascinating properties of light in these systems. This image is about solving Maxwell’s equations in a metamaterial system.
And here I show an example of light scattering off one of these metamaterials. This is a very sophisticated simulation that is run on a cluster up at the ATP in Sydney.
These structures allow us to control and confine light in very unusual ways. Here you see, effectively, a beam-shaping application where this metamaterial is used to beam-shape light out of the photonic circuit.
We are performing experiments on these devices and structures. Here is another example of a device that is fabricated on a scale of microns, about 10 microns across. This is a pyramid structure that Professor Min Gu will talk about.
And we are using optical fibres in very unusual ways. I show here an example of an optical fibre that initially had a scale of about 100 microns, the size of a human hair. It has actually been scaled down by a couple of orders of magnitude to create what we call a photonic wire. The dimensions of that wire are submicron, and in this regime these photonic wires behave very similarly to electronic wires in terms of controlling photons. These photonic wires represent a potential path to building photonic circuitry. You can see the general idea here of a wire sitting on a substrate and constructing what is a pretty sophisticated photonic circuit.
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I will end my presentation with a vision of the future for CUDOS, an artist’s impression of a photonic chip that comprises a number of different systems and structures, with novel optical fibres for coupling.
