AUSTRALIAN FRONTIERS OF SCIENCE, 2005
Walter and Eliza Hall Institute of Medical Research, Melbourne, 12-13 April
Nanophotonics a key to next generation information technology and biotechnology
Professor Min Gu, Director of the Centre for Micro-Photonics, Swinburne University of Technology
What I want to do in the next
20 minutes is to start with the topic that Professor Ben Eggleton just
started with, the all-optic logic device we are developing under the
Centre of Excellence umbrella, and then to go to a similar topic, information
processing. Then I want to cross the boundary towards biotechnology. So you
will see the same technology we are developing in photonics can be used for
various applications.
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When you talk about nanophotonics, you can start with this academic discussion of what nanophotonics is, since this is still a cutting edge research area. But what I want to emphasise is that, when we talk about nanophotonics, basically we have to talk about nanoscale science. In other words, we have to confine the light in the nanoscale region. Second, we are going to use this nanoscale probe to interact with the material, so this material must be nanostructure material. These are the two key components.
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So how do I illustrate this point? I want to use this slide to start with. This year is the Year of Physics, and we celebrate the Year of Physics for Einstein. One of Einstein’s theories is for this light interaction with material, the photon idea. That is very important; it will actually lay the foundation for nanophotonics.
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We also celebrate this year another guy, Ernst Abbe. This person actually laid the foundation for the microscope. I know that the people who are sitting in this audience use a microscope quite a lot, and this is the person who laid the foundation for what would be the smallest structure you can see. He died 100 years ago, and I have just come back from a celebration of him in Germany.
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What is his contribution? His contribution basically is to say, ‘If you are using a lens, what is the smallest scale at which you can confine the light in the focal region?’
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In other words, if you go to this using a light and focus the light to slow the lens, you can get a very small spot. So a lot of nanophotonics work is trying to reduce such a spot as much as possible, and using such a nanoscale spot to interact with the material to make a lot of fabrication devices.
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So what we can do, if we can confine such a light into a small scale, is to make next generation telecommunications devices, as Professor Ben Eggleton said at the start; we can do next generation compact discs; and we can also do next generation bio-imaging and sensing.
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One particular example I want to illustrate today is ‘multiphoton excitation’, using the lens I just illustrated. What that means is this: you all use a microscope so you all do this fluorescence labelling. So you need fluorescence to image. If you focus light through this material, then you will see that the light is not, as you expected, a small spot. But if you use this multiphoton technology which I have illustrated here, then you can see a very small spot here. Based on that spot, you can do a lot of things.
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First of all we do the photonic crystal fabrication which Professor Ben Eggleton just talked about. How can we make this small structure, which you see has many small holes in the material? And, particularly, one of the concepts we have just heard about is to make the 3D photonic crystal, and we have to start such a structure inside a bulk medium. In other words, we have to have a nanoprobe to produce such a structure.
This is one example. You have a bulk medium and if you have such a multiphoton, two-photon excitation as I have just illustrated, then you can produce this localised effect. You use a microscope like an optical pen. So you can move along this microscope and then can produce such a small structure. So this is one of the devices in my lab.
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So what can we do? First of all we illustrated this a couple of years ago with such a technology you can produce a micro-kangaroo, or nano-kangaroo. This is a three-dimensional one. So this is not like a two-dimensional structure; this is a three-dimensional one in other words, this is a stereo structure. And in the background of this is a human hair. Two years ago we demonstrated that, with this two-photon technology, under the microscope, we can produce such a 3D structure.
Here you see another example we produced using similar technology, a mini Opera House. The size of this is actually smaller than real size by one million times. So this is 10 micrometres, about 106 times smaller than the real Opera House. And again this is stereo, so you can see this is a three-dimensional structure and you can see a lot of nanofeatures that we can produce in such a small scale.
So with such technology now we can produce three-dimensional photonic crystals. This shows one example: we have produced a three-dimensional photonic crystal, using similar technology to what I have just illustrated here you can produce this periodic structure. But if you compare with Professor Ben Eggleton’s talk, this is a three-dimensional one. We can produce such a three-dimensional structure.
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So, with just the one technique for the photonic crystal, what that means is that we have to manipulate light. This is very important. So we made this structure and we want to see whether this works or not. Here is an example to show that when you are shining the light through such a periodic structure you can see that some of the light actually can be suppressed, and some of the light actually can be reflected. So that means that such a structure, a new concept of 3D photonic crystal, can be used to manipulate the light. This is the proof that it works.
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So let me switch on that. If you take this optical device which Professor Ben Eggleton just illustrated, one of the aims is to increase the information transfer. If you increase the speed, then you have a lot of information. So you have to have a place to store the information. Imagine you have a lot of information, then do you store it? The current compact disc won’t work.
Just to start with a compact disc, you know when you use a compact disc the maximum capacity is 700 megabytes. Now, this is old.
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So, in order to increase this capacity, basically you have to move the light from the red to blue the DVD people have probably heard about the DVD technology: as you move from red laser to blue laser you can increase the capacity. You can also increase it if, as I said, you confine the light. And one of the ways you can confine the light is to increase this angle. If you do this, that is DVD technology.
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You have heard a lot of DVD. For DVD basically you reduce the spot size, trying to put many spots on the CD; you double the size and eventually that is a DVD. And you are also using a blue laser.
But in the end, if you think about what we are doing, all the current technology, the spot we will put on the CD is very, very shallow. So this is 0.0001 millimetres. And if you calculated the size of a DVD or CD side, the diameter and the thickness, and the total volume, those spots which are the information you are interested in recording on the CD occupy less than 1 per cent of the volume. In other words, most of the material on the CD or DVD is not used for information recording.
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So what can we can do? Using technology similar to that which I just talked about, using this diagram which I will talk about, using this as an optical pen, now we can record information inside the volume.
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So you can do this: you can record information not on the surface as you do currently on a DVD or CD, you can record it layer by layer. In this example, every 5 micrometres you can record the pictures on this slide into the bulk medium.
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You can do it erasable we are developing this one. You can record, erase it and then record again. So this becomes erasable.
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In this example, you can see we can record 26 letters; every five micrometres we are going to record this into a bulk medium – not on the surface but stacked inside the medium.
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In that way you can develop this 3DCD technology, for which Swinburne has created a company we call 3DCD. And there you see a comparison of what we expected as the 3DCD capacity and the DVD capacity. But again we only use the volume of the CD, the physical size.
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So if we want to think about the next 10 years, if this photonic chip works, then we have vast information. This 3DCD is not enough, so we have to think about more. If we think about more, we have to now use nanostructure material.
One of the projects I am doing now is using quantum dots I know a lot of biologists are also using quantum dots for biological imaging. You can use quantum dots to record information according to the size of the quantum dots. If we can do that, we can now expand the 3D recording into multidimensional recording, and, also in three dimensions, the volume.
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So that is the vision. We want to see eventually such a CD, where you can stack a lot of bits in the volume and each bit can also produce different colour information.
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In that way we can reach the density as we want to do of 1 petabyte. Shown there is the current CD size or DVD size compared with 1 petabyte. (You can see there that 1 petabyte is 1000 terabytes and another three orders of magnitude of gigabytes.) So this is another big project that we are trying to initiate, we are doing.
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This topic is the all-optical device, and I can see that if we can make this photonic chip work then we can produce a new information technology which means you can generate information using the photonic crystal concept like those photonic transistors, similar to electronic transistors but operating with a photon. We can do these photonic circuits you have heard a lot about, and process the signal, and we can use optical fibre to transmit its information in terms of speed of light. And, finally, we should record all these things somewhere, and that is the multidimensional optical storage which I just talked about.
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In the last few minutes of my talk, let me cross the boundary, because as the President just said, this is trying to cross the boundary to see interdisciplinary things.
I show here that if you do this multiphoton excitation, you can confine the light into a very small region. In other words, if this part is a biological sample, now you can gather the information at a particular position. So, if you want to do imaging, you can gather the image information at a particular position inside the biological sample. So clearly you can do a lot of things for a living sample.
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But I am sure in this Institute there are such things as a multiphoton microscope, so it is bigger things. It is a lot of money. You have to spend probably more than half a million dollars, probably towards one million, in order to make this multiphoton system work. You need a conventional microscope, you need a laser, you need a lot of things. This is a typical example for the multiphoton fluorescence microscopy.
But if you want to do such things, as I said, it is very useful for tissue image inside the tissue. In other words, we can use such technology for early cancer detection in the clinical environment. That is, if we want to do that we have to miniaturise these things. In what we are doing now, we want to replace all of the current technology by this optical fibre technology. So this is an example of the geometry we can use to replace these things by a micro-optics device.
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And if you can do that, as you can see here you can get, for example, very nice colour information at different tissues. This is rat muscle tissue. You can see clearly distinct colour information. That means you can use this colour information for comparison of the cancer/non-cancer information.
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In another particular example, this is prostate tissue. Not only can you see fluorescence information this is a broad peak of fluorescence but you also see some other optical signals which usually people would not be able to see with the conventional microscope, which means there is a second harmonic generation. So you can see a very strong peak from one particular position, and second harmonic. So simultaneously you can get new information which you wouldn’t be able to get using the conventional microscope, and then you can compare this signal to see the cancer development at a very early stage, because the second harmonic generation is sensitive to the molecular shape. If you move to other positions you won’t see such things. So this is a very sensitive imaging technology.
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To summarise my talk: nanophotonics actually is a very good tool, and I have just illustrated one example using multiphoton technology. And you can see that you can do a lot of things, including the all-optical device, the nano-CD which I was talking about as a petabyte CD and also nanosensing imaging for biological applications.
