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Light-matter interactions on the nanoscale

Greg Blackman looks at the possible uses of nanophotonics, a branch of science that deals with structuring a material in such a wayas to impart unusual optical properties

How do opals get their iridescent sheen? The answer lies in the way light interacts with their structure. The gemstones are comprised of microscopic silica spheres ordered in such a way that the light bouncing off them interferes to give a brilliant colour. Scientists are trying to reproduce this effect artificially and design materials at the nanoscale that interact with light to give remarkable optical properties.

‘Nanophotonics is essentially about trying to change the way that light and matter interact,’ explained Professor Jeremy Baumberg, director of the Nanophotonics Centre at the University of Cambridge’s Cavendish Laboratory in the UK.

The scientists at the Nanophotonics Centre are developing all sorts of novel materials with nanophotonics, from textiles that change colour when stretched to biological sensors with applications in healthcare. Nanophotonics is also showing great promise as a way to integrate photonic components into the next generation of computer chips.

‘From a nanoscientist’s perspective, light is actually a big thing,’ Professor Baumberg said – the wavelength of light is several hundred nanometres. Nanophotonics aims to improve the way that light interacts with materials by changing the material’s structure at the nanoscale to get new optical properties.

One of the materials the Nanophotonics Centre has developed is a polymer opal. The centre has engineered a method of arranging polymer spheres into a crystal structure that produces similar iridescent colours to natural opals. The technology has been used to make textiles that change colour when stretched and the fabrics have even been featured in the Paris fashion show – London-based designer Amy Winters has incorporated the polymer opals into some of her clothes.

Opals can’t be manufactured; they’re made deep underground over millions of years. The group at the Nanophotonics Centre has developed a process that replicates this on a large scale and the scientists are trying to scale it up now for commercialisation.
So far, the researchers have produced some demonstrators and have even managed to make a kilometre of the textile. They’re now trying to do this at high speed, which is what is needed for industrial applications.

The polymer opals are synthesised by making small spheres around 100nm across that self-assemble. The spheres are dried out, processed and put through rollers. As the particles move past each other, they organise themselves, a bit like oranges in a crate, explained Professor Baumberg. The process generates a continuous sheet of material around 50µm thick containing millions and millions of spheres all in the right place, which can be done relatively cheaply.

The scientists at the Nanophotonics Centre are also engineering optical sensors that can trap light at an even smaller scale, around 1nm. The sensing technology is at an earlier stage of development than the polymer opals, but Professor Baumberg said that the researchers have demonstrated various potential uses for the technology.

The centre has just started to work with Addenbrooke’s hospital in Cambridge, UK, looking at applying the technology in clinical applications, including sensing extremely low concentrations of neurotransmitters present in urine. The neurotransmitters are indicators of health, but so far haven’t been accessible because they are too sparse to detect.

‘We’ve started to be able to measure all the different neurotransmitters at the same time in urine at clinical levels,’ explained Professor Baumberg. ‘What we can start to do is to make something [a sensor] where we can subtract what’s going on over time. It’s been very difficult to make sensors like that. Typically sensors are slow and don’t give you lots of quantitative information. We’re trying to do it in blood as well, and it’s that sort of technology we think will be really good for personalised healthcare.’

The device is a tag-free sensor, so doesn’t require molecular markers. It uses Raman spectroscopy to identify certain molecules present in a biological sample. ‘It’s a much simpler sensor technology [than using markers] and it’s very generic,’ commented Professor Baumberg. ‘You don’t have to always change the chemistry.

‘This way of trapping the light to a very small location [using nanophotonics] improves the sensitivity [of these devices] by a billion fold,’ he added. At the same time, the sensors are reproducible. ‘It’s this combination of tag-free and the reproducible way of making this optical sensor at the nanoscale that’s really important,’ he said.

The sensors are made by connecting tiny particles of gold with barrel molecules containing holes. The light is then trapped in between the gold particles. It’s more difficult to engineer these sensors than the polymer opals, but the concept of bottom-up assembly is the same. Here, the particles are designed with sticky ends so that they assemble correctly. ‘We know it’s possible, because it is what nature does, but we’re hopeless at it [bottom-up assembly] at the moment,’ commented Professor Baumberg.

Photonic integrated circuits

Another area of nanophotonics research is in photonic integrated circuits (PICs), and putting photonic components on computer chips. Sending signals via light as opposed to through electronic circuitry has the potential for higher performance computer chips, but there is a size discrepancy between the electronic and photonic components in PICs.

Professor Masaya Notomi at the NTT Nanophotonics Center in Japan has been working on using photonic crystals to guide light inside silicon chips. He will speak at the International Conference on Optical MEMS and Nanophotonics, held in Glasgow, Scotland from 17 to 21 August, about his group’s work on nanophotonics for large-scale photonic integration.

A photonic crystal is an artificial periodic structure where the periodicity is almost the optical wavelength inside the media. The advantage of these structures is that their optical properties mean they can make photonic bandgaps and, through this, effectively a photonic insulator. Photonic insulators don’t exist in nature, but photonic crystal structures can be engineered to be an insulator to confine light.

Professor Notomi commented that one problem with photonic integration compared to electrical integration is that it is very difficult to confine light. ‘We believe photonic crystals will be able to overcome this,’ he said.

The size of confinement is comparable to the wavelength of light in the material. If silicon is used, then the size of confinement is around a few hundred nanometres. The size of conventional photonic devices is much larger, typically several hundred microns or millimetre scale. Using photonic crystals, however, would reduce the size of photonic devices by almost three orders of magnitude, stated Professor Notomi.

‘Photonic crystals also allow us to strongly enhance light-matter interactions, and therefore reduce the power consumption of a PIC,’ he added. At the NTT, the group has demonstrated low power consumption by applying photonic crystals for different PIC devices.

The other goal is to integrate large numbers of devices on a small chip; otherwise photonic integrated circuits are not viable, Professor Notomi stated. ‘There have been a number of nanophotonic studies carried out recently that said that nanophotonics is promising for integration, but there are very few reports about large-scale integration,’ he said. ‘We want to demonstrate that nanophotonics can be integrated, and the easiest way to do this is by demonstrating arrays.’

The group has recently produced cavity arrays and multi-bit optical memories, also made of cavity arrays (the group fabricated a 100-bit memory using photonic crystals, reported in Nature Photonics).

The importance of being able to integrate large numbers of photonic devices on a single chip is so that many-core architectures with numerous co-processors can be engineered. At the moment, conventional photonic devices forming PICs are several hundred microns in size, meaning that one hundred of these almost covers the chip.

‘We believe we will need to fit approximately one million photonic devices on each chip,’ stated Professor Notomi. ‘That is a huge amount, because for the moment the largest number of devices per chip has only been a few hundred, using conventional integration techniques. So, we definitely need to introduce some breakthrough technology, and we think that nanophotonics is very promising.’

Although the research is still at a fundamental level, the size of the devices, which are around a single micron, would make it sufficient to fit one million onto a single chip. Also, the power consumption is very small, around a femtojoule per bit, so even if a million devices were integrated, the total power needed is not that large, Professor Notomi said. Initial results are promising, but Professor Notomi added that really large-scale integration has not been achieved yet.

Along with the small power consumption and engineering arrays, the third issue with photonic integration is material integration. In the case of electrical circuits only silicon and some metal are needed, but for photonic integration a lot of different materials are required because each device – the laser, detector, switch, etc – is based on different materials. All of this has to be integrated on the same chip, which is difficult.

To try and solve this, the NTT group has combined sophisticated etching and re-growth techniques to make a nanoscale functional material embedded in a photonic crystal. This approach is used for making lasers and memory.

Recently, the group demonstrated another approach consisting of combining a semiconductor nanowire with a photonic crystal. ‘We have put a functional nanowire at a specific point in a photonic crystal and we found that we can confine light strongly inside the nanowire. We can use this to make a laser or photodetector,’ said Professor Notomi. This technology was reported in Nature Materials.

Professor Notomi said that the group has successfully integrated single devices like the laser or memory, but that the next target is to integrate different devices on the same chip, i.e. make a photonic circuit consisting of nanodevices. ‘In the case of optical memory, we have integrated a number of nanodevices but each device is almost identical,’ he said. ‘Integration of different types of device is more challenging, but by doing this we could show some small-scale photonic integrated circuitry based on nanophotonics.’

He added that the timeline for real photonic networks inside a processor chip will be 10 or 15 years away.

The potential for nanophotonics is huge, but at the moment only relatively simple structures can be engineered, commented Professor Baumberg. The trick is to make more complicated structures that are more like nanomachines, he said.

The promising technology for achieving this is something called DNA origami. ‘We want to take a whole variety of different materials – supposing we want to take a bit of a semiconductor and connect it near a piece of gold and a piece of silver. Just three simple components that we want to put in the right location with each other, that’s been impossible up until now,’ Professor Baumberg said.

DNA origami uses a long piece of DNA, which can be bought commercially at a specified sequence of base-pairs. Staple strands are then added that bind closely to a particular portion of the DNA, forcing it to fold up in a specific geometry. The folded DNA will contain handles where nanoparticles can be attached. ‘It’s a bit more like Lego or origami or folding something up, and you can make it [the DNA construct] however you want,’ Professor Baumberg noted.

‘That technology is just breaking through in the last year, there are starting to be more and more people who are realising that you can make nanoconstructs with it, but nobody’s done anything terribly effective with it,’ he continued. ‘We’ve made sensors with it so far, but still at a very simple level. The question is how far can you go?’