Nanophotonics is living up to the hype. The study of light on the nanoscale might have been a ‘buzzword’ within optics circles a couple of years ago, but this tiny science is now moving away from the world of theoretical science and new research facilities are popping up in laboratories around the world.
And, with it, nanophotonics brings a myriad of new nano-prefixed buzzwords, including nanocapacitors, nanoforests, nanorice and nanoshells. But the real buzz is around the applications that using light as a tool on the submicron scale could open up.
The idea of creating light from nano or ‘meta’- materials could pave the way for a new wave of miniaturisation, similar to that seen in the semiconductor industry towards the end of the 20th century. Such advances are already allowing researchers to create lab-on-a-chip systems, where components such as miniature lasers, sensors and MEMS apparatus can be built on a semiconductor wafer. And, in the coming years, such systems could be used in a huge variety of applications, such as medical research and diagnosis, where these tiny tools could be used for instantaneous sample processing and analysis, and nanophotonics could produce tiny yet efficient solar cells, which should be popular in the current eco-chic age we live in.
As nanophotonics research begins to step up a gear, the field is branching into two categories: first of all, there is nanophotonic engineering, which is the fabrication of micro-photonic structures on silicon, such as lab-on-a-chip devices and their counterparts; secondly, we have physical nanophotonics, which is the science of manipulating light on the nanoscale.
These two categories are also creating a geographical divide – with the US and Japan leading the way for nanophotonic engineering, while Europe is fast becoming the home of physical nanophotonic research. Over in the US, at the University of California- Berkeley, Professor Xiang Zhang’s research lab is producing tiny optical components that sound like they would be more at home in the pages of a Marvel comic book.
One of the recent major breakthroughs for the group was a ‘hyperlens’, which brought researchers one step closer to nanoscale optical imaging, as it can produce magnified images of objects smaller than the wavelength of the imaging light. Made of silver and aluminium oxide, the hyperlens can capture, preserve and magnify the minute details contained in evanescent light waves. Speaking in March 2007, Zhang said capturing information carried by evanescent waves is ‘the holy grail of optical imaging.’
‘The hyperlens shows a new way to beat the diffraction limit, which would allow biologists to not only see a cell's nucleus and other smaller components but to study the movement and behaviour of individual molecules in living cells in real time,’ Zhang added. ‘In technology, this could eventually lead to higher density integrated circuits and DVDs.’
The Berkeley group has also developed an alternative approach to far-field ‘superlenses’, where the conversion from evanescent to propagating waves is achieved through scattering on a corrugated surface. Although it is still too early to tell whether the hyper or the superlens will prove most useful, Berkeley is one of the US hotspots for sub-wavelength optical imaging.
The US facilities are not only producing tiny optical components but also using the optical properties of a range of nanoparticles and exploiting the way changing the shape of these devices can alter the way they react to light. For example, when you shrink down to the nanoscale, a metal ball which is a few nanometres across will interact with light in a completely different way to a large metal ball. The light hitting the nano-sized ball sends waves of electrons called plasmons sloshing across its metal surface but, because of the nanoparticle’s size, only certainsized waves are allowed.
So nanoparticles can be tuned to absorb or emit specific frequencies, depending on their size, which makes them useful for tagging biomolecules or other sensing applications. The problem is that some nanoparticle shapes, such as nanorods and nanospheres, end up responding to a range of frequencies of lights as plasmons of various sizes can move around on their surfaces.
But researchers have come up with some weird and wonderful shapes to try to overcome this problem. Rice University in Houston, Texas has, quite aptly, created nanorice, which are riceshaped nanoparticles whose unique shape means that they can be tuned much more precisely to specific frequencies – making them easier to track than nanorods or nanospheres [1]. The team applied for a patent for its nanorice particles from the World Intellectual Property Organisation last year.
The Rice team also created nanoshells a couple of years ago, which contain a tiny core of nonconducting silica, covered by a thin shell of material. The nanoshell was then coated with a layer of molecules sensitive to pH levels, called paramercaptobenzoic acid, or pMBA. When placed in solutions of varying acidity and illuminated with laser light, the nanoshellmolecule device made small but detectable changes to the properties of the scattered light that can be used to determine the pH of the nanodevices’ local environment, to an accuracy of 0.1pH. The team hopes the nanoshells will give biologists a method for measuring accurate pH changes over a wide range, inside living tissue and cells, in real-time.
Nanowires, above. The Rice team has also created nanoshells, left.
But nanowires are still proving useful for other applications, with the Atwater group at the California Institute of Technology focusing on producing solar cells made up of nanowires. These consist of lots of nanowires, each around two or three microns in diameter, arranged in a forest-like configuration. Although the Atwater group’s research is still in the early stages, it is hoped the nanoforests could lead to the development of new materials that harvest sunlight cheaply and efficiently.
And the theoreticians are still busy modelling and investigating how different shaped particles react to light on the nanoscale to find what labs should be looking into next. The University of Pennsylvania recently produced a paper on an optical circuit containing a tapestry of subwavelength nanometre-scale metamaterial structures and nanoparticles, which could provide a mechanism for tailoring, patterning and manipulating local optical electric fields in a subwavelength domain, leading to the possibility of optical information processing on the nanometre scale.
By exploiting the optical properties of metamaterials, these nanoparticles may play the role of ‘lumped’ nanocircuit elements such as nanocapacitors, nanoinductors, and nanoresistors, which are analogous to microelectronics[2].
But these theoretical ideas need to be tested in the real world and building such nanostructures is no mean feat. Back in the UK, Southampton University is about to open its doors to a range of cutting-edge nanofabrication kits in the university’s new Mountbatten Building, due to open in mid-2008.
Professor Nikolay Zheludev, deputy director of the Optoelectronics Research Centre at the University of Southampton, says: ‘This is a truly interdisciplinary research endeavour at the interface between “nano” and “meta”, where ideas of nanoscopic electrodynamics and plasmonics will be combined with that of highly-advanced solid state and molecular physics on the platform of cutting-edge nano-diagnostics and nanoimaging and will be underpinned by tremendous recent advances in nanofabrication.’
Essential to the team’s work will be a new £120m cleanroom complex – which is where the nanostructures will be made and researchers will have access to nanofabrication equipment, including an e-beam lithography system with a 10nm resolution, a FIB/SEM system, one of the first helium-ion microscopes in Europe, quantum dot and nanowire growth, nanoimprint tools, CVD carbon nanotube growth and an atomic force and scanning near-field optical microscopes.
But lithography techniques have to continually evolve and improve to keep up with the demands of an increasingly smaller scientific world. This is where industry has to keep pace with the world of research, as manufacturers move from e-beam to Extreme UV (EUV) patterning and are even using X-rays to start patterning tinier and tinier structures.
But as the wavelengths come down, the complexity of the optics involved increases. Dr Samir Ellwi, VP of strategic technology at Powerlase, says: ‘It is possible to pattern using soft X-ray wavelengths, but the optics start getting too complex as the wavelengths decrease and these smaller wavelength systems bring in unknown issues, such as reflectivity and efficiency of the optics.’
Ellwi adds: ‘The EUV wavelength of 13.5nm is the best choice for projection lithography whereby the optics are mature, and this is due to the fact that researchers in this field have put great effort into choosing the right materials to reflect this wavelength with a high efficiency.’
And the researchers show no sign of stopping when trying to pattern smaller structures, as Zheludev adds: ‘From the technological prospective the discipline [of nanophotonics] is driven by the following idea: if a complex electronic circuit can be much smaller than the wavelength of electromagnetic radiation handled by the circuit, why not aim to develop photonic circuits smaller than the wavelength of light?’
At this rate, we’ll be talking about picophotonics before the decade is out.
REFERENCES
[1] Hui Wang, Daniel Brandl, Fei Le, Peter Nordlander, Naomi J. Halas, NanoRice: a hybrid nanostructure, Nano Letters 6, 827-832 (2006).
[2] Nader Engheta, et al. Circuits with Light at Nanoscales: Optical Nanocircuits Inspired by Metamaterials, Science 317, 1698 (2007).
