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Smaller than ever

Exciting times lay ahead in the world of nanophotonics, according to Nick Morris

Nanophotonics is one of the current ‘buzz words’ in the optics industry. The prospect of using light as a tool at the submicron scale opens up a new wave of possible applications, and the idea of creating light from nano or ‘meta’-materials could pave the way for a new wave of miniaturisation, as seen in the semiconductor industry in the last decades of the 20th century. Such advances are already allowing researchers to create so-called lab-on-a-chip systems, where components such as miniature lasers, sensors, and MEMS apparatus can be built onto a semiconductor wafer. In the coming years such systems will surely become ubiquitous in a huge variety of applications, especially in fields such as medical research and diagnosis, where these tiny tools will allow for virtually instantaneous sample processing and analysis.

Nanomaterials open up the possibilities for many new application fields above traditional materials. For example, scientists working at Rice University in Houston, Texas, have used a combination of laser light and nano-particles to create the world’s first nano-scale pH meter. The team, lead by Naomi Halas, professor of chemistry and director of the Laboratory for Nanophotonics at Rice, created the pH sensors using nanoparticles whose light scattering properties change with varying acidity. The findings were first discussed in the journal Nano Letters.

The scientists built the sensors using nanoshells – optically-tuned nanoparticles invented by Halas. Each nanoshell contains a tiny core of non-conducting silica, covered by a thin shell of material. The team coated each nanoshell 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 nanoshell-molecule 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 new technique will give biologists a method for measuring accurate pH changes over a wide range, inside living tissue and cells, in real-time. ‘Almost every biologist I’ve spoken to has come up with one or two things they’d like to measure with this,’ says Halas. The technique could eventually be used for applications such as determining whether tumour cells are malignant, without invasive surgery. With current methods, a sample of tissue has to be removed via biopsy and examined under a microscope. An ‘optical biopsy’ would need nothing more invasive than an injection to measure the pH inside the tumour.

One of the most exciting application areas for nanophotonics is for photonics-on-a-chip systems, combining nanophotonics with semiconductors in a single unit. Such systems will be particularly useful for information and communication applications, where currently electronic data is transferred to optical data for high-speed transmission, then converted back to electronic data for processing. Optical circuitry could both speed up processing time and increase data density, and eventually lead to fabled quantum computing systems.

Recently the European Union committed two million euros for a three-year project to develop a re-configurable photonic firewall-on-a-chip. Called WISDOM, which stands for WIrespeed Security Domains Using Optical Monitoring, the new system will be able to perform security checks and algorithms directly at high optical data communications rates. A consortium led by the Centre for Integrated Photonics, in the UK, is developing WISDOM. The optical sub-systems that are being developed under WISDOM will use integrated photonics technology to build a photonic firewall. The sub-systems will be based on research on high-speed (greater than 40Gb/s) optical logic gates and optical processing circuits provided by project partners Avanex, CIP and Tyndall.

In June 2006 the Association of Industrial Laser Users held a workshop, hosted by Exitech, investigating the most recent advances in the field, titled ‘Laser applications for micro and nano engineering’. The sessions covered various areas, from cutting-edge applications, to presentations of the various issues involved with laser-machining at small scales.

Tony Flaherty, from the National Centre for Laser Applications, part of the Physics department of the University of Ireland, gave a presentation titled ‘From micro to nano: techniques for laser processing of surfaces’, in which he discussed how scientists working at NCLA were working to overcome problems with the behaviour of materials at the nanoscale when exposed to laser processing techniques, particularly for imprinting shaped holes and channels into materials for bio-sensor applications. Flaherty explained that NCLA was manipulating phase-mask design, and even changing the chemical structure of the material being processed to make it easier to manipulate at the small scales needed.

Excitech’s Philipp Grunewald gave a working example of laser materials processing on the nanoscale, in the production of photovoltaic panels – a very significant growth area, considering the increasingly important need for sustainable sources of energy. In his presentation Grunewald explained how both through-substrate machining and front-side machining were used to produce separate energy-producing regions in each photovoltaic panel. Currently Exitech’s production methods are being used to develop next-generation solar panels for the UK Government’s Department Of Trade and Industry, BP Solar, and the European Union, among others.

Nanophotonics and nanotechnology also formed a significant element of the ‘Hot topics’ discussed at Photonics Europe, held in Strasbourg earlier this year. Clivia Sotomayor Torres, from University College Cork, Tyndall National Institute, Ireland outlined a European research agenda (PHOREMOST) to address near- and long-term needs of photonic functional components, building on the critical mass existing in Europe in this emerging area. Here, the main driving force is the expectation to access the molecular scale dispensing with electrical contacts.

Sandip Tiwari, from Cornell University, spoke on the national nanotechnology infrastructure network in the USA, and its support of photonics research and development. The network provides hands-on access to external users, remote usage, staff support, low cost usage, knowledge infrastructure, and brings together an extensive coordinated array of instruments for fabrication, synthesis, and characterisation together with other infrastructure resources. In the final session Kenji Kitamura, director, Opto-Single Crystal Materials Research, NIMS, Japan and Teruo Kishi, president, National Institute for Materials Science, discussed nanotechnology policy in Japan. In nanotechnology and materials science, five fields for application were identified: information technology, environment, biotechnology, generic technology, and materials. Financially, government funding for research and development on nanotechnology and materials science has seen significant increases and reached ¥94bn in 2004.

These are just a few of the many and varied range of diverse applications of nanophotonics. But this is a technology still in its infancy. As more companies begin to take advantage of the wealth of research currently being carried out in the field, the range of uses for nanophotonic devices is sure to grow exponentially.

Surveying the market for new nano devices, Gary Colquhoun, in charge of European industry development at SPIE Europe, says: ‘The potential marketplace for nanotechnology touches every aspect of our modern lives. Devices are already benefiting from the impact of nanotechnology research. Interdisciplinary connections, discussions and partnerships will be critical and SPIE Europe will continue to bring together those who seek to uncover with those who seek to utilise photonics technologies. In this spirit, SPIE Europe is hosting a show titled “Microtechnologies for the new Millenium” in May 2007 in Gran Canaria, where we will ultimately discover just how far down our devices have gone.’

However, Colquhoun sounds a word of caution with regard to research funding and investment: ‘Nanotechnology solutions are too far away from the comfort zone of your typical investor that usually wants a three-year turnaround for revenue from their investment. Furthermore, the investment funds required for new lithography equipment, software tools and new materials for molecular scaling will be significant. The steep investor funds required may limit the number of participants at the manufacturing level.’