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22 March 2011 Swinburne Magazine

Light Harvesting Provides Key to New Generation of Super Sensors

Bounce light around in a small enough space and you can magnify its intensity a thousand-fold, maybe more. The secret, says Professor Saulius Juodkazis, is having the right ‘landscape’ for the lightwaves to bounce around in.

This is the principle behind Professor Juodkazis’s research, which aims to develop the next generation of super sensors. It’s something like catching dust motes in a beam of sunlight, shrunk down to nanoscale and magnified many times over.

Professor Juodkazis says in the right environment, redirected and amplified light waves could be capable of detecting just a few molecules of targeted substances in either liquid or gas. To create this environment he uses gold particles, each only 10 to 100 nanometres in size (one nanometre is one-billionth of a metre).

“When you shine a light the nanoparticle acts as an antenna. It collects light from a much larger area than the particle itself,” he says. Electrons within the nanoparticles follow the oscillations of the lightwaves and can be driven towards sharp edges or gaps in the nanoparticles. This ‘herding’ of electrons enhances the light field.

These ‘hotspots’ of light can reveal molecules passing by simply because they are exposed to light thousands of times more intense than ambient light. His current research is identifying the most effective shapes and arrangements of nanoparticles to help ‘harvest the light’.

Big home for tiny particles

Professor Juodkazis leads Swinburne University of Technology’s Applied Plasmonics group at the Centre for Micro-Photonics. He joined Swinburne in 2010 to develop the university’s expertise in plasmonics, having worked in this field in Japan for the past 12 years at the universities of Tokushima and Hokkaido.

“Plasmonics is a bit of a jargon term. It simply means very small particles and how they interact with light. Those small particles are usually metals like silver and gold. Swinburne already has a strength in optical microphotonics and microfabrication. Now we are going even smaller,” he says.

A new plasmonics laboratory is under construction as part of the recently opened Advanced Technologies Centre at Swinburne’s Hawthorn campus. It will be the first laboratory in the world to combine both two- and three-dimensional fabrication and modification of nanoparticles by techniques using focused electrons and ions. The equipment that makes this possible is worth about $4 million and was jointly funded by the university and Victoria’s Science Agenda Investment Fund.

Professor Juodkazis says different wavelengths of light react with nanoparticles in different ways, so tailoring the nanoparticles in different shapes and sizes will generate different intensities of light and sensing capabilities.

Adding a third dimension creates more corners and therefore more opportunities to amplify the light field.

Although Swinburne’s plasmonics laboratory and research centre are still in the capacity-building phase, Professor Juodkazis says the research has the potential to underpin the next generation of sensors.

Development of a new sensing device is already in the design stage as part of a collaboration with Swinburne’s surface chemistry and sensing research group, which is involved in the detection of herbicides in soil and water pollutants.

Minute sensors, big challenge

Professor Juodkazis says fabrication of functional sensors to include the nanoparticles will be a major challenge. “The sensors will need to be in the order of one millimetre square which is huge when you’re talking about nanoparticles.”

Using gold particles of 10 to 100 nanometres, a one-millimetre-square sensor could contain many tens of thousands of nanoparticles – even more in layered or three-dimensional configurations.

Next-generation sensing devices will require nanoparticles to be embedded into a substrate or patterned over the surface, which must then be attached to electrodes and incorporated into more complex micro-architectures. This will allow the simultaneous delivery of light while performing electrical measurements.

Professor Juodkazis says current electrochemical sensors lack this functionality. But adding light-based sensing capacity will significantly enhance both detection capacity and functionality.

“Our vision would be to have a sensor that works in air, where the concentrations of molecules are usually much lower than in liquid. The sensor on a microchip device would detect what’s passing the sensor, what’s in the environment.”

He offers a possible futuristic scenario based on research priorities from his years in Japan, where there are strong initiatives to address issues of an ageing society: a new-generation sensor that people could use for the personal testing of blood or urine samples in their own home. Sensors would be capable of detecting a wide range of variables, and results could be uploaded directly to medical centres for interpretation and advice to patients. “It would be on-the-spot, fast, with no need to travel, which is very important for an ageing society, and it could be sensitive to so many things,” he says.

In another context, the light-enhancement technology using nanoparticles could also be applied to solar power generation and design of new solar panels. Novel concepts of trapping light by nanoparticles can be tested using smaller areas, although the scaling problems from nanometres to the size of actual solar panels represents a significant increase in difficulty for fabrication.

Explosive findings

Amplifying the power of light on a nanoscale is also behind another project Professor Juodkazis has been working on, initially in Japan and now at Swinburne, in conjunction with the Australian National University (ANU).

An Australian Research Council Discovery Grant has funded this project, which seeks proof of experimental and theoretical work, published in 2006, which has the potential to create materials never before found on Earth.

The theory is based on using tightly focused laser pulses on crystals. It was developed by ANU’s Professor Eugene Gamaly to explain the results of laser experiments conducted in Japan in 2006.

Research teams in Australia, Japan and the US are testing predictions of laser pulses applied to different crystals and glasses. A standard bench-top laser common in many research laboratories and manufacturing operations is used to generate a high pressure ‘micro-explosion’ within the crystal.

The process mimics at a nano-level the kind of seismic forces that have shaped the earth and other planets, melting and reforming materials under intense pressure. The laser is focused tightly and single short pulses of light are directed into the bulk of a crystal, delivery energy in the fastest possible way. The resulting pressure within the crystal is 20 to 30 times the pressure that exists within the Earth’s core.

The Australian research team is led by Professor Andrei Rode at ANU and the crystal of choice is sapphire – the hardest of nature’s oxides. Professor Rode says publication of some very promising research results is pending.

While there is growing excitement about a possible scientific breakthrough and the potential to create a previously unknown material, it could be between eight and 20 years before any laboratory discoveries are implemented in any real way.

Professor Juodkazis says the technology involved in the experimentation has been driven to its maximum limits in generating micro-explosions.

“We are looking at those spots within the crystals that have been irradiated. If we create new materials there may be potential new uses, although we are dealing with nanoparticles in very exotic phases.”

He says it is a big jump from where they are now, still in the proof-of-concept stage, to creating applications for new materials. “But it is important that we are starting to develop a fundamental understanding of how new nanomaterials can be synthesised.”

Source: Swinburne Magazine /...

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