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| CSIRO | SOLVE | Issue 9 | NOV 06 | |
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ARTICLE
NANOTECHNOLOGY:
Light Fantastic By Gio Braidotti
In building the world's smallest electrical circuits, CSIRO has created a material with unusual optical effects capable of tuning and controlling visible lightSomewhat gingerly, Dr Tim Davis removes the glass disk housing the world's smallest electrical circuits from a protective case and holds it in the palm of his hand. Casually he mentions that each fingernail-sized unit etched into the glass contains a staggering 32 billion circuits. He then holds the disc up to the light to demonstrate its orange glow, which is at the heart of the innovation. Dr Davis's nanocircuits are resonating at the same frequency as visible light. That resonance means his circuits can interact with light in much the same way that electrical circuits in radios and mobile phones are able to tune radio waves. Dr Davis then points out the other significant aspect – his electrical circuits are powered by light.
He explains that these nanocircuits represent a convergence of different ideas from physics, optics and material science: "Obviously there is a lot of interest in shrinking electronics to make them go faster. But this is also a way to potentially make components for high-speed electronics that are powered by light. There are also novel optical effects that come into play." The nanocircuits can behave like a lens, bending and focusing light, depending on the way the circuit is designed. Dr Davis offers the example of a unit that behaves like a mirror, but with the circuits redirecting the reflected light to the viewer, irrespective of the angle at which the reflecting surface is held. One early application being mooted is the development of extremely high-resolution microscopes. Dr Davis says that shrinking the circuits was instrumental in achieving the optical effects: "Theoretical physics has long proposed that shrinking capacitors and inductors should produce circuits where the electric current is oscillating at ever higher frequencies, much higher than radio waves," he says. But the circuits required to resonate with visible light have to be truly miniscule. "Each circuit is about 130 nanometres (or billionths of a metre) high and 160 nanometres in diameter." By comparison, human hair is a massive 100,000 nanometres thick, although Dr Davis prefers a different comparison: "One circuit is actually smaller than the wavelength of visible light, which is about 600 nanometres."
To build the tiny resonant circuits, Dr Davis made use of the electron beam lithography facilities at CSIRO Manufacturing and Materials Technology in Melbourne, and adapted a printing process that CSIRO uses to stamp images with advanced optical properties. "We coat a glass disk with an electron-sensitive polymer before passing a beam of electrons over the surface in a specific pattern," says Dr Davis. "The electrons expose the surface, producing an array of little holes that go down to the surface of the glass. We use a thin-film evaporator to add sequential coatings of silver, magnesium fluoride and more silver. We then lift off the polymer which removes all the layers except for the three tiers of materials in the etched holes." Each stack constitutes a circuit with a capacitor and an inductor separated by a magnesium fluoride dielectric (or non-conductive) layer. "Because the nanocircuits resonate at the frequency of light, we can design the electrical properties to control how light reflects off the circuits," says Dr Davis. "That's what I was originally attempting to do – build an unusual optical material that interacts with light in ways that you can't otherwise find in nature." Physicists refer to artificial materials with novel optical properties as 'metamaterials', and there is growing worldwide interest in learning how to fabricate them, says Professor Yuri Kivshar, who is a world leader in the study of the behaviour of light and the head of the Nonlinear Physics Centre at the Australian National University. "Metamaterials display amazing properties that can reverse fundamental principles of physics," says Professor Kivshar. "It would be great to use the opportunity to downscale optical devices, taking advantage of a metamaterial's unusual properties for novel imaging and sensing technology." "What makes Tim Davis's work so interesting is the leap in scale: from microwaves, which is what most researchers are working on, to optical wavelengths. It's an impressive achievement." Keen to collaborate with Dr Davis, the ANU physicists are preparing to do some numerical simulations that could provide new designs for the nanocircuits. Given that nanocircuits were purely theoretical only a year ago, Dr Davis estimates that second- and third-generation prototypes are at least 10 years down the track. Nonetheless, with each translation of theory into practice, science steadily progresses towards the sci-fi scenario of nano-machines and components powered by light and running on tiny electronic circuit boards. Towards that goal, there are two innovations that Dr Davis would like to build into the world's fastest resonant electrical circuits: "The first is diode that would allow the circuits to pump energy from light, like a solar cell does, but using different mechanism. The other is a transistor that would allow the circuit to mimic a computer chip."For further information contact: |
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