Cheap, Self-Assembling Optics

Researchers have made new nano building blocks for optical computing and solar-cell coatings.


Researchers at the University of California, Berkeley, have created nanoscale particles that can self-assemble into various optical devices. By controlling how densely the tiny silver particles assemble themselves, the researchers can make several different kinds of devices, including photonic crystals. The self-assembling materials could be made cheaply and on a large scale. As a result, the silver nanoparticles could be used to make metamaterials, color-changing paints, components for optical computers, and ultrasensitive chemical sensors, among many other potential applications.

Led by Peidong Yang, a professor of chemistry at Berkeley, the researchers have demonstrated that they can use the nanoparticles to increase the sensitivity of arsenic detection by an order of magnitude. They also made a very robust kind of photonic crystal called a plasmonic crystal. These new structures are "similar to photonic crystals, but better," says Peter Nordlander, a professor of physics at Rice University, who was not involved in the work. Photonic crystals allow some wavelengths of light to pass while filtering out others. They're used commercially to coat lenses and mirrors and in optical fibers; they could also be used in optical computers.

The silver nanoparticles that make up Yang's structures are octahedra with sides of about 150 nanometers; they are very regular in shape and size. Crystal structures made up of these nanoparticles can be made when the particles are simply placed in a test tube filled with water and allowed to pack together. When the water evaporates, a crystal structure remains.

Yang says that the simplicity of his group's process is important. Most nanostructured materials are made from the top down using lithography, which makes them hard to manufacture cheaply and on a large scale. In contrast, Yang's particles are grown in solution. And most self-assembled structures are made up of relatively small particles, says Paul Braun, a professor of materials science and engineering at the University of Illinois, Urbana-Champagne. Larger particles like those used by Yang's group have better optical properties, he says. "This is the first paper demonstrating high-quality self-assembly of metal particles [of this size]," says Braun of Yang's work, which was published in Nano Letters.

When the silver nanoparticles are loosely packed, the structures behave like photonic crystals, allowing some wavelengths of light to propagate and stopping others. When the nanoparticles are densely packed, the structures take on entirely new optical properties, behaving as so-called plasmonic crystals. At the edges of the silver particles, surface energy waves called plasmons become concentrated. Just as photonic crystals allow some photons to pass while restricting others, the new crystals control the flow of the energy contained in light in the form of plasmons. Nordlander says that this phenomenon enables the Berkeley structures to interact with light much more strongly than traditional photonic crystals do. For this reason, he says, the structures should have even more applications than photonic crystals.

Technology tunes into our emotions

Computers that read facial expressions may help in caring for the aged (Image: iStockphoto)
A technology that can recognise anxiety in people is being developed by an Australian computer scientist.

Australian National University researcher Gordon McIntyre says the technology could be applied in a range of areas from aged care to driver safety.

McIntyre, a PhD student from the Research School of Information Services and Engineering, is working on a computer system that detects anxiety by analysing a person's speech and facial expressions.

Changes in speech rhythm and pitch and any quavering in the voice are picked up by speech recognition software.

While changes in facial expressions are tracked using artificial neural networks, which mimic how the brain processes information.

In developing the project, McIntyre plotted 65 landscape points on the face that change during various emotional states, such as the eyebrows, lips and nose.

The computer determines emotions by measuring changes in the location of these landscape points compared to an average or expression-free face.

McIntyre says work by body language scientists such as the University of California, San Francisco's Professor Emeritus Paul Ekman, suggests an anxious face will often show contracting eyebrows, a tightening of the upper lip and a deepening of the furrow between the nose and lips.

"We build up an average shape of a face from a database," McIntyre says. "And then measure the difference between an average face and one that is subject to the emotion."

Two types of anxiety

McIntyre, who is working with psychology colleagues to develop the program, says there are two types of anxiety.

Long-term anxiety is more easily recognised via facial expressions, while short-term anxiety is revealed through speech.

One of his major hurdles is the lack of anxious face samples from which to develop a template image of the emotion.

McIntyre plans to create his own database with the help of ANU's School of Psychology.

He hopes to run a series of experiments next year where anxiety will be induced in participants through computer games or dialogue.

Their speech and faces will be recorded for inclusion in McIntyre's database.

According to McIntyre, Australian Bureau of Statistics data show one in every 10 Australians suffers from an anxiety disorder.

He says a computer that can detect anxiety could be used to help train medical practitioners to recognise the condition and to monitor patients not able to communicate their needs clearly.

"The important thing is to get something working to show people what can be done and then let them look at the situations where it might fit," he says.

MO Laptop

If you are very particular about the design of your book, then this product is surely for you. “Mo” is the redefined notebook PC designed for customizing each and every cover to fit the style of the owner. You just need to touch the screen in order to read or make any kind of changes in your book. This product allows for adaptation to users specific needs and interests as well as offering a convenient way to organize files. The customize cover allows the user to show his/her identity and can build a long lasting relationship with the product. It has unique backward folding design that allows you to use it as an alarm.