November 21/28, 2005

Butterflies master photonics

Biomimicry -- the practice of mining nature's billions of years worth of evolutionary experience for solutions to technological challenges -- is increasingly popular and fruitful. In many cases nature has already solved problems that humans are just beginning to tackle.

Researchers from Exeter University in England have found that the nanoscale structure of the wings of certain African butterflies closely matches the most advanced photonic materials under development in laboratories around the world.

The wings of the African swallowtail butterflies Princeps nireus are black with fluorescent blue patches formed from two-dimensional photonic crystal positioned above distributed Bragg reflectors. Fluorescent pigment in the photonic crystal structure of the butterfly wing absorbs light from blue skies and emits darker blue light.

Photonic crystal has regularly spaced nanoscale air holes or plugs of other material that block specific wavelengths of light. Distributed Bragg reflectors are highly efficient mirrors made from extremely thin alternating layers of material.

The basic design is the same as experimental ultra-high-efficiency light-emitting diodes. Light-emitting diodes use semiconductor material that converts electrical current to light of a particular wavelength.

The photonic crystal-distributed Bragg reflector combination concentrates light and directs it outward. Such light-emitting diodes would be much more efficient than today's models, which scatter light in all directions, causing much of the light they produce to be reabsorbed by the devices' material.

(Directionally Controlled Fluorescence Emission in Butterflies, Science, November 18, 2005)

Search engines share the wealth

Search engines have a tremendous influence over what information is accessed on the Web. Some researchers have been concerned that the way search engines rank pages could lead to a vicious cycle that benefits popular sites at the expense of new and less popular sites -- a rich-get-richer phenomenon.

Researchers from Indiana University and the University of Bielefeld in Germany have found that the combination of the way search engines return results and the way people use those results counterbalances the rich-get-richer phenomenon to send more traffic to less popular sites than they would receive from random surfing.

It turns out that the number of hits a search engine returns and the wording of the query introduces a bias toward less popular sites, and this bias more than offsets the bias toward popular sites introduced by search engine page ranking algorithms.

The research could be useful for optimizing Web sites for search engines, forecasting Web traffic and designing search engines.

Separately, researchers from Indiana University, the University of Barcelona in Spain and the University of Bielefeld in Germany have uncovered sampling biases in studies of the underlying structure of the Web. The work shows that scientists do not yet have a clear picture of the Web's structure, which is important for developing effective ways of indexing, searching and surfing the Web.

(The Egalitarian Effect of Search Engines; Decoding the Structure of the WWW: Facts Verses Sampling Biases, posted on the Computing Research Repository (CoRR))

Finger tap interface detailed

The acoustics research that is helping law enforcement officials locate gunshot sources could also be used to turn everyday objects into virtual keyboards and command consoles.

Scientists from Sensitive Object S.A. and the French National Center for Scientific Research (CNRS) have detailed the physics behind their invention -- unveiled last year -- of an acoustics sensor and analyzer that localizes finger impacts on a surface and maps them to computer commands. The system taps time-reversal acoustics to analyze reverberations within a solid object in order to trace a sound back to its point of origin.

The system works with rigid materials like glass, metal, hard plastics and ceramics, and can work with irregularly-shaped objects. The system is made up of a single acoustic receiver and a personal computer, and can identify contact points as small as a square centimeter.

The system can be used to turn a desk top into a computer keyboard, a coffee table into a remote control, and a spot on a wall into a light switch.

(In Solid Localization of Finger Impacts Using Acoustic Time-Reversal Process, Applied Physics Letters, November 14, 2005)

Hologram stats ID microbes

Rapidly identifying microorganisms is important for medicine, security and biological research. While using tiny biochips to ID pathogens is an exciting emerging method, the trusty microscope is getting a makeover that promises to keep it in the game.

University of Connecticut researchers are using statistics to improve their digital holographic technique for identifying microorganisms.

The system uses a type of digital holography that captures three-dimensional microscope images in a single exposure, which makes it possible to image moving and growing microorganisms. The system uses statistical analysis to identify features within the images that allow the bugs to be classified. The statistical analysis method makes classification possible even though many microorganisms have similar shapes.

In the researchers' tests, the system correctly identified 87 of 100 positive test images, and misidentified 2 of 100 false test images.

The method could eventually be used to diagnose bacterial infections, detect biological weapons, track microorganisms in wastewater treatment, and monitor plankton in the ocean.

(Shaped Tolerant Three-dimensional Recognition of Biological Microorganisms Using Digital Holography, Optics Express, November 14, 2005)

Bits and pieces

Plastic, cage-like crystals promise lightweight hydrogen storage; a silver superlens could boost optical storage capacities; a six-legged molecule picks up and deposits sets of individual atoms for nanofabrication; optoelectronic interconnects speed parallel processing supercomputers; a new way of electrifying carbon nanotubes produces very bright infrared light; a laser beam shot through ultracold atoms guides slow light from another laser.