ARCHIVED - Fast-Changing Materials
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May 04, 2004— Ottawa, Ontario
An international research collaboration between the National Research Council (NRC) and the British Council is producing striking new findings using ultrafast lasers. Research has uncovered new physical phenomena and intriguing possibilities for the modification of materials using femtosecond laser pulses (see sidebar for more information on femtosecond lasers).
|From left to right Misha Ivanov, NRC-SIMS, Lucien Gaier, Imperial College, London, UK, Peter Knight, Imperial College, London U.K.|
How fast is a femtosecond?
Consider the following analogy. Last month, many of us adjusted our clocks for Daylight Savings Time, in this case, moving clocks ahead by an hour. Since the minute hand of clocks makes one revolution per hour, moving clocks ahead can be done quickly with just quick turn of the dial. But, imagine if you had to move an imaginary "femtosecond hand" to bring the clock forward. An hour equals 360 million billion femtoseconds, meaning that you would have to turn the hand 360 million billion revolutions each spring. And, finally one makes the change just as fall arrives and then the "femtosecond hand" needs be moved 360 million billion times the other way!
Researchers from the NRC Steacie Institue for Molecular Sciences (NRC-SIMS) and Imperial College in London, England, have been investigating the abilities of extremely fast laser pulses (10-40 femtoseconds) to modify transparent materials such as fused silica (glass). The goal is to change the properties of materials, i.e. create glass with a higher refractive index to better guide light, using the modification process as a way of machining precise and extremely small photonic structures into these materials. The project holds significant impact for fields as diverse as telecommunications and biotechnology. (see sidebar for further background).
A recent paper published by the team in the Journal of Physics B has clearly struck a chord. In the first week it was published, the paper was downloaded approximately 100 times, an extraordinary number of times given the audience size for this specialized journal.
Everything is Up For Grabs
As Internet and connection speeds grow faster and carry more information, there is demand for improved infrastructure to carry this data. Optical networking, using light waves to carry information, has long been established as a fast and effective way of moving information. Increasingly, photonics is also being applied in biotechnology research to speed analysis of samples, a field known as biophotonics.
Researchers worldwide are looking for ways of making transmission faster and more effective. Problems being addressed include decreasing the loss of light during transmission, incorporating a number of different photonic structures into a single device and creating unique structures, such as wave guides that can turn a corner as opposed to having to use something to reflect and transmit the signal down the line. Effective and precise modification of materials used in photonics is key to this research effort.
According to Peter Knight of Imperial College and President of the Optical Society of America in 2004, results achieved by the group mean that "everything is up for grabs in the theoretical understanding of the physics and dynamics of ultrafast modification." The key to this new physical regime is ultrashort femtosecond laser pulses, which create results that ordinarily would require energies as much as 1000 times higher.
Modification of the materials starts with ionization, in this case widespread ionization. Ionization is the process whereby an atom gains an electrical charge by loosing its electrons.
Traditionally, widespread ionization, also known as avalanche ionization, has been triggered through the application of a high intensity electrical field. This stimulation causes electrons to "wiggle", almost as if they were being tickled. Continued movement causes electrons to bump into each other and into the overall lattice structure, creating thermal energy which creates more free electrons and ultimately damages the host material.
Working with extremely short laser pulses also creates free electrons, which sets into motion ionization, but there is no time for the traditional avalanche effect to develop. Instead of an avalanche which sweeps through and damages an entire area, the team has described this new and, according to Knight, "completely unexpected effect", as a forest fire.
|Forest-fire simulation. From left to right, ionized regions (dark) make up 10%, 25% and 50% of lattice.|
In forest fires, one is apt to see numerous intense pockets of burning, behaviour confirmed by images of last summer's fires in Kelowna, B.C., which, inexplicably, razed certain parts of neighbourhoods while leaving nearby houses intact. In the forest fire ionization model, the "fires" burn quickly and intensely along the edges of "islands" containing a large number of electrons, islands that are left behind in the resulting firescape. These islands range in size from 1 nanometres to tens of nanometres in size.
"The process is one where bonds in materials are being broken very fast but are being re-created almost as fast, much the same way as a sand pile moves, shifts and changes shape as quickly-moving grains of sand combine and recombine into many different shapes. The results we've been getting with short-pulse lasers suggest that all conventional methods of materials modification are incomplete," Knight said.
Overall, the process has resulted in materials with a higher density than before, creating a higher refraction index and, ultimately, the ability to guide light. Meanwhile, the "islands" also seen in experiments create a kind of topography that could be very useful for other applications, such as nanosensors. In this case, researchers are painstakingly creating nano-sized droplets of gold or other substance on a surface. The results of this laborious building-up process can be achieved automatically with the new forest-fire ionization process.
Misha Ivanov of NRC-SIMS applauded the strong relationship between the two partners. "The MOU with the British Council has allowed us to bring together people with very different strengths and create results that would have been impossible to achieve on our own," he said. The current project is one of seven R&D currently being conducted under the NRC-British Council MOU.
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