Air Force Office of Strategic Research (AFOSR) funded researchers at the University of Rochester are using laser light technology to create new forms of metal that may guide, attract and repel liquids and cool small electronic devices. Laser regimes can also produce materials suitable for biomedical applications, particularly medical applications where a metal or metal-clad device is to be implanted into humans or animals. The alterations to the metal or metal cladding act to improve the biocompatibility of the metal. Lasers can also produce materials with increased catalytic properties by making sufficient macro-, micro-, or particularly nanostructural changes so as to have increased catalytic surface areas.
Chunlei Guo and Anatoliy Y. Vorobyev filed U.S. Patent Application 20080216926 for materials processing regimes obtained with laser processing using ultra-short laser pulses of subpicosecond (i.e., up to hundreds of femtoseconds (fs)) duration, and to the altered materials obtained through such materials processing regimes. The application details various methods for altering materials by exposure of the materials to one or more pulses of a fs duration laser, while other aspects of the invention are directed to materials altered by the laser blasts. These macro-, micro-, and nanostructured materials have a variety of applications, including aesthetic applications such as jewelry or ornamentation; biomedical applications, especially medical applications involving biocompatibility bioperformance; catalysis applications; and modification of the optical and hydrophilic properties of materials.
Nanograph (2): showing laser induced periodic surface structures (LIPSS) in the irradiated area after 20,000 laser shots in the gold target. Nanograph (3)close up : Nanobranches and supported spherical nanoparticles in the LIPSS.
In contrast to the common belief for femtosecond laser ablation that the thermal energy remaining in the ablated sample should be negligible, Guo and Vorobyev found that a significant amount of residual thermal energy is deposited in metal samples following multi-shot femtosecond laser ablation. This suggests there might be a significant enhancement in laser light absorption following ablation. To understand the physical mechanisms of laser energy absorption, they measured the change in absorptance of gold due to structural modification following multi-thousand shots of femtosecond laser ablation.
They were able to show that besides the known mechanisms of absorption increase via micro- and macro-structuring, there is also a significant absorption enhancement due to nanostructuring. They found that nanostructuring alone can enhance the absorptance by a factor of about three. The physical mechanism of the total enhanced absorption is due to a combined effect of nano-, micro-, macro-structural surface modifications induced by femtosecond laser ablation. Virtually, at a sufficiently high fluence and with a large number of applied pulses, the absorptance of gold surface can reach a value close to 100%.
Guo and his team of researchers at Rochester University looked at the reverse process of light absorption or light radiation and transformed the incandescent lamp into a bulb that glows twice as brightly as a regular light source, while consuming the same amount of energy.” The key to creating this super-filament is an ultra-brief, ultra-intense beam of light called a femtosecond laser pulse. The laser burst lasts only a few quadrillionths of a second. That intense blast forces the surface of the metal to form nano-structures and micro-structures that dramatically alter how efficiently light can radiate from the filament.
In addition to increasing the brightness of a bulb, Guo's process can be used to tune the color of the light as well. Last year, his team used a similar process to change the color of nearly any metal to blue, gold, gray, in addition to the black. They controlled the size and shape of the nano-structures -- and thus what colors of light those structures absorb and radiate -- to change the amount of each wavelength of light the filament radiates.
In addition to this research, Guo and his team have been working on creating technology that may enable the Air Force to create an additional kind of metal. They are able to do this by using the femtosecond laser once again to alter the surface of metal and create unique nano- and micro-scale structures on the metal.
"During its brief burst, the laser unleashes as much power as the entire electric grid of North America does, all focused onto a spot the size of a needle," according to Guo. The unique nano-structures which are created from the laser affect the way liquid molecules interact with metal molecules. The liquid spreads out over the metal because the nano-structures attach themselves to the liquid's molecules more readily than the liquid's molecules bond to each other. The end result is the formation of a new kind of metal that can cool the plane's electronic brain and heat pumps and allow the craft to retain dominance over any enemy that is also in flight.
The metal can push water uphill, defying gravity by using the capillary effect. It’s the same effect that happens when you put a piece of fabric in a wine glass, for example, and hold it with your hand. The water binds to the material and rises itself, as a combined action of capillarity and evaporation.
Currently, the researchers need half an hour to change the surface of metal that is approximately the size of a quarter. Their goal is to make the process quicker so they can meet the ever increasing demands of warfighting.
AFOSR continues to expand the horizon of scientific knowledge through its leadership and management of the Air Force's basic research program through funding research like Dr. Guo's. The fs laser regime can be applied to metals such as gold, silver, titanium, aluminum, platinum, stainless steel, and copper.
