Trap And Zap: Harnessing The Power Of Light To Pattern Surfaces

Tag : High Power Laser

The method, which creates lines and dots that are 1,000 timesnarrower than a human hair, may enable the creation of biologicalcomputers as well as micromachines with applications in medicine,optical communications, computing and sensor technologies.
The technique, created by mechanical and aerospace engineeringassistant professor Craig Arnold and graduate student Euan McLeod,is similar to poising a magnifying lens over a scrap of paper andangling the lens to focus sunlight and ignite the paper. In placeof the lens, the researchers use a microscopic plastic beadfloating in water to focus light from a powerful laser and burndesigns onto a blank microchip. Their findings are reported onlineJune 8 in the journal Nature Nanotechnology.
While others have passed laser light through various microscopicobjects to pattern surfaces, they have struggled to maintain aconsistent distance between the bead and the surface of themicrochip. If this distance changes, the laser light is focused indifferent ways across the surface and the resulting pattern isinconsistent. Arnold and McLeod established an innovative way toensure that the bead is always the same distance from themicrochip, which allows them to draw on the surface with highlevels of precision.
"One of the biggest challenges in probe-based nanopatterningis regulating the distance between your probe and the surface ofthe microchip," said Arnold. "We used a special laser totrap the bead and keep it close to the surface without touchingit."
The researchers used the technique to "draw" featuresthat were about 100 nanometers (a billionth of a centimeter) insize.
The key innovation is the use of a second, highly focused laser,which points directly down onto the bead. This intense light exertsa physical force on the bead, trapping it in the beam and pushingit down toward the surface. The surface pushes back with a constantforce, and the bead settles at a height that balances the opposingforces. The original laser is then pulsed at the bead, whichfocuses the light to "zap" the surface directly below. Bymoving the bead along a computer controlled trajectory whilerepeating the laser pulse, a desired pattern is created.
The technique offers particular advantages on curved or irregularsurfaces because the bead tracks the surface, moving up when thereis a bump and dropping when it moves over a dip. While otherfabrication techniques, such as electron-beam lithography, can alsobe used to pattern uneven surfaces, they are extremely expensiveand must be performed in a vibration- and oxygen-free environment.The new Princeton technique can be performed in a regularenvironment, making it accessible for use with biological materialsand other systems that require the presence of oxygen.
"The technique provides a very interesting new capability toexpand laser-assisted nanofabrication without involving movingmechanical parts and related hardware complications," saidCostas Grigoropoulos, mechanical engineering professor atUniversity of California-Berkeley. "I do expect that thisnovel technique will advance nanopatterning since it offers anelegant and highly effective means for parallel, optically drivenand controlled nanofabrication."
In addition to burning away parts of a chip, Arnold and McLeod'smethod has the potential to deposit materials on surfaces, ratherlike gold-plating. This could provide a new means of creatingthree-dimensional structures, including miniscule guides thatmanipulate light and nanoscale electrical-mechanical devices. Suchdevices have many potential uses in ultrasmall sensor systems andlow-power computer processors.
"In the future, we imagine the use of multiple beads ofdifferent shapes and sizes -- in essence a nanopatterning toolkit-- for researchers to pick and choose during the course offabrication," said Arnold. He and McLeod are currently workingto pattern a surface using an array of many beads moving inparallel, each trapped and controlled by a different laser beam.
The research was supported by Princeton University and the AirForce Office of Scientific Research.