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Scanning electron microscope
images of woodpile-type photonic crystal structures fabricated
with 520 nanometer excitation at (a) higher power and at (b) lower
power using DABP. Magnified images of the structures are shown
below their respective overview images.
Image by Wojciech, H. et al.
A Georgia Tech research team led
by Joe Perry - shown here with researcher Vincent Chen - has
produced three-dimensional polymer line structures as small as 65
nanometers wide using new two-photon absorbing molecules.
Georgia Tech Photos: Gary Meek |
"Being able to obtain line widths down to 65
nanometers, which is substantially below prior published work of 100
nanometers, opens up new applications for multi-photon lithography,"
said Joseph Perry, a professor in the Georgia Tech School of Chemistry
and Biochemistry and the Center for Organic Photonics and Electronics.
The technique scans a laser beam across a substrate
coated with a polymer resin containing a unique dye to create a
desired hardened polymer structure. The laser writing process takes
advantage of the fact that the chemical reaction of cross-linking
occurs only where molecules have absorbed two photons of light. Since
the rate of two-photon absorption drops off rapidly with distance from
the laser's focal point, only molecules at the focal point receive
enough light to absorb two photons.
The fabrication method and dye were described in
the March 19 issue of Optics Express. The research was supported by
the Office of Naval Research APEX Consortium and the National Science
Foundation, through the Science and Technology Center for Materials
and Devices for Information Technology Research.
Seth Marder and Stephen Barlow, also researchers in
the School of Chemistry and Biochemistry and the Center for Organic
Photonics and Electronics, synthesized the unique molecule called DAPB,
4,4'-bis(di-n-butylamino)biphenyl, to initiate the chemical reaction
leading to the hardening of the polymers when exposed to laser light.
"We needed a dye with good two-photon absorption at
a wavelength of 520 nanometers, so we tried DAPB," explained Perry. "DAPB
proved to be very effective in this kind of lithography."
The molecule developed by Marder and Barlow is
about ten times more efficient at absorbing light by two photon
absorption than commercial ultraviolet photoactive materials. That
efficiency allowed Perry and graduate students Wojciech Haske and
Vincent Chen, research scientist Joel Hales and postdoctoral associate
Wenting Dong to create 3D patterns with nanoscale lines at light
intensities low enough to avoid damaging the polymers.
For the experiments, a film of the polymer resin
containing DAPB was formed. When the film was exposed to the focused
laser, DAPB was excited and triggered cross-linking, leaving the
insoluble scanned structure on the surface of a substrate when placed
in a developer solution.
Since Perry controls where the Ti: Sapphire pulsed
laser scans with a computer program, the polymers can be cross-linked
in any pattern including 3D stacks of straight lines that are
connected and sturdy. The laser beam is turned on to expose lines of
polymer and off when no line should be drawn.
Conventional lithography involves creating a
specific pattern on a mask for each new layer and exposing each layer
to light and developing it. With this new technique, three-dimensional
layered nanostructures can be created simply by having a computer
program scan a different pattern for each layer. Mask templates become
unnecessary and the coating, exposing and developing processes only
have to be conducted once.
"We can create essentially any pattern we want. For
this work, some of the patterns look like walls or lines suspended
across walls and some are like a tall stack of crisscrossed lines,"
noted Perry.
Perry and Marder co-founded a company in 2003
called Focal Point Microsystems that is working to commercialize this
fabrication technology.
"We can write very small lines and create
stacked-up grids of lines called photonic crystals," explained Perry.
"This work shows that we can fabricate functional photonic
micro-devices with tailored transmission capabilities."
It takes only 10 minutes to create a 20 micron by
20 micron structure with 30 layers, Perry added. Perry envisions using
this technology to create compact micro-spectrometers on a chip for
use in telecommunications and sensors. It may also be used as a
compact way to separate the multiple wavelengths traveling through a
fiber optic cable.
This type of simple, table-top technology may also
be useful to fabricate customized types of circuits with many layers,
which would be extremely expensive with standard methods because each
layer would require a special mask.
"With the combination of the right molecule and
short wavelength light, we've demonstrated that we can obtain
nanoscale features. We're at 65 nanometers now and we're still trying
to go smaller," said Perry. |
