CMDITR Review of Undergraduate Research - Pluto - University of ...

Two-Photon Induced Free Radical Polymerization
Mitchell J. Witkowski, University of Washington
Jen Lab, Dept. of Materials Science & Engineering, University of Washington
Introduction
Two-photon absorption is a statistically rare,
nonlinear optical event. The probability for a
molecule to absorb two photons goes as the
square of the intensity of the incident light. By
using a femtosecond mode-locked laser, focused
into a small volume, enough photons are present
for the event to occur in a reasonable amount of
time. A specially designed chromophore, with a
high two-photon cross section, absorbs these two
photons and is induced to fluoresce a single,
high-energy photon. Note that these single
photons are produced from within a volume
smaller than the diffraction limit of the laser due
to the quadratic dependence of two-photon
absorption. This single photon then causes
photo-polymerization of the resin by generating
free radicals. 1-6
Objective
The primary objective of this research is to
develop and run an experiment to determine the
voxel volume-filling rate of the acrylate used in
two-photon initiated free radical polymerization.
It is important for determining limiting operation
powers of the mode locked laser used for two
photon microfabrication, decreasing the
running time to fabricate an object, and
achieving higher resolution of the objects
that are fabricated. The secondary
objective is to support manufacture
devices and structures for chemical
sensors, photonic devices, and biological
applications. Support manufacturing is
important so that other groups working
with micro and nano scale devices have
the access to microstructures.
Methods
Beginning with the primary
objective, the experiment to determine the
filling rate of the volume of the voxels
was developed to create the smallest
volume of polymer with the smallest
exposure time. There are multiple
types of photo resin used; the primary
photo resin had nothing added to it, while
the other photo resins had different
amounts of platinum based porphyrin
Table 1.
ranging from 2% to 0.1%. The exposure time
was gradually increased to determine whether or
not there was a visible trend between volume and
exposure time. With this design, the only
measurement that needs to be taken is the
volume of the voxel created by the exposure to
the laser. The exposure time is measured by the
shutter, which is controlled by the computer.
Running the experiment required preparing
clean glass slides, knowing how to use
AutoCAD 2004, LabVIEW, python, and the
controlling software for the piezoelectric
Nanocube, and then removing the excess photo
resin from the slide after the sample has been
created. Preparing the slide required placing two
pieces of tape on the center of the slide about one
centimeter apart, placing a very small drop of the
photo resin in-between the two pieces of tape,
placing a glass cover on top of the photo resin
(being careful not to allow the photo resin to
move under and over the tape), and then putting
a small drop of immersion oil on top of the cover
glass. The slide was then placed onto a stage
underneath the focal lens. The focal lens was
then lowered until it became immersed in the oil
and focused on the substrate where fabrication
CMDITR Review of Undergraduate Research Vol. 1 No. 1 Summer 2004 61