Determination of the Electro-Optic Effect in PDMS-TiO2 Hybrid Nano-composites
by: Kevin Rocheleau, Summer 2015
Lead Mentor: C. Middlebrook (ECE) / Co-Mentor: M. Mullins(CM)
Project Summary: Polymeric materials for embedded waveguide applications have been
pursued primarily due to their compatibility within the printed circuit board fabrication
processes, operational temperatures, and low absorption at operating wavelengths. Many
families of polymers have been used in waveguide applications and several classes of
polymeric materials have been implemented for photonic waveguides in ongoing research
[1-7]. Optical polymers are limited in the range of their refraction index (RI) (nominally
1.56) and the lack of ability to alter the RI with an applied electric field. Combining the
advantages of polymeric and inorganic optical materials can be a potential route to
prepare nano-composites with a higher range of RIs and can be utilized as active optical
components (switches, modulators, etc.). This project will focus on a new material based
on polymer-metal oxide nano-composites for active and inactive optical components. The
material composition is a PDMS (Polydimethylsiloxane) combined with titanium dioxide.
Refractive indices as high as 1.74 have been measured using these composites [8]. It is
hypothesized that having the TiO2 matrix within the organic host of the PDMS will allow
for a change in the refractive index with an applied electric field. This research project
seeks to determine the optical properties of polymer-metal oxide nano-composites
waveguides. The attenuation and the electro-optic coefficient of the fabricated
waveguides will be investigated, characterized, and measured as a function of the
PDMS:TiO2 ratio.
Background and Theory: The real part of a materials complex RI determines the
refraction while the imaginary portion is proportional to the absorption coefficient and
the attenuation. Dynamically controlling the refractive index is desirable and necessary in
order to implement an active optical device. Active optical devices, such as a MachZehnder Modulator, split the input light wave into two separate equal length paths. One
path serves as a reference path and the other
path is where the RI is varied thus introducing
retardation between the two light waves and
resulting in interference/modulation during
recombination of the paths. The postulate of the
proposed research work is believed to be similar
to that of acousto-optics where a perturbation of
the RI is caused by sound and with the propagation of light through this perturbed timevarying inhomegenous medium. In the proposed structure the electric field travels
through the polymer-metal oxide nano-composite setting up a dynamic strain involving
molecular vibrations that take the form of a standing wave. The RI of the medium then
becomes periodic with the same periodicity as the electric field. If the electric field is a
traveling wave the periodic modulation of the RI also moves at a velocity equal to the
field’s velocity. Since the electric field velocity is significantly smaller than that of light,
the periodic RI modulation is essentially stationary. The propagation of light within and
through this medium can be treated as electromagnetic propagation through a periodic
medium in the form of a grating changing the light’s path and accumulating a phase delay
with respect to the reference wave.
Goals and Research Plan: The four main goals in this research plan are: 1) Fabricate
nano-hybrid composite waveguides, 2) Measure the attenuation and absorption of the
nano-hybrid composite at the operating wavelength, 3) Determine experimentally the
electro-optic effect of the nano-hybrid composite (specifically related to the use within
optical waveguides), and 4) Perform data analysis on the experimental data to determine
the electro-optic coefficient.
Fabrication: (May/June 2015):
Fabricate PDMS-TiO2 nanocomposite utilizing the sol-gel
method. Titanium Isopropoxide
(TTIP) is used, and the PDMS is
a Xiameter Silanol fluid. Due to
the viscosity of the material
waveguide fabrication can be
performed through soft-imprint
lithography. The soft lithography
process is shown in Figure 1 and
consists of three main steps
including
master
mold
fabrication, PDMS mold fabrication, and using a PDMS mold to then fabricate the
waveguides.
Attenuation/Absorption Measurements (June 2015): The attenuation and absorption will
be determined in two ways and compared for validation. Direct measurement through
ellipsometery will be compared to using standard cutback procedure, where the insertion
loss of a waveguide is measured while the waveguide length is reduced.
Electro-optic Effect Determination (July/August 2015): In order to obtain experimental
validation measurements a Mach Zehnder interferometer wave guiding structure will be
constructed and gold contacts will be deposited over a select region in one path to apply
an electric field. Experimentation will be performed in order to see a change in the output
profile with and without an applied voltage.
Data Anlysis (August 2015): Based on the results of the previous section and specific
information on the configuration of the device, data analysis will be performed in order to
determine the electro-optic coefficient of the PDMS-TiO2 Hybrid Nano-composites.
Outcomes: The potential discovery of a measurable and significant electro-optic effect in
a new class of hybrid composite polymers provides new research and development
opportunities for directly integrated active optical devices within embedded waveguides
and single layer flexible optical circuits. This project will also allow me to further my
knowledge in the area of photonics and explore graduate research possibilities.
Additionally, this will provide an opportunity to interact with graduate students and two
faculty mentors. It is expected that at the conclusion of this research project a proceeding
paper or poster will be submitted to a national conference, providing additional
enhancement to my undergraduate research experience.
References
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
K. L. Kruse and C. T. Middlebrook, "Fan-out routing and optical splitting techniques for compact
optical interconnects using single-mode polymer waveguides," Journal of Modern Optics, pp. 110, 2014.
K. L. Kruse and C. T. Middlebrook, "Laser-written polymer waveguides for embedded printed
circuit board computing applications," in SPIE OPTO, 2014, pp. 899104-899104-9.
K. Kruse, et al., "Chemical inertness of UV-cured optical elastomers within the printed circuit
board manufacturing process for embedded waveguide applications," in SPIE OPTO, 2014, pp.
898811-898811-11.
K. Kruse and C. Middlebrook, "Laser-direct writing of single mode and multi-mode polymer step
index waveguide structures for optical backplanes and interconnection assemblies," Photonics and
Nanostructures-Fundamentals and Applications, 2014.
N. Riegel, et al., "Using the 3D beam propagation method to model the effects of lithographic
roughness on the attenuation of highly multimodal polymer waveguides," Journal of Vacuum
Science & Technology B, vol. 31, May 2013.
R. Nicholas, et al., "Optimal VIA Placement in Under Filled Embedded Multimode Waveguides,"
Optics and Photonics Journal, vol. 2013, 2013.
J. Dingeldein, et al., "Optical-electrical printed wiring board for high-speed computing
applications," Optical Engineering, vol. 52, Mar 2013.
Q. Lu and M. E. Mullins, "In situ Synthesis of High Refractive Index PDMS/Metal Oxide
Nanocomposites," MRS Online Proceedings Library, vol. 1400, pp. null-null, 2012.