Walter Scott, Jr. College of Engineering

Graduate Exam Abstract

Abstract: A microfluidic refractometer was designed based on
previous optofluidic intracavity spectroscopy
(OFIS) chips utilized to distinguish healthy and
cancerous cells. The optofluidic cavity is
realized by adding high reflectivity dielectric
mirrors to the top and bottom of a microfluidic
channel. This creates a plane-plane Fabry-Perot
optical cavity in which the resonant wavelengths
are highly dependent on the optical path length
inside the cavity. Refractometry is a useful
method to determine the nature of fluids,
including the concentration of a solute in a
solvent as well as the temperature of the fluid.
Advantages of microfluidic systems are the easy
integration with lab-on-chip devices and the need
for only small volumes of fluid. The unique
abilities of the microfluidic refractometer in
this thesis include its spatial, temperature, and
wavelength dependence. Spatial dependence of the
transmission spectrum is inherent through a
spatial filtering process implemented with an
optical fiber and microscope objective.
A sequence of experimental observations guided the
change from using the OFIS chip as a cell
discrimination device to a complimentary
refractometer. First, it was noted the electrode
structure within the microfluidic channel,
designed to trap and manipulate biological cells
with dielectrophoretic (DEP) forces, caused the
resonant wavelengths to blue-shift when the
electrodes were energized. This phenomenon is
consistent with the negative dn/dT property of
water and water-based solutions. Next, it was
necessary to develop a method to separate the
optical path length into physical path length and
refractive index. Air holes were placed near the
microfluidic channel to exclusively measure the
cavity length with the known refractive index of
air. The cavity length was then interpolated
across the microfluidic channel, allowing any
mechanical changes to be taken into account.
After the separation of physical path length and
refractive index, it was of interest to
characterize the n(T) relationship for phosphate
buffered saline. Phosphate buffered saline (PBS)
is a water-based solution used with our biological
cells because it maintains an ion concentration
similar to that found in body fluids. The n(T)
characterization was performed using a custom-
built isothermal apparatus in which the
temperature could be controlled. To check for the
accuracy of the PBS refractive index measurements,
water was also measured and compared with known
values in the literature. The literature source
of choice has affiliations to NIST and a
formulation of refractive index involving
temperature and wavelength dependence, two
parameters which are necessary for our specialized
infrared wavelength range. From the NIST formula,
linear approximations were found to be dn/dT =
-1.4×10-4 and dn/d? = -1.5×10-5 for water.
A comparison with the formulated refractive
indices of water indicated the measured values
were off. This was attributed to the fact that
light penetration into the HfO2/SiO2 dielectric
mirrors had not been considered. Once accounted
for, the refractive indices of water were
consistent with the literature, and the values for
PBS are believed to be accurate. A further
discovery was the refractive index values at the
discrete resonant wavelengths were monotonically
decreasing, such that the dn/d? slope for water
was considerably close to the NIST formula. Thus,
n(T,?) was characterized for both water and PBS.
A refractive index relationship for PBS with
spatial, temperature, and wavelength dependence is
particularly useful for non-uniform temperature
distributions caused by DEP electrodes. First, a
maximum temperature can be inferred, which is the
desired measurement for cell viability concerns.
In addition, a lateral refractive index
distribution can be measured to help quantify the
gradient index lenses that are formed by the
energized electrodes. The non-uniform temperature
distribution was also simulated with a multi-
physics FEM software package. This simulated
temperature distribution was converted to a
refractive index distribution, and focal lengths
were calculated for positive and negative gradient
index lenses to a smallest possible length of
about 10mm.
Adviser: Dr. Kevin L. Lear
Co-Adviser: N/A
Non-ECE Member: Dr. Kristen Buchanan, Physics
Member 3: Dr. Branislav Notaros, ECE
Addional Members: N/A
R. Pownall, G. Yuan, C. Thangaraj, J. Kindt, T. W. Chen, P. Nikkel and K. L. Lear, "DC and AC performance of leaky-mode metal-semiconductor-metal polysilicon photodetectors," Proc. SPIE 7598, 759811 (2010).

R. Pownall, J. Kindt, P. Nikkel, and K. Lear, "AC Performance of Polysilicon Leaky-Mode MSM Photodetectors," J. Lightwave Technol. 28, 2724-2729 (2010).

J. Kindt, M. Naqbi, T. Kiljan, W. Fuller, W. Wang, D. W. Kisker and K. L. Lear, "Automated optical cell detection, sorting, and temperature measurements," Proc. SPIE 7902, 790222 (2011).
Program of Study: