This document discusses research on functionalizing microtoroid whispering gallery mode resonators with mesoporous silica thin films to enhance their gas sensing sensitivity. Mesoporous silica was deposited on microtoroids using different surfactants like F127 and calcined at temperatures from 90-500°C. Characterization with SEM, TEM, and ellipsometry showed the successful fabrication of thin films with 50% porosity and 7-8nm pore diameters. Preliminary ethanol vapor sensing experiments demonstrated that the mesoporous coating enhanced sensitivity over 2 orders of magnitude compared to bare microtoroids, likely due to the increased surface area and electric field interactions within the pores.

1. Design & Synthesis of Low-Loss Optical Materials for
Nano/Micro Photonic Devices
Yutong Liu
Abstract: Whispering gallery mode (WGM) optical micro-resonators have attracted intense interests
because combination of high quality factors (Q) and small mode volumes of modes in WGM resonators
significantly can enhance the lightmatter interactions, making them excellent cavities for achieving low
threshold and narrow linewidth lasers.[1-10] Surfactant-templated meso-porous silica materials are rapidly
becoming important in many fields of chemistry for hosting reactants or catalysts in confined space.[11-20]
In this research, microtoroid surface were functionalized by meso-pore silica thin film so that the sensitive
was significantly enhanced. Multiple kinds of surfactant were employed to fabricate meso-pore thin film
and best recipe and procedure was determined. Optical Microscopy results assist to select the best surfactant.
SEM and TEM results illustrated that meso-pore thin film was successfully fabricated with desired inter-
pore distance and pore diameter. Ellipsometry results reveals that this thin film has about 50% porosity.
Chemical composition in meso-pore thin film and sufficient calcination temperature was given by FTIR
test. A preliminary experiment shows that the meso-pore covered microtoroid has significantly enhanced
sensitivity for ethanol vapor when compare to common microtoroids.
Key words: Whispering gallery mode resonator, Surfactant templated Mesoporous silica, Gas sensing.
1 Introduction
Whispering gallery mode (WGM) optical micro-
resonators have attracted intense interests in the
past decades.[1-10] The combination of high
quality factors (Q) and small mode volumes of
modes in WGM resonators significantly
enhances the lightmatter interactions, making
them excellent cavities for achieving high
sensitivity chemical and biological molecules.[1-
6] With introduction of resonant micro-cavities
that use whispering gallery mode (WGM)
recirculation, sensitivity to single binding events
can be materialized.[8] In this research, we
significantly increased the sensitivity of
microtoroid by coating it with a specific diameter
sol-gel meso-pore thin film.
Surfactant-templated meso-porous silica
materials are rapidly becoming important in
many fields of chemistry for hosting reactants or
catalysts in confined space.[11-15] Fine control
of the pore size, wall structure, surface
functionalization, defects, and morphology is
needed for fine-tuning the pores as nano-
reactors.[17] Controls in surfactant packing and
liquid crystalline phase transformation can lead to
various tailored synthesis strategies.[19] Post
synthesis treatments further make more stable
meso-porous materials.[20]
In whispering gallery mode resonator, with radius
increasing from center to the edge, the electric
field increases first and then decreases, the
maximum value locates at the vicinity of the ring
edge.[5, 7, 15] Coating Mesoporous silica on
microtoroid has several advantages including
higher electric field distribution at analyte
location, much bigger surface area, having
selectivity for molecule size and capability to be
functionalized by Dye or Enzymes. [15-16]
2 Experimental
The fabrication process of microtoroid is
composed of four steps: photolithography;
pattern transfer into the silicon dioxide layer;
selective, dry etch of the exposed silicon; and
selective reflow of the patterned silica. [2, 5] The
process details are as follows: First,
photolithography is performed to create disk-
shaped photo-resist pads (160 um in diameter) on
a (100) prime grade silicon substrate with 2 um of

2. oxide. An additional bake follows in order to
reflow the photo-resist, smoothing the edges in
the process. The circular disks of photo-resist act
as an etch mask during immersion in buffered HF
solution at room temperature. Acetone is then
used to remove residual photo-resist and organic
contamination. The remaining SiO2 disks act as
etch masks during exposure to XeF2 gas at 3 torr.
XeF2 was specifically chosen for the purpose of
isotropic selective removal of silicon. As a result,
the edges of the SiO2 disks are equally undercut,
leaving circular silicon pillars supporting larger
SiO2 disks. Finally, microtoroids were obtained
by reflowing the disks with a CO2 laser. [7]
As for mesoporous silica, according to the
previous references, C16TAB, BrijC10, P123 and
F127 were chosen to be tested as surfactants
under different treatment temperature, standing
time and coating conditions.[17, 19] The ultimate
fabrication method of Meso-porous silica films
with four different surfactant were prepared
following the same procedure:
Tetraethylorthosilicate (TEOS, 98%) was
hydrolyzed under acidic conditions (HCl, 36.5-
38%), and then ethanol (Ethanol, 99.8%) was
added into the hydrolyzed TEOS at room
temperature. Finally, C16TAB,
(CH3(CH2)15N(Br)(CH3)3 , 25wt%), BrijC10
(C16H33(OCH2CH2)10OH, 4wt%), P123
(OH(CH2CH2O)77(CHCH3CH2O)29-(CH2CH2-
O)77H, 10 wt %) and F127
(OH(CH2CH2O)77(CHCH3CH2O)29-(CH2CH2-
O)77H, 10 wt %) as structure directing agents
were added. The final reactant mole ratios were 1
TEOS : 6 H2O : 0.05 HCl : 10 Ethanol : 0.1- 0.2
Surfactant.[11-16] The mixture was stirred for
further 2 hours at 70 ℃ to achieve
oligomerization.[17] After the solution had
cooled to room temperature, thin films were
coated onto fabricated microtoroids by spin
coating. (1500 rpm, 30 s, optimized to 2 steps,
500 rpm, 5s first, and then 3000 rpm, 30s for thin
film or 7500rpm, 30s for toroid coating).[18]
Later, coated microtoroids were heated to 500 ℃
(optimized to 300 ℃ in our research) for 12
hours in the furnace with heating speed of 1℃
/min for surfactant calcination.[19] At last, the
microtoroids were coated by meso-pore
structure.[20]
After fabrication, Optical Microscopy, Scanning
Electron Microscopy, Transmission Electron
Microscopy were employed to test the parameters
of the meso-pore thin film. Then, Ellipsometry
test and Fourier Transform Infrared Spectroscopy
were applied to measure the optical property of
the thin film.
Finally, our mesoporous silica coated
microtoroids were applied in ethanol vapor
sensing, with the comparison of common
microtoroids. Light is coupled into the sensing
device from a continuous wavelength, narrow
linewidth tunable laser centered at 660 nm using
a tapered optical fiber waveguide. The
transmitted signal from the optical sensing device
is detected on a photo-detector and monitored in
real-time. One drop of ethanol was released 3 cm
away from toroid and evaporated in room
temperature. Resonant wavelength shift was
monitored.
3 Result and Discussion
Optical Microscopy Result:
Figure 1. F127 Surfactant Thin Film. Part (a) represents 0
hour fabrication aging and 0 hour spin coating aging, Part (b)
represents 0 hour fabrication aging and 24 hour spin coating
aging Part (c) represents 48 hour fabrication aging and 0
hour spin coating aging Part (c) represents 48 hour
fabrication aging and 24 hour spin coating aging.
(a) (b)
(c) (d)

3. Figure 1 illustrates F127 Surfactant thin film with
different fabrication and spin coating aging time.
We can see the surface is quite smooth without
obvious cavities, which promise less mechanical
stresses and cracks after calcination.
Figure 2. P123 Surfactant Thin Film. Part (a) represents 0
hour fabrication aging and 0 hour spin coating aging, Part (b)
represents 0 hour fabrication aging and 24 hour spin coating
aging Part (c) represents 48 hour fabrication aging and 0
hour spin coating aging Part (d) represents 48 hour
fabrication aging and 24 hour spin coating aging.
Figure 2 illustrates P123 Surfactant thin film with
different fabrication and spin coating aging time.
Although the surface is quite smooth without
obvious cavities right after fabrication and spin
coating, many cavities can be seen after 24 hours
aging.
Figure 3. CT16AB Surfactant Thin Film. Part (a) represents
0 hour fabrication aging and 0 hour spin coating aging with
100% surfactant. Part (b) represents 0 hour fabrication aging
and 24 hour spin coating aging with 100% surfactant. Part
(c) represents 0 hour fabrication aging and 0 hour spin
coating aging with 50% surfactant. Part (d) represents 0 hour
fabrication aging and 24 hour spin coating aging with 50%
surfactant.
Figure 3 illustrates CT16AB Surfactant thin film
with different surfactant concentration and spin
coating aging time. Part (c) and (d) demonstrate
that half surfactant can significantly eliminate
cavities on the thin film surface.
Figure 4. BrijC10 Surfactant Thin Film. Part (a) represents 0
hour fabrication aging and 0 hour spin coating aging with
100% surfactant. Part (b) represents 0 hour fabrication aging
and 24 hour spin coating aging with 100% surfactant. Part
(c) represents 0 hour fabrication aging and 0 hour spin
coating aging with 50% surfactant. Part (d) represents 0 hour
fabrication aging and 24 hour spin coating aging with 50%
surfactant.
Figure 43 illustrates BrijC10 Surfactant thin film
with different surfactant concentration and spin
coating aging time. Part (c) and (d) demonstrate
that half surfactant can significantly reduce
cavities on the thin film surface.
Ellipsometry Result:
Cauchy model with uniaxial anisotropy was
applied to fit our data. Strong stress induced
birefringence is observed in samples without
surfactant inside, i.e. samples after calcination,
which leads to a thickness difference between
Longitudinal axial and 2 plane axels.
Figure 5 illustrate Ellipsometry results of 4
different surfactant thin film. From previous
research, the index of refraction of glass and air
is about 1.45 and 1.0 respectively. All of
surfactant thin film shows index of refraction
around 1.25, which means our meso-pore thin
film has about 50% porosity according to
effective medium approximation. [21]
(a) (b)
(c) (d)
(a) (b)
(c) (d)
(a) (b)
(c) (d)

4. Figure 5. Ellipsometry of 4 different surfactant thin film.
Part (a) F127, 500 ℃ calcination, 1500 rpm, Part (b) P123
500 ℃ calcination, 1500 rpm, Part (c) CT16AB 500 ℃
calcination, 1500 rpm, Part (d) BrijC10 500 ℃ calcination,
1500 rpm.
Figure 6 illustrate Ellipsometry results of F127
surfactant thin film under different conditions:
Part (a) and (b) demonstrate that 500 ℃
calcination more fully, which corresponding to a
higher porosity, however, 300 ℃ calcination is
good enough with much shorter heat treatment
time. Part (c) and (d) prove that although the
porosity of meso-pore structure is only about 20%
in 105 ℃ calcination, however, 90 ℃ cannot
make calcination happen, which means
temperature between this area is a boundary.
Figure 6. Ellipsometry of F127 surfactant under different
conditions. Part (a) 500 ℃ calcination, 1500 rpm, Part (b)
300 ℃ calcination, 1500 rpm, Part (c) 105 ℃ calcination,
1500 rpm, Part (d) 90 ℃ calcination, 3000 rpm.
SEM Result:
Figure 7 is the SEM image of sol-gel coated
microtoroid. We can tell the thickness of meso-
pore thin film is much thicker than 1 um, which
can crack more easily.[8, 11] In this case, Spin
coating speed was optimized from 1500 rpm, 30
s to 2 steps, 500 rpm, 5s first, and then 3000 rpm,
(a)
(b)
(c)
(d)
(a)
(b)
(c)
(d)

5. 30s for thin film or 7500rpm, 30s for toroid
coating so as to decrease the thickness and
increase surface smooth.
Figure 7. SEM image of sol-gel coated microtoroids. (300 ℃
calcination, 1500 rpm), Part (a) is after the calcination while
Part (b) is after reflow.
TEM Result:
Figure 8. TEM image of sol-gel meso-pore structure. Part (a)
reveals the 3-Dimensional information and Part (b) gives us
structure parameters.
Figure 8 is TEM image which demonstrate that
sol-gel meso-pore structure was fabricated
successfully. [13-14, 17-18] We can tell the
meso-pore structure was formed in 3-Dimenion
evenly form Part (a). Part (b) provide us with
structure parameter: the diameter of meso-pore is
about 7-8 nm and inter-pore distance is around
14-16nm.
FTIR Result:
Figure 9 illustrate FTIR results of our sample.
According to the FTIR bonding parameters, the
dip of wavenumber between 800 and 1000 cm-1
correspond to C-H bond stretching while dip
between 2850 and 2950 cm-1 associate with Si-
O-Si bond stretching. Comparing 120 ℃
calcination and 90 ℃ calcination, the difference
between C-H bonding area and coherence in Si-
O-Si demonstrate that surfactant calcination has
taken place and our meso-pore thin film structure
has been formed successfully. [14]
Figure 9. FTIR results of sol-gel coated microtoroids.
Ethanol Vapor Sensing Application:
Figure 10. Shift result of sol-gel coated microtoroids. Part (a)
is pure microtoroid and Part (b) is sol-gel coated microtoroid.
Figure 10 shows the result of Ethanol vapor
sensing contrast between pure microtoroid and
sol-gel coated microtoroids. Before ethanol vapor
introduced, the resonant wavelength is stable.
After ethanol vapor introduced, the resonant
wavelength red shift. However, moving air
fluctuation lead to fluctuation Ethanol vapor
concentration, which is reflected by multiple
peaks. As Ethanol decrease with evaporation, the
concentration decay, which leads to a decaying
shift amplitude. Comparing to regular toroids,
(a) (b)
(a)
(b)

6. meso-pore coating enhance Ethanol vapor
sensing sensitivity over 2 orders of magnitude.
Following reasons may explain this phenomenon:
Firstly, meso-pore structure has bigger surface
area than smooth microtoroid.[11-15] At the
same time, the maximum value locates at the
vicinity of the ring edge. In whispering gallery
mode resonator, with radius increasing from
center to the edge, the electric field increases first
and then decreases, the maximum value locates at
the vicinity of the ring edge. So mesoporous
structure allows ethanol inside the toroids where
there is higher electric field strength, which lead
to a much higher sensitivity. [5, 7, 15]
4 Conclusion
Surface functionalized microtoroid with meso-
pore thin film has an enhanced sensitivity.
According to our orthogonal experiments, F127
was chosen as our surfactant, best recipe was
determined and procedure was optimized: Spin
coating was optimized from 1500 rpm, 30 s to 2
steps, 500 rpm, 5s first, and then 3000 rpm, 30s
for thin film or 7500rpm, 30s for toroid coating
so as to decrease the thickness and increase
surface smooth. Calcination temperature was
optimized from 500 ℃ to 300 ℃ in our research
to save time. SEM and TEM results illustrated
that meso-pore thin film was successfully
fabricated in 3-Dimensions with desired
thickness and pore diameter. Ellipsometry results
demonstrated that this thin film has about 50%
porosity. Chemical composition in meso-pore
thin film and sufficient calcination temperature
was given by FTIR And over 2 orders of
magnitude of sensing sensitivity over common
microtoroid was demonstrated by measuring
resonant wavelength shift from Ethanol vapor. In
the future, we may extend our sensing toroid from
ethanol vapor to other chemical vapor such as
acetone and water and compare the differences
between multiple chemicals. And we may
measure quantitative relation between chemical
amount and wavelength shift. Also, we may
extend the condition from gas sensing to
biomolecule sensing in aqueous conditions.
Acknowledgement
I gratitude systematic instruction from Dr. Yang,
inspiring discussion with Dr. Özdemir. I also
appreciate patient training from Steven Huang
and encouragement from all the members from
Dr. Yang’s group. This research rotation was
supported by School Fellowship of Washington
University and Dr. Yang’s group funding.
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