AN ABSTRACT OF THE THESIS OF
Vishvas Chalishazar for the degree of Master of Science in Electrical and Computer Engineering
presented on January15, 2016.
Title: Nanostructured Optical Devices for Biosensing Applications
Abstract approved:
_____________________________________________________________________
Li-Jing Cheng
We demonstrated a sensitive biosensor based on plasmonic metal composite nanostructures.
The device can be fabricated and duplicated with ease using a novel nanoprinting technology
which allows precise transfer of metal nanostructures without complicated lithography processes.
The design featured by its unique material composition and sub-wavelength structures
maximizes the exposure of localized plasmonic hot spots to analyte and, therefore, enhances the
refractive-index sensitivity with a plasmon resonance peak in visible range. A refractive-indexsensitivity of ~ 502 nm/RIU and a figure-of-merit of ~ 30.1 RIU-1 has been achieved. The new
sensing technology will find applications in medical diagnostics and environmental monitoring.
©Copyright by Vishvas Chalishazar
January15, 2016
All Rights Reserved
Nanostructured Optical Devices for Biosensing Applications
by
Vishvas Chalishazar
A THESIS
submitted to
Oregon State University
in partial fulfillment of
the requirements for the
degree of
Master of Science
Presented January 15, 2016
Commencement June 2016
Master of Science thesis of Vishvas Chalishazar presented on January 15, 2016
APPROVED:
Major Professor, representing Electrical and Computer Engineering
Director of the School of Electrical Engineering and Computer Science
Dean of the Graduate School
I understand that my thesis will become part of the permanent collection of Oregon State
University libraries. My signature below authorizes release of my thesis to any reader upon
request.
Vishvas Chalishazar, Author
ACKNOWLEDGEMENTS
Firstly I want to thank Dr. Larry Cheng for his continued faith and support in all my endeavors.
His expertise in the field has always been my motivation as well as my guiding line which has
kept me on track. I would specially thank him for the hours that he has given brainstorming with
me whenever I have hit a dead end. This would not have been possible without his continued
actions to motivate me.
Secondly I would like to thank Dr. Alan Wang, Dr. Ted Brekken and Dr. Leonard Coop for
being my committee members.
I want to thank all my friends in Corvallis who have constantly shown faith in me and have
helped me in many ways. Especially Pratim Chowdhury and Anirban Roy for being my
inspiration and constant tech support at all times. And last but not the least I want to convey my
deepest gratitude to Janhavi Kulkarni who has been the emotional pillar in my life through my
journey at Oregon State University.
I most importantly want to thank my parents and my brother for showing faith in me and
encouraging me to follow my dreams. I will be eternally in their debt for providing me this
opportunity to explore the possibilities that I never imagined would present itself.
TABLE OF CONTENTS
Page
1. Introduction ............................................................................................................................. 1
1.1 Optical Biosensors .......................................................................................................... 1
1.2 Surface Plasmon Resonance (SPR) ............................................................................ 1
1.3 Refractive Index Sensitivity (RIS), Quality Factor (Q-Factor) and Figure of
Merit (FOM) ............................................................................................................................ 3
1.4 Previous Works ................................................................................................................ 6
1.5 Research Goal ................................................................................................................ 14
2. Device Design and Parameter Optimization. ............................................................... 16
2.1 Effect of Period of the grating structure. ................................................................. 17
2.2 Effect of Interlayer material and thickness ............................................................. 17
2.2.1 Choosing metal over dielectric as the interlayer material ............................ 17
2.2.2 Optimizing the Interlayer Thickness ................................................................. 18
2.3 Effect of the Bottom Au layer. ................................................................................... 18
2.4 Effect of Laterally Etching the Interlayer................................................................ 22
2.5 Parameters for the two layered double stack device ............................................. 23
3 Device Fabrication and Optical Setup ............................................................................. 25
3.1 Fabrication Procedures and Processes ..................................................................... 25
3.2 Optical Setup .................................................................................................................. 29
4 Results and Analysis ............................................................................................................ 30
4.1 Device Fabrication Results and Analysis of Single-Layer Structures ............. 30
4.2 Device Fabrication Results and Analysis of Double-Layer Structures ........... 31
5. Conclusion............................................................................................................................. 37
References .................................................................................................................................. 38
LIST OF FIGURES
Figure
Page
Figure 1.1. Different configuration for SPP excitation: (a) Kretschmann setup, (b) Two-Layer
Kretschmann setup[11]. .................................................................................................................. 3
Figure 1.2 Reflectance spectra of a SPR device under two different background refractive indices.
......................................................................................................................................................... 4
Figure 1.3 An example for calculating the value of Full Width Half Maximum.[12] .................... 5
Figure 1.4 (a) Transmission spectra obtained from simulation and measured from the device
(b)&(c) SEM image of the top and bottom layer of device structure fabricated [14] .................... 7
Figure 1.5 (a) SEM images of silicon mold (top) and gold layer with nano-slits (bottom) and (b)
experimental results of shift in resonance peak at various environmental refractive indexes [15] 7
Figure 1.6 Scanning Electron Microscopy image of the gold nano-mushroom array [16] ............ 8
Figure 1.7 (A) Resonant wavelength shifts for different concentrations of background glycerol
solution (B) Corresponding RIS calculation plot. [16] ................................................................... 8
Figure 1.8 (a) Schematic of the H-shaped device structure (b) change in the resonant wavelength
by changing the refractive index of the sensing medium. [17] ....................................................... 9
Figure 1.9 Schematic of the LC circuit equivalent for an H-shaped unit.[17]................................ 9
Figure 1.10 Schematic (a) and SEM image (b) of the broken H shaped device. [18] .................. 10
Figure 1.11 (a) Top view SEM images of the manufactured device (b) Angled view SEM image
of the manufactured device (c) Simulated and measured 0-order reflectance spectra [19] .......... 11
Figure 1.12 (a) Schematic of the device fabrication process (b) schematic of the two beam
interference lithography tool (c) SEM image top view of the device and (d) Cross-section SEM
image of the device.[20] ............................................................................................................... 12
Figure 1.13 (a) Reflectance spectra of the device at different background refractive indexes and
(b) Measured and simulated reflectance spectra of the device. [20]............................................. 13
Figure 2.1(a) Schematic of the single layered stacking device design (b) The reflectance spectral
response of the single layered device with etched interlayer. ....................................................... 17
Figure 2.2: (a), (b) & (c) Absorption profiles for different values of period, interlayer thickness
and Bottom Au layer thickness at 1.33 Refractive Index respectively. (d), (e) & (f) Simulated
values of Refractive Index Sensitivity (RIS), Figure of Merit (FOM), Q-factor and Resonant
Wavelength peak for different Periods, interlayer thickness and bottom Au layer thickness
respectively. .................................................................................................................................. 20
LIST OF FIGURES (CONTINUED)
Figure
Page
Figure 2.3 Poynting Vector and Electric Field profiles(at the resonant wavelengths)in the unit
cells of various nanostructures. (a) A device with top and bottom gold layers and an etched
interlayer aluminum. (b) A device with top and bottom gold layers but un-etched interlayer
aluminum. (c) A device with top and bottom gold layers but etched dielectric interlayer (d) A
device with only top gold layer, an etched interlayer aluminum but no bottom gold layer.(e)
Corresponding reflectance spectra for the structures (a), (b), (c) and (d). .................................... 22
Figure 2.4 (a) Schematic of double layered stacking device and (b) Reflectance spectral response
of the double layered device with etched interlayer. .................................................................... 24
Figure 2.5 Simulated reflectance spectrums of the two devices proposed in this research. ......... 24
Figure 3.1 SEM image of the silicon master mold from Lightsmyth Technologies. [22] ............ 25
Figure 3.2 Schematic of the fabrication of single layered device in order of the processes. ........ 28
Figure 3.3 Schematic of the fabrication of double layered device in order of the processes. ...... 28
Figure 3.4 Schematic of the optical setup for reflective spectral characterization. ...................... 29
Figure 4.1 SEM image of cross-section view of the single layered device. ................................. 30
Figure 4.2 (a) Top view of the two layered double stack device design (b) SEM image of crosssection of the two layered double stack device with non etched Aluminum (c) Enlarged Crosssection of the same device with non etched Aluminum and (d) Cross-section of the same device
with etched Aluminum. ................................................................................................................. 32
Figure 4.3 (a) Simulated and (b) Measured reflectance spectra of the two layered double stack
with non-etched aluminum interlayer device design at different glucose concentrations. ........... 34
Figure 4.4 The measured reflectance spectra of the etch aluminum interlayer device at two
different refractive indices. ........................................................................................................... 34
Figure 4.5 Shift in resonant wavelength Vs refractive index linear fit curve to measure the RIS
value from the slope. ..................................................................................................................... 35
Figure 4.6 Trends for (a) Q-factor and (b) FOM for increasing glucose concentrations.............. 35
LIST OF TABLES
Table
Page
Table 1.1 List of previous devices, their performance parameters & resonant wavelength range…
....................................................................................................................................................... 13
1
1. Introduction
1.1 Optical Biosensors
A high performance optical biosensor is a transducer able to sensitively transform the signal
resulting from interaction of the analyte with a specific biological element into an optical signal.
In particular, optical biosensors based on surface plasmon resonance (SPR) which utilize
evanescent wave mechanism can perform tests in real-time using a minimum volume of samples
and reagents, without fluorescent labelling or amplification steps. Taking advantage of the
interaction of light with plasmonic materials and sub-wavelength metallic structures, SPR is
sensitive to the changes in the surrounding refractive index upon the docking of target molecules
on the device surface and, thus, is considered to be one of the most promising methods to
achieve sensitive label-free biosensing. A variety of plasmonic devices have been developed and
incorporated in practical use for rapid detection in areas of food safety, mobile healthcare and
clinical diagnosis [1]. However, the detection sensitivity, especially in visible range, and the
complexity of the device fabrication are the major issues needed to be improved.
1.2 Surface Plasmon Resonance (SPR)
Surface plasmons (SP's) or surface plasmon polaritons (SPP's) present when conduction
electrons near the surface of metal or a metallic nanostructure undergo resonant oscillations
coupled to an external optical field. When the existence conditions of SPR are satisfied the light
is confined to the surface and slowed down as a result of the coupling to the free electrons,
leading to sub-wavelength light confinement and enhancement near the metallic surface[ 2].
Because of the confinement of SP at the interface between a metal and dielectric material, SPR is
2
very sensitive to the surface chemistry and the refractive index of the surrounding environment.
The adsorption of molecules to a metal surface significantly changes the oscillation of SP and
therefore modulates the light output. The modulation can be measured by either the change of
transmissive or reflective light under normal incident. Such optical response provides the basis of
SPR-based biosensors. During the course of this research we have suggested multiple device
structures to tune and optimize the coupling of the incident light energy to improve sensing
performance. The performance of the SPR based sensor will be gauged by three different
parameters: refractive index sensitivity (RIS), Q-factor and figure of merit (FOM).
Conventional SPR biosensors generally operate in a total internal reflection mode through a
Kretschmann configuration which utilizes a prism to increase the momentum of the incoming
light to couple propagating surface plasmon on a metal surface (Figure 1 a, b)[3,4,5]. This
configuration requires a prism and complicated optical alignment and has difficulty in realizing
large dynamic range of detection, and small probing areas. By forming 1D grating or 2D periodic
sub-wavelength structures on a metal surface, it is possible to couple light into the surface
plasmon by providing the necessary momentum matching. This phenomenon can be employed
for biosensor applications because they can perform with reduced detection volumes while
operating at normal illumination [6,7,8,9]. Because of encouraging optical confinement in the
visible and near infrared range, SPR's in this particular range are of extremely high interest and
may contribute tremendously in the future of biosensors [10].
When necessary momentum matching is provided, the incident light gets coupled to the
surface plasmons and gives rise to the evanescent waves. These waves are generated by inducing
oscillations in the surface charges due to the oscillating electric field. Evanescent waves have
spatially decaying radiation fields that penetrate both metal and environmental dielectric material.
3
This penetration of the evanescent waves in the surrounding material makes it extremely
sensitive to the change in the surrounding material. Larger the penetration depth higher the
sensitivity towards the change in the background material.
Figure 1.1. Different configuration for SPP excitation: (a) Kretschmann setup, (b) Two-Layer
Kretschmann setup[11].
1.3 Refractive Index Sensitivity (RIS), Quality Factor (Q-Factor) and Figure of
Merit (FOM)
SPR sensors functioning as an optical refractive index sensor are quantitatively compared by
their refractive index sensitivity. RIS defines in equation (2) characterizes the magnitude in shift
of the resonant wavelength versus the change in surrounding RI and has a unit of nm/RIU, with
RIU being the refractive index unit
RIS =
∆ λ (nm)
∆n (RIU)
(2)
Take the spectral shift shown in figure 1.3 for instance. A 20 nm shift of the resonance
wavelength ∆λ in response to a RI change ∆n of 0.05 RIU yields an RIS of 400 nm/RIU.
4
Figure 1.2 Reflectance spectra of a SPR device under two different background refractive
indices.
The second most important measure of the performance of any SPR based sensor is the
quality factor or Q-factor. Q-factor is a dimension less quantity that describes the resonance
sharpness which is physically determined by the photon lifetime stored in the resonator. As
shown in figure 1.3 the Q-factor can be defined as the ratio of the resonance wavelength λ and
the full width at half-maximum (FWHM) bandwidth of the resonance
Q − factor =
λ (nm)
FWHM (nm)
(3)
A high-Q resonance has a narrow linewidth able to filter the spectral noise more effectively,
which leads to a lower spectral deviation from the resonance center. Thus, Q-factor plays an
important role in the detection limit of the sensor.
5
Figure 1.3 An example for calculating the value of Full Width Half Maximum.[12]
The third measure for the performance is figure of merit (FOM) which is defined as the ratio
of RIS to the FWHM of the resonance and has a unit of RIU-1.
FOM =
RIS (nm/RIU)
FWHM (nm)
(4)
The higher the FOM, the better is the performance of the sensor. A higher FOM can be
achieved by increasing RIS or by reducing FWHM of the resonance which essentially indicates a
high-Q resonance. This makes FOM a better judge for the device performance than the other two
because it considers both the previous measures[13].
6
1.4 Previous Works
Recent studies have demonstrated several new SPR designs for bio-sensing applications.
Most of them have shown resonance in near infra-red (IR) or mid-IR range. Creating SPR
structures with both high Q resonance in the visible range and decent RIS remains challenging.
The design fabrication methods and performance of an array of representative sensors are
described as follows and summarized in Table 1.
Silver hole array structures [14]
The device consists of 200nm thick silver nanohole array on a glass substrate fabricated
using a template stripping technology which allow repeatable duplication of nanostructure with
ease. The subwavelength Ag hole array were formed by depositing Ag thin film on a silicon hole
array template. The pattern was then transferred to an epoxy coated glass substrate The device
exhibits well defines Fano-type resonant profiles with a Q factor of 71 and RIS of ~ 495 nm/RIU
at resonant wavelength of 710 nm as shown in Figure 1.4. The result yields a FOM of ~50 RIU-1.
7
Figure 1.4 (a) Transmission spectra obtained from simulation and measured from the device
(b)&(c) SEM image of the top and bottom layer of device structure fabricated [14]
Gold nano-slits [15]
The gold nano-slits grating array as shown in figure 1.5 (a) exhibits a resonance in visible
range with an RIS of ~ 320 nm/RIU at normal illumination. Tilting incident angle by ± 7 0
diminishes RIS to ~ 230 nm/RIU but enhances the intensity of the transmission spectra by 4.5
times and therefore increases FOM by 1.8 times comparing with the result measured with normal
incidence.
(a)
(b)
Figure 1.5 (a) SEM images of silicon mold (top) and gold layer with nano-slits (bottom) and (b)
experimental results of shift in resonance peak at various environmental refractive indexes [15]
Gold nano-mushroom array [16]
Figure 1.6 shows a nano-mushroom array formed by depositing thin layer of gold on
polymeric nano-pillar structures created by interference lithography The device displayed a
8
strong localized surface plasmon resonance (LSPR) and was reported to have an RIS of ~
465nm/RIU with the resonance at ~ 920 nm (Figure 1.7).
Figure 1.6 Scanning Electron Microscopy image of the gold nano-mushroom array [16]
Figure 1.7 (A) Resonant wavelength shifts for different concentrations of background glycerol
solution (B) Corresponding RIS calculation plot. [16]
9
H-shaped slit in gold sheet [17]
This device consists of an H-shaped slit carved into a 100nm thick gold layer on a glass
substrate. Figure 1.8 shows the schematic of the structure and its spectral response in the IR
range. The resonance mechanism was explained by an equivalent LC resonator circuit. As shown
in figure 1.9 the gaps and area with the surrounding metal of the H-shape slit acts as a set of
capacitors and inductors resulting in a resonator circuit operating in IR frequency. The resonant
wavelength can be estimated using a simple circuit analogy.
Figure 1.8 (a) Schematic of the H-shaped device structure (b) change in the resonant wavelength
by changing the refractive index of the sensing medium. [17]
Figure 1.9 Schematic of the LC circuit equivalent for an H-shaped unit.[17]
10
H shaped broken slits [18]
Another H shaped structure but with disconnected slits instead was demonstrated to
perform as a LSPR-based sensor with a resonance in the IR range. Figure 1.10 shows the
schematic as well as the SEM image of the device structure. This structure was realized using
focused ion beam lithography. The resonance was attributed to the fact that the side quadrupole
antennas coupled to each other through a dipole antenna in between offset from the centre which
creates a broken symmetry. While the theoretical RIS and FOM were 725 nm/RIU and 7.4 RIU -1
respectively the experimental RIS and FOM were measured to be 588 nm/RIU and 3.4 RIU-1
respectively.
Figure 1.10 Schematic (a) and SEM image (b) of the broken H shaped device. [18]
11
U-shaped gold cavity structures [19]
The U-cavity structure in figure 1.11 exhibits high sensitivity by trapping light in hollow
cavities with coupled ridge hotspots to U-cavity. The structure was fabricated by electron-beam
lithography, large-angle metal evaporation and removal of sacrificial e-beam resist. Figure 1.11
(a) & (b) shows the SEM images of the U-cavity device, and figure 1.11 (c) shows the measured
and simulated reflectance spectra. Sharp and strong resonance reflectance dips were
experimentally observed with a narrow FWHM of 14 nm. A high RIS of 791 nm/RIU and high
FOM of 57 RIU-1 were achieved due to the optical vortices and the large area of electric field
hotspots exposed to the sensing medium.
Figure 1.11 (a) Top view SEM images of the manufactured device (b) Angled view SEM
image of the manufactured device (c) Simulated and measured 0-order reflectance spectra
[19]
12
Double layer gold grating device [20]
A double layered metal grating structures (DMGs) was fabricated by direct deposition of
5 nm - thick nickel and 100 nm - thick gold thin film on a polymeric grating structure created by
using interference lithography on a quartz substrate as shown in Figure 1.12 (a) and (b). Figure
1.12 (c) and (d) shows the SEM images of the fabricated device. The corresponding spectral
response in figure 1.13 shows a large RIS of ~ 525 nm/RIU and FOM of 38 RIU-1 .
Figure 1.12 (a) Schematic of the device fabrication process (b) schematic of the two
beam interference lithography tool (c) SEM image top view of the device and (d) Crosssection SEM image of the device.[20]
13
Figure 1.13 (a) Reflectance spectra of the device at different background refractive
indexes and (b) Measured and simulated reflectance spectra of the device. [20]
Table 1.1 List of previous devices, their performance parameters & resonant wavelength range.
Device structure
RIS value
(nm/RIU)
FOM (RIU-1)
Wavelength
range for the
resonance (nm)
Experim
ental (E)
or
Theoreti
cal (T)
Ref
1
Silver hole array
~ 495
50
700 - 750 (Vis)
E
[14]
2
Gold slits grating
array
~ 230
48
650 - 700 (Vis)
E
[15]
3
Gold nano
mushroom array
~ 465
N/A
900 - 920
E
[16]
4
H-shape slit in
gold sheet
~ 1170
2.1
1300 - 2000 (IR)
T
[17]
5
H-shaped broken
slits in gold sheet
~ 588
5.3
1600 - 2000 (IR)
E
[18]
6
U-shaped gold
cavity
~ 791
57
1500 - 1600 (IR)
E
[19]
7
Double-layer gold
grating
~520
38
800 - 900 nm
(Vis-NIR)
E
[20]
(Vis -NIR)
14
Most of the plasmonic sensors reported to date are designed to operate in NIR or IR range in
which a larger resonance shift can be acquired comparing with those operating in visible range.
The primary reason for this is that the plasmon energy density residing in the background
dielectric environment actually depends on the real part of the metal permittivity which increases
nearly linearly with wavelength. That leads to a higher RIS at longer wavelengths [21]. However,
the relatively high cost of the optical components operating in IR range, including light sources,
photo detectors and spectrometers, drives us to seek for better plasmonic sensor designs that
support sufficiently large RIS while operating in visible range.
1.5 Research Goal
The goal for this research was to explore the new design of plasmonic biosensors for
advancing refractive index sensitivity (RIS) in visible range and to develop nanofabrication
technology for facilitating device manufacturing. A couple of subwavelength plasmonic
structures were theoretically and experimentally investigated to achieve the criteria. The sensor
performance quantified by its RIS, Q-factor and FOM was optimized by tuning the device
structural geometries and dimensions with the attempt to enhance the magnitude of electric-field
hotspots and to maximize the exposure of the hotspot to the surrounding dielectric medium.
This research is also intended to provide an insight on how the structures along with the
choice of material can affect the plasmonic resonance and the absorption of light. The ease of
fabrication lies in the roots for any biosensor design and so the main agenda of this research also
circles around the same idea. Even though there are a lot of limitations on the fabrication
capability for realizing the proposed nanostructure, the clear intention is to make it as simple and
elegant as it can get. During the course of this research, the fabrication of proposed double stack
device design was attempted by multiple methods in our endeavor of making the process
15
extremely simple with least number of the processing steps. Surface functionalization of the
sensor that supports selective biosensing will be developed in the next phase of this research.
16
2. Device Design and Parameter Optimization.
For this research we propose a periodic double stack nanostructure aiming to improve the
sensitivity of refractive index sensing. As shown in figure 2.1(a) the structure of each unit cell
consists of two gold (Au) layers connected by a thin aluminum (Al) support in between. The top
and bottom Au layer are 100nm and 40nm thick, respectively. The interlayer Al of 160nm
thickness is laterally etched. The entire structure lies on a plastic substrate through a 50 nm thick
adhesion polymer. The whole nanostructure slightly lifts from the surface as the result of a
pattern transfer process. The choice of dimension for each layer will be discussed in detail ahead
in this chapter. The significance of lateral etching and the choice of Al instead of a dielectric
material as the supporting interlayer will be explained in detail. All these parameters are
optimized to sharpen the resonance peak that lead to high Q-factor and FOM. During the
optimization process RIS was considered the key parameter in deciding the range of values for
each parameter. All the simulation results presented in this chapter are performed using
Lumerical FDTD Solution., including reflection and transmission spectra, and electric-field and
Poynting vector profiles.
(a)
(b)
17
Figure 2.1(a) Schematic of the single layered stacking device design (b) The reflectance spectral
response of the single layered device with etched interlayer.
2.1 Effect of Period of the grating structure.
The periodic feature of the proposed structure supports propagating surface plasmon
resonance. The effect can be observed in the absorption spectrum in Figure 2.2(a) in which the
resonant wavelength varies with the size of the period. The absorption peak is sharp and deep
when the structural period is between 450 nm and 550 nm. A period of 500 nm was selected to
form the device. This condition yields a large absorption peak at the wavelength around the
visible range, an RIS > 500 nm/RIU and a relatively narrow linewidth (high Q-factor) resulting
in a large FOM.
2.2 Effect of Interlayer material and thickness
There are two aspects to this section. First is choosing the right material for the interlayer
and the second is choosing the optimized thickness of the same material. At this point it is
important to mention that the optimized thickness varies with the choice of material. But
considering the fabrication capability, it is necessary to choose the correct pair of material and
thickness that can be accommodated in our limits.
2.2.1 Choosing metal over dielectric as the interlayer material
In this subsection we would like to shine some light on the conventional design that has
stacking metal layers with a dielectric material as the interlayer. With a silicon dioxide (SiO2)
interlayer, a majority of electric field penetrates through the entire structure even with the
presence of the bottom Au layer as observed in the Poynting vector profile in figure 2.3(c).In the
18
proposed design with aluminum as the interlayer material, light energy at a resonance
wavelength can be redirected by the interlayer conductive structure, concentrated to and then
blocked by the bottom gold layer. As a result, light energy is trapped on the structure with
coupled high-intensity electric-field hotspots on the top Au layer. Comparing the Poynting vector
profiles in figure 2.3 (a) and figure 2.3 (c), the leakage of energy as transmission is observable in
the device with SiO2 interlayer. The device with an Al interlayer yields a stronger electric field
hotspots around the nanostructure (|E|2~300) comparing with the counterpart with a SiO2
interlayer (|E|2~200).
2.2.2 Optimizing the Interlayer Thickness
An interlayer thickness in the range of 10 nm - 200 nm is considered due to the limitations
of fabrication capability and mechanical strength of the nanostructure. Any thickness beyond this
range might not hold the structure after lateral etching. Also the structure will be fabricated
through a transfer technology using a Si mold which has a trench depth of about 350 nm
available to accommodate all the three layers. The total thickness of the structure cannot be taller
than 300 nm that is the maximal thickness required for a transfer process. The optimized
interlayer thickness selected from the simulated data in figure 2.2(e) is chosen to be 150 nm. The
design fit into the picture almost perfectly because there is a very sharp absorbance in the visible
range (figure 2.1 (b)). The selected interlayer thickness has an acceptable process window. The
spectral response does not change significantly under a thickness variation of +/- 10 nm.
2.3 Effect of the Bottom Au layer
The bottom Au layer in the proposed structure was found to play an important role in the
absorption of light in a narrow wavelength range. The addition of the thin bottom Au layer, even
19
discontinued, prevents the incident light from leaking out as transmission from the bottom at a
certain wavelength, leaving most of the energy absorbed on the top layer. This can be observed
from the Poynting vector profile in figure 2.3 (a). It can be clearly seen that no Poynting vectors
present below the bottom layer. Comparing with figure 2.3 (d), the structure without bottom gold
layer allows the energy to leak out as transmission. The energy density in the electric field
hotspots around the nanostructure is extremely low without the bottom Au layer. No clear
reflectance valley is observed (figure 2.3(e)). This also suggests that there is very minimal
absorption in the device with no bottom gold layer (figure 2.3 (d)).
To determine a proper thickness of the bottom Au layer, we again follow the same
procedure that we did for the period. Figure 2.2 (c) suggests that a minimum of 40 nm thick gold
layer is needed for the absorption to be maximized. Further increase in thickness does not affect
the spectral feature as shown in figure 2.2(f). We conclude that a minimum bottom Au layer
thickness of 40 nm is required to sustain high values of RIS, Q-factor and FOM.
20
1200
0.6
0.4
600
0
300
RIS (nm/RIU)
900
0.2
800
60
700
40
600
20
(c)
1
0.6
0.4
600
0.2
0
20
40
60
80
100
Bottom Au Thickness (nm)
400 450 500
Period (nm)
550
600
35
(e)
600
25
400
15
200
5
850
40
800
30
20
750
10
700
100
150
200
250
Interlayer Thickness (nm)
900
300
350
FOM (1/RIU)
10
Q-factor
300
900
0.8
Wavelength (nm)
20
800
Resonant Peak (nm)
Wavelength (nm)
0.8
1200
400
300
1
(b)
100
300
500
700
Interlayer Thickness (nm)
30
600
300
(f)
520
RIS (nm/RIU)
400
500
Period (nm)
500
FOM (1/RIU)
0
40
Q-Factor
0.2
(d)
0
35
25
510
15
500
FOM (1/RIU)
0.4
600
RIS (nm/RIU)
0.6
Resonant peak (nm)
900
Resonant Peak (nm)
Wavelength (nm)
0.8
300
300
600
1
(a)
5
50
730
40
30
720
20
Q-Factor
1200
10
710
0
0
20
40
60
80
100
Bottom Au Thickness (nm)
Figure 2.2: (a), (b) & (c) Absorption profiles for different values of period, interlayer thickness
and bottom Au layer thickness at 1.33 Refractive Index respectively. (d), (e) & (f) Simulated
21
values of refractive index sensitivity (RIS), figure of merit (FOM), Q-factor and resonant
wavelength peak for different periods, interlayer thickness and bottom Au layer thickness
respectively.
22
(e)
Figure 2.3 Poynting Vector and Electric Field profiles(at the resonant wavelengths)in the unit
cells of various nanostructures. (a) A device with top and bottom gold layers and an etched
interlayer aluminum. (b) A device with top and bottom gold layers but un-etched interlayer
aluminum. (c) A device with top and bottom gold layers but etched dielectric interlayer (d) A
device with only top gold layer, an etched interlayer aluminum but no bottom gold layer.(e)
Corresponding reflectance spectra for the structures (a), (b), (c) and (d).
2.4 Effect of Laterally Etching the Interlayer
The use of a narrow metal interlayer structure to connect the top and the bottom gold layer
was found to effectively manipulate light and enhance the absorption at the resonance
wavelength. This unique property is not observable when the metal interlayer is as wide as the
two Au layers. This will further magnify electric field hotspots on the top Au layer resulting a
23
strong resonance. To implement the structure, we need to etch it laterally at least up to a certain
extent. The resonance was observed to significantly improve when the interlayer width is
reduced to at least 60% which is 70 nm - 80 nm of lateral etching from each side. The device
without a narrowed interlayer exhibits no resonance peak at any interlayer thickness. The electric
field and Poynting vector profiles in figure 2.3 (a) and (b)suggest that having the widths of the
interlayer and Au layers identical reduces absorption leaving weak electric field hotspots coupled
around the nanostructure. Lateral etching is also advantageous in exposing a large area of electric
field hotspots to the surrounding medium resulting in the improvement in refractive index
sensitivity.
2.5 Parameters for the two layered double stack device
Figure 2.4 (a) shows an alternative device structure designed with the attempt of achieving a
similar spectral response while simplifying the fabrication process and figure 2.4 (b) exhibits the
corresponding reflectance spectra. This structure is different from the previous design in the way
that additional double stack metal structures are present in the trench of the grating. The
dimensions of this device structure are as follows: Both top and bottom Au layers are about 100
nm thick and the Al interlayer is about 160 nm thick. The dimension of the metal layers in the
trench and over the feature remains the same. Figure 2.5 shows the reflection spectra for the
both designs (schematics shown in figure 2.1(a) and 2.4(b)). The spectral responses of both
structures show identical resonance peaks at 730 nm wavelength contributed by the top portion
of the nanostructure.
24
(a)
(b)
Figure 2.4 (a) Schematic of double layered stacking device and (b) Reflectance spectral response
of the double layered device with etched interlayer.
Figure 2.5 Simulated reflectance spectrums of the two devices proposed in this research.
25
3 Device Fabrication and Optical Setup
3.1 Fabrication Procedures and Processes
A nanoprinting process was developed to facilitate the fabrication of the double stack device.
This method enables precise and repeatable transfer of nanoscale structures. The silicon mold
used in this process was a 1D grating structure acquired from Lightsmyth Technologies and has
500nm period, 220nm feature width and 350nm groove depth. A scanning electron microscopy
(SEM) image of the silicon mold is shown in figure 3.1.
Figure 3.1 SEM image of the silicon master mold from Lightsmyth Technologies. [22]
26
The silicon mold surface is treated to promote demolding process. Two types of demolding
agents were tested; one is perfluoro-silane (Trimethyl(trifluoromethyl)silane), the other chlorosilane (Trichloro(octadecyl)silane) (OTS). The treatment of perfluoro-silane was found to make
the evaporated metal layers on the mold surface extremely rough while OTS treated mold yields
a smooth deposited metal layer. The procedures for this step are as follows:
Protocol for surface silanization:

Clean the silicon mold with acetone, methanol and de-ionized water (AMD clean).

Dry the mold using nitrogen gas.

Treat the mold with oxygen plasma for 5 minutes.

Keep the mold in a glass petri dish on a hot plate at 1000C. Drop 2μL of OTS in the
petri dish and cover the petri dish with aluminum foil so that the reaction can take
place in the dark.

Remove the mold after 15-20 minutes, rinse the mold in iso-propanol, and sonicate it
for 5 minutes.

Remove the mold and blow dry it using nitrogen gas.
The fabrication process is summarized in figure 3.2. The next step is to evaporate all the
stacking layers of metal on to the mold. It is not desired to have the metal deposited over the
entire mold so a shadow mask with 2 mm opening was used to define the area of the metal
structure. The three metal layers were deposited using Veeco thermal evaporator in an order of
100 nm top Au layer, 150 nm interlayer Al, and 50 nm bottom Au layer.
The deposited metal layers present on top of the grating mold were then removed using
scotch tape. To ensure that the metal layers in the trench stay intact during the stripping process,
we remove the scotch tape in the direction perpendicular to the grating orientation.
27
The metal structure staying in the trench of the mold was then transfer to a polyethylene
terephthalate(PET) sheet through a UV curable polymer layer. The UV curable polymer is
prepared from a prepolymer mixture containing glycerol propoxytriacrylate(GPTA) (OTA-480,
Allnex) 1% v/v photoinitiator 2-hydroxy-2-methyl-1-phenyl-1-propanone (HMPP). A drop of 1
μL prepolymer mixture was placed on top of the silicon mold and sandwiched by a PET sheet on
a piece of glass slide as a back support. The physical contact was able to spread out the
prepolymer over the area of the mold containing the evaporated metal. The polymer was cured
using a UV LED lamp with 375nm wavelength for 7 minutes at an irradiance of 135 mW/cm-2.
After removing the glass back support, the PET sheet was detached from the grating mold to
bring the metal nanostructures out of the template. Demolding was performed along the direction
of grating to ensure high fidelity transfer of the nanostructures.
The transfer process is extremely easy, fast and repeatable with consistent accuracy. The last
step which is also the most crucial step is the lateral etching of the aluminum interlayer. After
oxygen plasma treatment for 5 minutes at 50 W power, the device was etched in a type A Al
etchant at 500C for 1 min. It is required to turn the device surface hydrophilic using oxygen
plasma treatment to proceed the lateral etching. The hydrophobic nature of the polymer and its
nanostructure make the etchant solution not accessible to the Al surfaces.
28
UV exposure
Top Layer
Removal
Metal
Evaporation
PET
GPTA
Si mold
Pattern
Transfer
Wet Etching
Figure 3.2 Schematic of the fabrication of single layered device in order of the processes.
As the process illustrated in figure 3.3, the second device can be fabricated in a simplified
process. In this process we transfer the grating patterns from the same silicon mold to a UV
curable polymer backed by a PET sheet using UV-assisted nano-imprinting technique. The
metal layer in the order of 100 nm thick Au, 130 nm thick Al and 100 nm thick Au were directly
deposited on the polymeric grating structure.
UV exposure
PET
GPTA
Pattern
Transfer
Si mold
Metal
Evaporation
Wet Etching
Figure 3.3 Schematic of the fabrication of double layered device in order of the processes.
29
3.2 Optical Setup
An optical setup illustrated in figure 3.4 was used for device measurement. A broadband
white light source produced from a halogen lamp was delivered through an optical fiber,
collimated, polarized, deflected by a 50/50 splitter, and focused by an achromatic lens to the
back focal plane of a 10X objective to produce a collimated light incident to the device. The
reflective light was collected by the same objective and measured by a spectrometer.
Spectrometer
Collimator
Collimator
50/50
splitter
White light
source
Achromatic Iris and
polarizer
lens
10X objective
Device flow cell
Figure 3.4 Schematic of the optical setup for reflective spectral characterization.
We also used a flow cell to manipulate fluid above the sensor for quantifying refractive
index sensitivity using solutions that produce various refractive indices. This flow cell was
formed by the device substrate and a PET sheet with a channel height defined by a 1 mm thick
silicone spacer. The solutions were delivered through the tubings connecting to the flow cell. If
necessary flow rate can be precisely controlled by using a syringe pump.
30
4 Results and Analysis
4.1 Device Fabrication Results and Analysis of Single-Layer Structures
Figure 4.1 shows the SEM images of the single layered stacking device with metal structures
present only on top of the thin features and not in grooves. The Al interlayers in the structure
were not laterally etched. Neglecting the polymer filled in the nanostructure for SEM imaging
purpose the structure side-walls are almost completely covered with UV cured polymer clearly
visible in the figure 4.1. This makes the interlayer material inaccessible to etchant, thus making it
impossible to etch the interlayer aluminum. We believe that this phenomenon occurs because the
evaporation becomes bulky as the metal layers grow thicker. This effect forms the evaporated
metal in the trenches of the silicon mold in a shape of trapezoids resulting in the structure shown
in figure 4.1 with side walls covered with polymer.
Figure 4.1 SEM image of cross-section view of the single layered device.
31
4.2 Device Fabrication Results and Analysis of Double-Layer Structures
Figure 4.2 shows the SEM images of the double layered stacking device in which the metal
structures are present both on the top of the grating structure and in the grooves. The structures
with and without lateral etching of Al interlayer are shown in figure 4.2 (c) and (d), respectively.
The resulting device structures are different from the design. The deposited metal structure
becomes bulky as it grows thicker. The top Au layer covers most of the sidewall of the Al
interlayer. The unwanted structure resulted from the non-directional metal evaporation produced
by the short working distance of the evaporation tool.
From the cross-sectional SEM images in figure 4.2(c) & (d) we can notice that the narrowed
Al interlayer was left after lateral etching to connect the top and bottom gold layers. Similar
structures can also be observed in the trenches. The wet etching process step is highly unreliable
due to the unwanted coverage of top Au layer over the Al interlayer sidewalls. Keeping this in
mind even though we observe very thin aluminum layer the top and bottom gold layers are still
suspended and retain their original position. We observe the metal in the trenches are shaped as
trapezoids. This occurs when the metal evaporation is not very directional due to the short
working distance of the tool. The metal deposition on the features also grow laterally along with
vertical direction and this phenomenon naturally blocks the trench area. The results of the nondirectional evaporation become more noticeable as the structure grows thicker. We also observe
rough metal surface from the top view image in figure 4.2(a). This rough surface may be
advantageous in improving RIS. The measured RIS value as observed in figure 4.3(b) is higher
than that obtained by simulations. We hypothesize that this could be the result of the rough
surface and the discrepancies between the simulation structure and the real device.
32
(a)
(b)
(c)
(d)
Figure 4.2 (a) Top view of the two layered double stack device design (b) SEM image of crosssection of the two layered double stack device with non etched Aluminum (c) Enlarged Crosssection of the same device with non etched Aluminum and (d) Cross-section of the same device
with etched Aluminum.
33
The transmission of the double-layer structure is completely zero over the spectral range
between 400 nm to 1000 nm. So any light not reflected from the device is considered to be
absorbed. The reflectance spectrums for a non-etched device are measured at various refractive
indices using different concentrations of D-Glucose aqueous solutions: DI water (~1.333), 10%
glucose solution (~1.3477), 20% glucose solution (~1.3635), 30% glucose solution (~1.3805),
and 50% glucose solution (~1.4181) [ 23 ]. Figure 4.3(a) and (b) show the simulated and
experimental spectra, respectively, under five different background refractive indices. The
theoretical RIS, Q-factor and FOM are calculated to be 401 nm/RIU, 64.74 and 38.56 RIU-1,
respectively. There are three major features observed in the reflectance spectrum for water in
figure 4.3(b). A peak at 665nm, a valley at 674 nm and another peak at 705 nm. The valley at
674 nm was used to measure all the performance parameters. The RIS was determined by the
slope of the linear fit to the measured wavelength shifts as plotted in figure 4.5. The average of
the measured RIS is about 502 nm/RIU. The averaged Q-factor and FOM are measured to be
41.54 and 30.115 RIU-1.
(a)
34
Figure 4.3 (a) Simulated and (b) Measured reflectance spectra of the two layered double stack
with non-etched aluminum interlayer device design at different glucose concentrations.
Figure 4.4 shows the measured reflectance spectra of the device with lateral etching of Al
interlayer at two different surrounding refractive indices. The results do not change drastically
from the non-etched device. We believe this is because the gold covers the side walls of
aluminum interlayer as can be seen in the SEM images of cross-section figure 4.2(c) and (d). The
coverage of the top gold layer encloses the cavity we intended to create in the Al interlayer
making the spectral responses of the devices with etched and non-etch Al interlayer almost
identical.
Figure 4.4 The measured reflectance spectra of the etch aluminum interlayer device at two
different refractive indices.
35
Figure 4.5 Shift in resonant wavelength Vs refractive index linear fit curve to measure the RIS
value from the slope.
Figure 4.6 Trends for (a) Q-factor and (b) FOM for increasing glucose concentrations.
The spectral response of the device with non-etched Al interlayer can vary with medium
refractive indices. Figure 4.6 (a) and (b) show that both Q-factor and FOM improve as the
glucose concentration increases. This trend is crucial as it affects the range of detection. For
36
realizing this structure as a biosensor its surface chemistry needs to be changed. This changes the
background refractive index around the structure. The trends in figure 4.6 (a) and (b) provide a
standard deviation to expect when we are operating in different range of background refractive
index.
37
5. Conclusion
In the course of this research we have successfully designed two sensor devices and have
implemented one of the designs that exhibits a high refractive index sensitive in the visible
region. The importance of electric field hotspot, its energy density and its positioning has been
proven critical in designing the structural parameters of the device. The device was measured to
exhibit an RIS of 502 nm/RIU, a Q-factor of 41.5 and a FOM of 30.1 RIU-1. The performance is
superior to most of the conventional device operating with the resonant wavelength below 800
nm wavelength.
Facile fabrication process was developed to pattern metal nanostructures with ease. This was
achieved by using the nanoprinting and UV polymer-assisted transfer techniques. The constraint
of non directional metal deposition has limited successful fabrication of the first design.
For the future work, we will seek for solutions to overcome the limitations on fabrication
capabilities and continue to implement the first design that takes advantages of the narrowed Al
support to enhance the device performance. We will also explore different designs that support
both high-Q resonance and large refractive index sensitivity. The resulting devices will be
functionalized with molecular probes to realize biosensing applications.
38
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