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Microfluidic refractive index sensor based
on polymer grating couplers
C. Prokop1,2,
S. Schoenhardt1,2, Christian Karnutsch1, Arnan Mitchell2
1 Institute for Optofluidics and Nanophotonics (IONAS), Department of Electrical Engineering and Information Technology, University of Applied Sciences, Karlsruhe, Germany
2 School of Electrical and Computer Engineering, RMIT University, Melbourne, Australia
1 [email protected] laser optics – 19 March 2014, Berlin, Germany
• Biological and chemical analysis of solutions and compositions of fluids
• Microfluidic lab-on-a-chip platforms offer:
• Low reagent and sample consumption
• High processing speed and precision
• High portability
• Low-cost
• Optical detection is the most sensitive in biochemical analysis
• Optical sensor combined with microfluidics – an optofluidic lab-on-a-chip sensor
2
Microfluidic refractive index sensor based on polymer grating couplers
Motivation
[email protected] laser optics – 19 March 2014, Berlin, Germany
Microfluidic refractive index sensor based on polymer grating couplers
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Route to efficient coupling of light into polymer photonic devices
• Surface grating coupler are particularly attractive
• High coupling efficiency
• Light coupling anywhere on the wafer
• Problem: Difficult to implement in polymer material due to low refractive index contrast
• Proposed solution: Increasing the refractive index contrast by air cavities
Grating coupler Waveguide Substrate Air cavity Polymer structure
Light source Detector
[email protected] laser optics – 19 March 2014, Berlin, Germany
Microfluidic refractive index sensor based on polymer grating couplers
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Proposed sensor design in cross-section, not to scale
• Device uses an grating coupler as a refractive index sensor element
• Depending on the refractive index of the analyte, the peak wavelength shifts
[email protected] laser optics – 19 March 2014, Berlin, Germany
Waveguide
Substrate
KMPR
SU-8
Air cavity Analyte channel
Grating coupler
• Simulation carried out in CAMFR (CAvity Modelling FRamework) [1]
• n1 < n3 < n2
[1] See CAMFR website http://camfr.sourceforge.net
Microfluidic refractive index sensor based on polymer grating couplers
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Simulation overview
[email protected] laser optics – 19 March 2014, Berlin, Germany
PML
PML
Air layer, n1
Analyte layer, n3
Guiding layer, n2 thguide
Λ thgroove
n2 = 1.57
n1 = 1.0
n3 = 1.33
• Simulation carried out in CAMFR (CAvity Modelling FRamework) [1]
• λ = 1550 nm
• thguide = 900 nm
• thgroove = 500 nm
• Period Λ = 1340 nm
• Filling factor = 0.5
• n1 < n3 < n2
[1] See CAMFR website http://camfr.sourceforge.net
Microfluidic refractive index sensor based on polymer grating couplers
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Simulation overview
[email protected] laser optics – 19 March 2014, Berlin, Germany
Microfluidic refractive index sensor based on polymer grating couplers
7
Simulation results for a grating coupler optimized for an analyte with n = 1.33
[email protected] laser optics – 19 March 2014, Berlin, Germany
8
Microfluidic refractive index sensor based on polymer grating couplers
Simulation results for a grating coupler optimized for an analyte with n = 1.33
Microfluidic refractive index sensor based on polymer grating couplers
[email protected] laser optics – 19 March 2014, Berlin, Germany
Sensitivity of 300 nm/RIU
Microfluidic refractive index sensor based on polymer grating couplers
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Materials: SU-8 and KMPR
• Epoxy based negative near-UV photoresists
• Developed for high aspect ratios in very thick photoresist layers
• SU-8 is very similar to KMPR
General
Optical
properties
• nSU-8: 1,575 at 1550 nm
• nKMPR: 1,547 at 1550 nm
Characteristics
regarding
optofluidics
• Optically transparent
• Structurable by photolithography or nanoimprint lithography
• Inert to most fluids
• High chemical and plasma resistance
[email protected] laser optics – 19 March 2014, Berlin, Germany
Microfluidic refractive index sensor based on polymer grating couplers
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Proposed fabrication method (1)
PFPE
PDMS
SU-8
Substrate
KMPR
Substrate
1. Cast PDMS from PFPE working stamp
2. Spin coat SU-8 on PDMS stamp
3. Pattern KMPR photolithographically
[email protected] laser optics – 19 March 2014, Berlin, Germany
4. Bond SU-8 film to KMPR structure
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Microfluidic refractive index sensor based on polymer grating couplers
Proposed fabrication method (2)
KMPR
SU-8
Substrate
KMPR SU-8
Analyte
5. Peel off PDMS stamp
6. Apply analyte
Substrate
PDMS
KMPR SU-8
Substrate
[email protected] laser optics – 19 March 2014, Berlin, Germany
12
Microfluidic refractive index sensor based on polymer grating couplers
Preliminary fabrication results – bonding SU-8
• Bonding of structured SU-8 films
• Trenches up to 150 x 300 µm
• SU-8 film thickness down to 500 nm
[email protected] laser optics – 19 March 2014, Berlin, Germany
100 µm
1 µm 1 µm
13
Microfluidic refractive index sensor based on polymer grating couplers
Preliminary fabrication results – bonding SU-8
[email protected] laser optics – 19 March 2014, Berlin, Germany
10 µm
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Microfluidic refractive index sensor based on polymer grating couplers
Preliminary fabrication results – grating coupler
Silicon master structure:
• Grating period: 1.35 µm
• Groove depth: 188 nm
• Various waveguide
lengths up to 500 µm
• Trench: 15 µm
• Fabricated by:
[email protected] laser optics – 19 March 2014, Berlin, Germany
20 µm
10 µm
15
Microfluidic refractive index sensor based on polymer grating couplers
Preliminary fabrication results – grating coupler
[email protected] laser optics – 19 March 2014, Berlin, Germany
PDMS grating coupler stamp
10 µm
SU-8 grating coupler
10 µm
16
Acknowledgements
• Sensor simulation shows a sensitivity of 300 nm/RIU
• Bonding technique for thin structured SU-8 layer down to 500 nm
• Grating coupler fabrication in SU-8
• New optofluidic devices and sensors based on air cavity approach
Microfluidic refractive index sensor based on polymer grating couplers
Summary
[email protected] laser optics – 19 March 2014, Berlin, Germany