Microfluidic refractive index sensor based on polymer grating couplers

$: Microfluidic refractive index sensor based on polymer grating couplers$
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

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Microfluidic refractive index sensor based on polymer grating couplers

Motivation

[email protected] laser optics – 19 March 2014, Berlin, Germany


3

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



4

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


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


5

Simulation overview


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


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Simulation overview



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Simulation results for a grating coupler optimized for an analyte with n = 1.33


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Simulation results for a grating coupler optimized for an analyte with n = 1.33



Sensitivity of 300 nm/RIU


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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



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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


4. Bond SU-8 film to KMPR structure

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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


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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


100 µm

1 µm 1 µm

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Preliminary fabrication results – bonding SU-8


10 µm

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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:


20 µm

10 µm

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Preliminary fabrication results – grating coupler


PDMS grating coupler stamp

10 µm

SU-8 grating coupler

10 µm

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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


Summary


Microfluidic refractive index sensor based on polymer grating couplers

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