Microfluidic refractive index sensor based on polymer grating couplers

Transcription

1 Microfluidic refractive index sensor based on polymer grating couplers C. Prokop 1,2, S. Schoenhardt 1,2, Christian Karnutsch 1, Arnan Mitchell 2 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

2 Motivation 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

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 Light source Detector Substrate Grating coupler Waveguide Air cavity Polymer structure 3

4 Proposed sensor design in cross-section, not to scale Waveguide Grating coupler SU-8 KMPR Air cavity Analyte channel Substrate Device uses an grating coupler as a refractive index sensor element Depending on the refractive index of the analyte, the peak wavelength shifts 4

5 Simulation overview Simulation carried out in CAMFR (CAvity Modelling FRamework) [1] n 1 < n 3 < n 2 PML Air layer, n 1 th groove Λ th guide Guiding layer, n 2 Analyte layer, n 3 PML [1] See CAMFR website 5

6 Simulation overview Simulation carried out in CAMFR (CAvity Modelling FRamework) [1] λ = 1550 nm th guide = 900 nm th groove = 500 nm Period Λ = 1340 nm n 1 = 1.0 n 2 = 1.57 Filling factor = 0.5 n 1 < n 3 < n 2 n 3 = 1.33 [1] See CAMFR website 6

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

8 Simulation results for a grating coupler optimized for an analyte with n = 1.33 Sensitivity of 300 nm/riu 8

9 Materials: SU-8 and KMPR General Epoxy based negative near-uv photoresists Developed for high aspect ratios in very thick photoresist layers SU-8 is very similar to KMPR Optical properties n SU-8 : 1,575 at 1550 nm n KMPR : 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 9

10 Proposed fabrication method (1) 1. Cast PDMS from PFPE working stamp PDMS PFPE 2. Spin coat SU-8 on PDMS stamp SU-8 Substrate 3. Pattern KMPR photolithographically KMPR Substrate 10

11 Proposed fabrication method (2) 4. Bond SU-8 film to KMPR structure PDMS SU-8 KMPR Substrate 5. Peel off PDMS stamp 6. Apply analyte Analyte SU-8 KMPR Substrate SU-8 KMPR Substrate 11

12 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 12

13 Preliminary fabrication results bonding SU-8 10 µm 13

14 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: 10 µm 20 µm 14

15 Preliminary fabrication results grating coupler PDMS grating coupler stamp SU-8 grating coupler 10 µm 10 µm 15

16 Summary 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 Acknowledgements 16