EVANESCENT FIELD ABSORPTION SPECTROSCOPY ON POLY(DIMETHYLSILOXANE) SINGLE-MODE RIB WAVEGUIDE INTEGRATED WITH MICROFLUIDIC SYSTEM J.S. Kee. 1,2* D.P. Poenar, 2 L. Yobas 1 and Y. Chen 1 1 Institute of Microelectronics, A*STAR (Agency for Science, Technology and Research), SINGAPORE and 2 Microelecronics Center, School of EEE, Nanyang Technological University, SINGAPORE ABSTRACT An integrated poly(dimethylsiloxane) (PDMS) single-mode rib waveguide with microfluidic system is demonstrated for evanescent field absorption spectroscopy. Based on the PDMS single-mode rib waveguide, a photonic circuit is de- signed and fabricated to permit multiple waveguides sensing. The applicability of the integrated photonic and microflui- dic PDMS chip for evanescent field absorption spectroscopy in visible light range (400 – 700 nm) was confirmed by ab- sorbance measurement of Fluorescein and Bromophenol Blue dyes. The lowest detection concentration of 0.1 µM for Bromophenol Blue was obtained. The demonstrations show that the integrated PDMS single-mode waveguide offers dis- posable, multiple sensing and visible light absorption detection. KEYWORDS: Evanescent Field Spectroscopy, Poly(dimethylsiloxane), Single-mode waveguide INTRODUCTION This paper reports on the evanescent field absorption spectroscopy by a monolithically integrated Poly(dimethylsiloxane) (PDMS) single-mode rib waveguide with microfluidic interface. Absorption measurement by the evanescent field of a waveguide has shown improved detection in small volume flow channel [1]. Integrated waveguide with microfluidic interface for evanescent field absorption spectroscopy has been demonstrated with chalcogenide [2] and SU-8 [3] waveguides. Here, we achieve the evanescent field absorption spectroscopy on our recently reported single- mode rib waveguide [4] integrated with microfluidic system in PDMS material for low cost and rapid prototyping. THEORY Evanescent field exists at the core-cladding interface of the waveguide as the light propagates along the waveguide core. The analyte that comes into this evanescent field of the guided mode propagation of the waveguide interacts with the light propagation and alters its intensity. This interaction of the evanescent field with the analytes adjacent to it at the core-cladding interface provides the sensing mechanism. Evanescent wave absorbance can be measured and calculated by the modified Lambert-Beer formula: A=log( Iref /Isample )= σεlc where Iref is the light intensity measured by the detector when a non-absorbing reference media is used, Isample is the light intensity measured by the detector when an absorbing analytes is used, σ is the fraction of the normalized mode power propagating in the evanescent field region of the waveguide, ε is the molar absorption coefficient, l is the optical pathlength and c is the concentration of the analytes. EXPERIMENTAL Figure 1 illustrates the integration of the PDMS single-mode rib waveguides with microfluidic system. The PDMS single-mode rib waveguide was used as a sensor for evanescent field absorption spectroscpy. Based on this PDMS single- mode rib waveguide, the photonic circuit was designed with the Apollo Photonics Solution Suite™. It consists of an 1×3 star coupler and S-bend waveguides for splitting one input waveguide to three output waveguides which can be indivi- dually accessed by separate microfluidic channel for multianalytes sensing. In addition, taper waveguide is used for dif- fraction effect reduction as the waveguide traverse across the microfluidic channel wall. Fluidic inlet/outlet Light output 1×3 star coupler Microfluidic Channel Light input Rib waveguide S-bend Figure 1: Schematic diagram of the integrated PDMS rib waveguides with microfluidic interface. The microfluidic channels flow analytes to the rib waveguide for evanescent field interaction. (Diagram not drawn to scale) 978-0-9798064-3-8/µTAS 2010/$20©2010 CBMS 1256 14th International Conference on Miniaturized Systems for Chemistry and Life Sciences 3 - 7 October 2010, Groningen, The Netherlands The PDMS single-mode rib waveguide was fabricated with two types of PDMS material; Gelest, OE 43 ( n=1.429) for the core layer and Dow Corning, Sylgard 184 ( n=1.412) for the cladding layer. The rib waveguide made use of air ( n = 1) as the top and side cladding. In utilizing the rib waveguide as an evanescent wave sensor, the analyte liquid with n ≈ 1.33 flows along and adjacent to the waveguide core and provides the top and side cladding. Figure 2 details the fabrication steps for both the PDMS photonic circuit and microfluidic layer. The fabricated PDMS chip was tested for evanescent field absorption spectroscopy of Fluorescein and Bromophenol Blue. A fiber optic from a visible light source (ANDO, AQ4303C) was butt-coupled to the input waveguide and the signal from the output waveguide was collected by a fiber optic connected to an optical spectrometer (Newport, OSM100-UV/Vis). The absorption measurement was further per- formed for various concentrations of Bromophenol Blue (0.1 to 25 µM). Figure 2: The fabrication process of the PDMS waveguide layer and microfluidic layer. RESULTS AND DISCUSSION As shown in Figure 3a, the experimentally characterized PDMS rib waveguide agrees well with the simulated results and corroborates the single-mode propagation in the waveguide. Based on this single-mode rib waveguide, the 1×3 star coupler was designed to efficiently splits the single input waveguide to three output waveguides (Figure 3b) as well as low loss S-bend and taper waveguides (total loss of 0.1 dB). Figure 4 demonstrates the successfully integrated sin- gle-mode rib waveguide with microfluidic channel. The 8 × 8 µm 2 waveguide has an evanescent field pene- tration depth of more than 0.5 µm and an interaction length of 13 mm. Input Output waveguide waveguide 0 200 Intensity (a.u.) b) a) Star coupler 10 µm Figure 3:. The photonics circuit was design based on the proven PDMS single-mode rib waveguide a) Comparison be- tween the mode profile captured by CCD with the simulation result (inset), both simulated and captured beam has con- curring Gaussian beam size diameter at 640 nm wavelength of 8.9 vs 9.1 µm and b) The simulation result of the 1×3 star coupler with the beam propagation method. Microfluidic Waveguide Channel Microfluidic Taper Microfluidic Channel Layer Rib waveguide Waveguides ncore =1.429 ncladding =1.412 Waveguide Layer a) 50 µm b) 10 µm c) 20 µm Figure 4: The microscope images of a) the top view of the integrated single-mode rib waveguide with the microfluidic channel b) the face end image of the waveguide c) the cross-section of the integrated microfluidic and waveguide layer. 1257 The achievability of the fabricated PDMS chip for evanescent absorption spectroscopy was confirmed by measuring the absorption spectra of both fluorescein and bromophenol blue. The absorption peaks of fluorescein and bromophenol blue are both detected at 490 nm and 590 nm. 0.8 Fluorescein Bromophenol Blue 0.7 0.6 0.5 0.4 0.3 0.2 0.1 Evanescent Absorption 0.0 -0.1 400 450 500 550 600 650 700 Wavelength (nm) Figure 5: Absorption spectroscopy of Fluorescein and Bromophenol Blue with the evanescent field of the rib waveguide. Figure 6 reveals the sensitivity of the PDMS single-mode rib waveguide detection of various concentration of Bromophe- nol Blue. The minimum concentration detected by the platform is at 0.1 µM. 0.8 0.1 µM 0.7 5 µM 10 µM 0.6 25 µM 0.5 0.4 0.3 0.2 Evanescent Absorption 0.1 0.0 400 450 500 550 600 650 700 Wavelength (nm) Figure 6: Absorption spectra of various Bromophenol concentrations. CONCLUSION The evanescent field absorption spectroscopy by monolithically integrated PDMS single-mode rib waveguides with microfluidic system offers disposable, multiplexing and high sensitivity visible light spectroscopy detection for lab-on-a- chip devices. REFERENCES [1] G. Pandraud, T.M. Koster, C. Gui, A. van den Berg and P.V. Lambeck , Evanescene wave sensing: new features for detection in small volumes , Sensors & Actuators A, 85, pp. 158-162, (2000). [2] J. Hu, V. Tarasov, A. Agarwal, and L. Kimerling, Fabrication and testing of planar chalcogenide waveguide inte- grated with microfluidic sensor , Optics Express, 15, 5, pp. 2307-2314, (2007). [3] A. Prabhakar, and S. Mukherji, Microfabricated polymer chip with integrated U-bend waveguides for evanescent field absorption based detection , Lab Chip, 10, pp. 748-754, (2010). [4] J.S. Kee, D. P. Poenar, P. Neuzil, and L. Yobas, Design and fabrication of Poly(dimethylsiloxane) single-mode rib waveguide , Optics Express, 17, 14, pp. 11739-11745, (2009). CONTACT *J.S. Kee, tel: +65-67780136; [email protected] 1258.