Design and fabrication of Poly(dimethylsiloxane ... - OSA Publishing

Design and fabrication of Poly(dimethylsiloxane) single-mode rib waveguide Jack Sheng Kee,1,2 Daniel Puiu Poenar,2 Pavel Neuzil, 1 and Levent Yobas,1,* 2

1 Institute of Microelectronics, A*STAR (Agency for Science, Technology and Research), SINGAPORE Microelecronics Center, School of Electrical and Electronic Engineering, Nanyang Technological University, SINGAPORE *[email protected]

Abstract: We have designed, fabricated and characterized poly(dimethylsiloxane) (PDMS) single-mode rib waveguides. PDMS was chosen specifically for the core and cladding. Combined with the soft lithography fabrication techniques, it enables an easy integration of microoptical components for lab-on-a-chip systems. The refractive index contrast, ∆ of 0.07% between the core and cladding for single-mode propagation was achieved by modifying the properties of the same base material. Alternatively, a higher refractive index contrast, ∆ of 1.18% was shown by using PDMS materials from two different manufacturers. The fabricated rib waveguides were characterized for mode profile characteristics and confirmed the excitation of the fundamental mode of the waveguide. The propagation loss of the single-mode rib waveguide was characterized using the cutback measurement method at a wavelength of 635 nm and found to be 0.48 dB/cm for ∆ of 0.07% and 0.20 dB/cm for ∆ of 1.18%. Y-branch splitter of PDMS single-mode rib waveguide was further demonstrated. ©2009 Optical Society of America OCIS codes:(130.5460) Integrated optics, Polymer waveguides; (220.4000) Optical design and fabrication, Microstructure fabrication; (230.7390)Waveguides, planar

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#109170 - $15.00 USD Received 25 Mar 2009; revised 15 May 2009; accepted 16 May 2009; published 29 Jun 2009

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1. Introduction Poly(dimethylsiloxane) (PDMS) has been widely used for the fabrication of microfluidics and lab-on-a-chip (LOC) devices due to unique characteristic such as biocompatibility, low cost, and rapid prototyping capability by soft lithography [1]. Most often, LOC devices depend on optical detection for sensing biochemical species [2,3] and the high transparency of PDMS in visible light thus motivates the monolithic integration of optical components with microfluidics in the same material. A range of LOC based microoptical components [4] such as 2D lenses [5], prisms [6], and waveguides [7,8] have been demonstrated. Among these components, PDMS waveguides have received considerable attention because of their potential for interfacing with other photonic or electronic devices and for producing mechanically robust microphotonic devices. To date, several reported PDMS waveguides have been fabricated with large sizes (tens to hundreds of micrometers), hence featuring multimode behaviour. Such multimode PDMS waveguides have been integrated either in LOC devices [8,9] with core size of 125 × 125 µm2 and 250 × 250 µm2, or in optical interconnects in an electro-optical circuit board [10] with core size of 70 × 70 µm2. In comparison, single-mode waveguides provide higher versatility as basic building blocks in complex microphotonics devices such as such interferometers and biosensors. One of the essential criterions for producing single-mode waveguide is to reduce the waveguide core dimension to small sizes, on the order of micrometers. In addition, the previously reported approaches of fabricating multimode waveguides in PDMS involved tuning the refractive index difference either by modifying the PDMS curing process, which requires precise control of process parameters [8], or by attaching a –CH3 group to the silicon backbone, which produce two distinct materials through chemical processing [9], or by adding silicone oil to increase the refractive index [10]. In this paper, we design the single-mode rib waveguide based on the geometrical adjustment of the rib width, total waveguide height and slab height as first proposed by Soref et al. [11]. The refractive indices for the cover, film and substrate used were of 1, 1.412, and 1.411, respectively for low refractive index contrast waveguide and 1, 1.429, and 1.412, respectively for high refractive index contrast waveguide. Based on the design results, we fabricated PDMS rib waveguides with small core size ( 4 µm . The precursor mixture in the SU-8 trenches was allowed to settle down to a uniform layer and was thereafter cured at 80°C for 2 hours. The second precursor mixture with added hexane was subsequently poured to form a thin layer and cured at room temperature (25 °C) for 48 hours. #109170 - $15.00 USD Received 25 Mar 2009; revised 15 May 2009; accepted 16 May 2009; published 29 Jun 2009

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Cross-linked PDMS macromolecules diluted with hexane form a guest/host matrix configuration. The hexane can be subsequently removed through vaporization by heating the cured PDMS at 90°C (the boiling point of hexane is 78°C). Thus, the vaporization of hexane yielded void in the PDMS and produce a lower density of PDMS. This second PDMS layer fused together with the first one and formed the waveguide cladding layer. A thick layer of pure PDMS has to be cured over this layer to prevent the chip from warping due to the compressive stress induced after the hexane evaporation. The amount of Hexane was capped at 10% w/w because the 20% w/w of Hexane shows no further decrease of the refractive index and yet produces severe warping after evaporation. The third layer of pure PDMS precursor mixture was then poured to form a thick substrate layer. For high refractive index contrast waveguide, the PDMS precursor mixture (OE-43, Gelest) was prepared at a weight ratio of base to curing agent 1:1. This PDMS precursor has a lower viscosity which allows spin coating on wafer to produce thin layer of PDMS down to 1 µm. The spin coated PDMS was cured at 55°C for 4 hours. Thereafter, the standard PDMS precursor mixture (Sylgard 184, Dow Corning) was cured over the thin layer of PDMS to form the cladding layer. The refractive index was measured using the prism coupling method (Model 2010, Metricon Corporation) with a refractive index accuracy of ± 0.0002 on a thin PDMS film spin-coated at 6900 rpm for 60 s. Pre-cured PDMS was mixed with hexane at different composition and its refractive index was measured and compared with that of pure PDMS.

Fig. 3. Schematic diagram of the fabrication process of the PDMS-based single-mode rib waveguide.

The propagation losses of the waveguides were measured with a cut-back method in which the PDMS waveguides are cut-back from 7.5 cm to 2.5 cm in 4 steps. The mode profile of the waveguide was studied using a collimated diode laser beam of wavelength 635 nm and buttcoupled into the waveguide through a 9/125 µm single-mode optical fiber. The image of the output end of the waveguide was focused with a 50 × objective lens and captured on a CCD camera (Exwave, Sony, Japan). The resulting image was analyzed using Origin Software. The Y-branch splitter was designed and fabricated based on the single-mode PDMS rib waveguide. The Y-branch was tested for splitting the power in the single-mode waveguide to a ratio 1:1 into both branching arm for a branch gap of 20 µm and 50 µm. 4. Results and Discussion

The measured refractive indices for both pure and hexane-modified PDMS materials are 1.412 and 1.411, respectively, resulting in a refractive index difference of 10−3 which has thus confirmed the effectiveness of the proposed scheme of refractive index tuning. Figure 4 shows the SEM images and microscope images of the fabricated single-mode waveguides. The distribution of the mode fields resulting from FDTD and Beam Propagation Method (BPM) simulations for a PDMS rib waveguide 8 µm wide, with 6.8 µm rib height and 3.4 µm


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a)

n 0 =1.0

b)

n 1 =1.412 n 2 =1.411

c)

Intensity (a.u.)

Fig. 4. Images of rib waveguides fabricated in PDMS: a) SEM image of an array of waveguides; b) Microscope image of the face end of waveguide c) SEM image of a single rib waveguide.

FDTD Mode 1

a)

0

Intensity (a.u.)

200

FDTD Mode 2

BPM Simulation

b)

10 µm

Fig. 5. Beam profile studies on straight PDMS single-mode rib waveguides at a z-position of 5.5 cm: a) Simulation results of both FDTD for mode 1 & 2 and BPM; b) Face end image of output waveguide captured by CCD.

slab height are shown in Fig. 5a. From the pictures shown in Fig. 5a, it can observed that only the fundamental mode is bounded whereas the 2nd order mode is a radiation mode. The nearfield pattern at the output of the waveguide was also studied, and the resulting cross-sectional intensity profiles in the xy-plane are shown in Fig. 5b. The Gaussian-fit diameters of the intensity profiles were approximately 8.9 µm in the x-direction and 8.8µm in the y-direction. These Gaussian diameters agrees well with the simulated results of 9.14 µm in the x-direction and 9.15 µm in the y-direction and thus confirmed that the waveguides works essentially as a single-mode waveguide at a wavelength of 635 nm. Next, we evaluated the propagation loss of the PDMS rib waveguide for both types of waveguides. The intensity of the mode profile captured at the waveguide end was measured as a function of the length of the waveguide to determine the propagation loss and the results are shown in Fig. 6. The propagation loss of the single-mode waveguide was measured to be 0.48 dB/cm for low refractive index contrast waveguides (∆ = 0.07%) and 0.20 dB/cm for high #109170 - $15.00 USD Received 25 Mar 2009; revised 15 May 2009; accepted 16 May 2009; published 29 Jun 2009

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refractive index contrast waveguides (∆ = 1.18%) at the wavelength of 635 nm. Propagation loss in a straight optical waveguide is generally attributable to the absorption of the material and scattering loss from the surface of the waveguide. PDMS has a low absorption and the simulation results indicated that the mode radiation loss should be in the order of only ~0.2 dB/cm. Therefore, the higher propagation loss measured in our low refractive index contrast PDMS single-mode waveguide might be due to the scattering from the sidewall roughness of the waveguide and low confinement factor in the waveguide [16,17]. This loss can be minimized through further optimization of the mold used in the soft lithography fabrication process. In addition, the improvement of confinement of light in the waveguide reduces the propagation loss as shown by the high refractive index contrast waveguides. The Y-branch power splitter demonstrated the feasibility of producing bending waveguides for complex microphotonics devices. The Y-branch has a linear branching length of 1 mm and a branch gap of 20 µm and 50 µm. As shown in Fig. 7, the output of the Ybranch allows the waveguide to split power equally (1:1) in both waveguide branches. 12

Low index contrast High index contrast

11

Loss (dB)

10

y = 0.48x + 7.91

9 8 7 6 5

y = 0.20x + 4.11

4 2

3

4

5 Length (cm)

6

7

Fig. 6. Measured propagation loss in the fabricated PDMS rib waveguides.

a)

10 µm

b)

10 µm

Fig. 7. Y-branch power splitter output face end image; a) Branch gap of 20 µm and ;b) Branch gap of 50 µm .

5. Conclusions

Single-mode PDMS rib waveguides have been designed, fabricated and characterized. A refractive index difference of 10−3 between the core and cladding for single-mode waveguide #109170 - $15.00 USD Received 25 Mar 2009; revised 15 May 2009; accepted 16 May 2009; published 29 Jun 2009

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has been produced by diluting the PDMS precursor with hexane. For a higher refractive index contrast, two PDMS precursor from different manufacturer were used for the core and cladding layer. The range of slab height for single-mode operation has been determined using FDTD simulations and further confirmed with BPM simulations. The mode profiles have shown single-mode propagation by both the simulation and measured data. The demonstration Y-branch power splitter based on the single-mode PDMS waveguides confirms that they can be used as basic building blocks for complex microphotonics devices.


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