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Fabrication and testing of planar chalcogenide waveguide integrated microfluidic sensor

Juejun Hu, Vladimir Tarasov, Anu Agarwal, Lionel Kimerling, Nathan Carlie, Laeticia Petit, and Kathleen Richardson

Open Access Open Access

Abstract

We have fabricated and tested, to the best of our knowledge, the first microfluidic device monolithically integrated with planar chalcogenide glass waveguides on a silicon substrate. High-quality Ge23Sb7S70 glass films have been deposited onto oxide coated silicon wafers using thermal evaporation, and high-index-contrast channel waveguides have been defined using SF6 plasma etching. Microfluidic channel patterning in photocurable resin (SU8) and channel sealing by a polydimethylsiloxane (PDMS) cover completed the device fabrication. The chalcogenide waveguides yield a transmission loss of 2.3 dB/cm at 1550 nm. We show in this letter that using this device, N-methylaniline can be detected using its well-defined absorption fingerprint of the N-H bond near 1496 nm. Our measurements indicate linear response of the sensor to varying N-methylaniline concentrations. From our experiments, a sensitivity of this sensor down to a N-methylaniline concentration 0.7 vol. % is expected. Given the low-cost fabrication process used, and robust device configuration, our integration scheme provides a promising device platform for chemical sensing applications.

©2007 Optical Society of America

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Figures (7)
Fig. 1.
Fig. 1. Schematic processing flow of the microfluidic sensor chip integrated with Ge23Sb7S70 waveguides.

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Fig. 2.
Fig. 2. Photo of the assembled microfluidic chip with fluid inlet and outlet tubing; the microfluidic channels and Ge23Sb7S70 waveguides are too small to resolve in the image.

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Fig. 3.
Fig. 3. (a). Schematic of the measurement setup for transmission loss in waveguide and absorption in sensor; (b) Near field image of 1550nm optical output from a Ge23Sb7S70 waveguide. The modal FWHM (Full Width at Half Maximum) is measured to be 4.3 μm by 1.0 μm. Inset: TM waveguide mode profile simulated using a finite domain technique20.

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Fig. 4.
Fig. 4. (a). SEM cross-section of a 50 μm wide SU8 microfluidic channel before being capped with a PDMS cover; Insets: the high magnification cross-sectional micrographs of the Ge23Sb7S70 waveguides formed by SF6 plasma etching sitting at the bottom of the channel, showing a vertical sidewall profile. The slightly darker area on the left side is an artifact due to electronic charge accumulation during SEM observation; (b) Fluorescent image of a microfluidic channel filled with FITC (fluorescein isothiocyanate) solution on a sensor chip, indicating successful fluid injection into the channel free of leakage.

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Fig. 5.
Fig. 5. (a). Absorption spectrum showing the N-H bond absorption at 1496 nm wavelength in N-methylaniline measured using our integrated evanescent sensor. The absorption is defined by taking the ratio of light transmission in the case of a microfluidic channel filled with pure carbon tetrachloride against the case when the channel is filled with N-methylaniline solution in carbon tetrachloride (0.33, volumetric concentration). (b) Transmission spectra of pure N-methylaniline and carbon tetrachloride (CCl4) measured using traditional UV-Vis spectroscopy. The absorption spectrum of N-methylaniline shows the same N-H absorption peak near 1496 nm while carbon tetrachloride is transparent in the wavelength range of interest.

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Fig. 6.
Fig. 6. Peak absorption of N-methylaniline solution in carbon tetrachloride measured using the waveguide evanescent sensor as a function of N-methylaniline volume concentration, indicating good linearity of the sensor response.

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Fig. 7.
Fig. 7. Schematic illustration of a fabrication method to eliminate optical coupling related fluctuation within our microfluidic channels: Input laser beam goes through a single-mode waveguide and then a Y-splitter; output power from the reference beam is used as a monitor for coupling variations.

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Equations (3)

Equations on this page are rendered with MathJax. Learn more.

αL = 10 log 10 I solvent I analyte
c min = 2 Lαη ( Δ F 0 F 0 ) 2 + ( σ j RF 0 exp ( − AL ) ) 2
αL = 10 log 10 ( I sensor I ref ) solvent ( I sensor I ref ) analyte ( in dB )
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