Abstract

We report measurements on planar optical waveguides having an aqueous core and a low-index nanoporous dielectric cladding. Spin-on deposition of the nanoporous dielectric results in a thin (~0.8 µm) low-index cladding layer on a higher-index fused silica substrate, which produces leaky waveguide modes; however, for the aqueous layer thickness needed in most microfluidic applications, a large number of low-loss modes exist. We demonstrate that such a waveguide can be used for efficient collection and transport of fluorescence generated within the aqueous core and show that the use of these nanoporous materials offers advantages over the principal alternative, Teflon® AF.

© 2004 Optical Society of America

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References

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Adv. Mat.

H. Kim, J. Wilds, C. Kreller, W. Volksen, P. Brock, V. Lee, T. Magbitang, J. Hedrick, C. Hawker, and R. Miller, �??Fabrication of multi layered nanoporous poly(methyl silsesquioxane),�?? Adv. Mat. 14, 1637�??1639 (2002).
[CrossRef]

Anal. Chem.

P. Dasgupta, Z. Genfa, S. Poruthoor, S. Caldwell, S. Dong, and S. Liu, �??High-sensitivity gas sensors based on gas-permeable liquid core waveguides and long-path absorbance detection,�?? Anal. Chem. 70, 4661�??4669 (1998).
[CrossRef]

P. Dasgupta, G. Zhang, J. Li, C. Boring, S. Jambunathan, and R. Al-Horr, �??Luminescence detection with a liquid core waveguide,�?? Anal. Chem. 71, 1400�??1407 (1999).
[CrossRef] [PubMed]

M. Holtz, P. Dasgupta, and G. Zhang, �??Small-volume raman spectroscopy with a liquid core waveguide,�?? Anal. Chem. 71, 2934�??2938 (1999).
[CrossRef]

Appl. Opt.

Appl. Phys. B

P. Dress and H. Franke, �??A cylindrical liquid-core waveguide,�?? Appl. Phys. B 63, 12�??19 (1996).
[CrossRef]

Appl. Spectrosc.

Chem. Mater.

H.-C. Kim, C. R. Kreller, K. A. Tran, V. Sisodiya, S. Angelos, G. Wallraff, S. Swanson, and R. D. Miller, �??Nanoporous Thin Films with Hydrophilicity-Contrasted Patterns,�?? Chem. Mater. 16, 4267�??4272 (2004).
[CrossRef]

IEEE J. Lightwave Technol.

E. Anemogiannis, E. Glytsis, and T. Gaylord, �??Determination of guided and leaky modes in lossless and lossy planar multilayer optical waveguides: Reflection pole method and wavevector density method,�?? IEEE J. Lightwave Technol. 17, 929�??941 (1999).
[CrossRef]

IEEE Sensors Journal

R. Manor, A. Datta, I. Ahmad, M. Holtz, S. Gangopadhyay, and T. Dallas, �??Microfabrication and characterization of liquid core waveguide glass channels coated with Teflon AF,�?? IEEE Sensors Journal 3, 687�??692 (2003).
[CrossRef]

A. Datta, I. Eom, A. Dhar, P. Kuban, R. Manor, I. Ahmad, S. Gangopadhyay, T. Dallas, M. Holtz, F. Temkin, and P. Dasgupta, �??Microfabrication and characterization of Teflon AF-coated liquid core waveguide channels in silicon,�?? IEEE Sensors Journal, 3, 788�??795 (2003).
[CrossRef]

J. Micromech. Microeng.

B. Helbo, A. Kristensen, and A. Menon, �??A micro-cavity fluidic dye laser,�?? J. Micromech. Microeng. 13, 307�??311 (2003).
[CrossRef]

Opt. Eng.

J. Lowry, J. Mendlowitz, and N. Subramanian, �??Optical characteristics of Teflon AF® fluoroplastic materials,�?? Opt. Eng. 31, 1982�??1985 (1992).
[CrossRef]

Rev. Sci. Instrum.

P. Dress and H. Franke, �??Increasing the accuracy of liquid analysis and pH-value control using a liquid-core waveguide,�?? Rev. Sci. Instrum. 68, 2167�??2171 (1997).
[CrossRef]

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Figures (7)

Fig. 1.
Fig. 1.

Experimental arrangement for holding two fused silica substrates coated with nanoporous films parallel to each other and a fixed distance apart. The thickness of spacer and wafers is not to scale.

Fig. 2.
Fig. 2.

Refractive index profile of waveguides having an aqueous core and a cladding consisting of a low-index nanoporous dielectric (NPD) film deposited on a fused silica substrate.

Fig. 3.
Fig. 3.

(a) Attenuation coefficient and effective index for individual modes calculated by the RPM method for a waveguide with the refractive index profile shown in Fig. 2. The properties of the nanoporous film assumed in the model are given in the inset box. The thickness of the water layer is 50 µm. The wavelength used was 550 nm, which is near the emission peak of the fluorescent dye. The refractive index of the water was taken to be 1.334 and that of fused silica to be 1.460. All materials were assumed to be lossless. Each point corresponds to an individual mode. (b) The same data as in (a), rearranged so that the number of modes with attenuation below a given value can be readily determined. Corresponding data for double-layer claddings is also included.

Fig. 4.
Fig. 4.

Experimental arrangement for fluorescence scanning

Fig. 5.
Fig. 5.

Expected variation of fluorescence signal with distance of pump spot from edge of waveguide, indicating certain features where transitions occur between factors dominating the signal.

Fig. 6.
Fig. 6.

Variation of fluorescence signal with distance of exciting pump spot from the edge of the waveguide for various nanoporous claddings and for structures without any cladding for comparison. (a) Linear vertical scale (b) Logarithmic vertical scale

Fig. 7.
Fig. 7.

Fit of the relative shoulder heights in Fig. 6 to Eq. 1

Tables (1)

Tables Icon

Table 1. Summary of properties of nanoporous films used in these experiments, along with possible alternatives for comparison. (The Teflon® AF numerical designations “1600” and “2400” refer to the glass transition temperature. The difference between the two formulations is the relative amount of dioxole monomer in the basic polymer chain.)

Equations (1)

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ξ = 2 π cos 1 ( n clad n core )

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