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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
1. Introduction
Liquid-core waveguides have been shown to offer advantages over conventional cells in absorbance spectroscopy, fluorescence spectroscopy, and Raman spectroscopy. In absorption spectroscopy, guiding of light in the liquid core makes possible a long path length [1]; in fluorescence spectroscopy, capture and transport of the fluorescence makes possible transverse illumination geometries that improve rejection of excitation light [2]; in Raman spectroscopy, confinement of the light to the liquid core increases the pump intensity and improves the efficiency of Raman generation [3]. The ability to efficiently guide light in liquids might also offer advantages for microfluidic dye lasers [4].
Liquid-core waveguides based on capillary tubes have been investigated for spectroscopic applications for several decades (e.g., [5]; see also [8] for a historical review). The earliest work studied glass (refractive index n~1.52) or silica (n~1.46) capillary tubes, but the choice of liquids was very limited because of the requirement that the refractive index of the liquid “core” exceed that of the capillary “cladding.” In particular, light could not be guided in water (n~1.33) or in aqueous solutions using these capillaries.
The development of the low-refractive-index amorphous fluoropolymers Teflon® AF 1600 (n~1.31) and Teflon® AF 2400 (n~1.29) [6] made it possible to investigate capillary waveguides with aqueous cores. Capillary tubes made from Teflon® AF [1]–[3], [7]–[9] and fused silica capillaries coated internally[10], [11] and externally [2], [12] with Teflon® AF have been used for a variety of spectroscopic experiments.
Interest in integrated microfluidic systems for detecting chemical and biological agents in aqueous samples has stimulated research into liquid-core waveguides that can be readily integrated on the same substrate with other microfluidic elements. Waveguide structures formed by etching channels in Pyrex and soda-lime glass [13] or silicon [14] and then coating the channels with Teflon® AF have been studied. Since efficient collection and transport of fluorescence emitted within the liquid is desired for many of these applications, the waveguiding properties of these structures have often been evaluated by filling the channel with a fluorescent aqueous solution, exciting the fluorescence using a pump beam that impinges transversely on the structure at some distance from the waveguide end, and recording the detected fluorescence signal as a function of this distance. That the presence of the Teflon® AF layer promotes waveguiding has been demonstrated by comparing coated channels to similar channels made without the Teflon® AF coating.
Teflon® AF has long been the only option for a low-refractive-index cladding for an aqueous-core waveguide; however, it is not an ideal material for this application. Teflon® AF has poor adhesion to commonly-used substrates, requiring that additional adhesion-promoting steps be added in fabrication [13], [14]. The interface between the Teflon® AF cladding and the aqueous core offers little possibility for chemical functionalization, as might be desired for certain biosensor applications. In addition, the refractive index contrast between core and cladding is at most Δn=ncore-nclad≈0.04. In the important application of collection and transport of fluorescence generated within the aqueous core, the fraction of fluorescent photons that are captured by total internal reflection and transported along the waveguide increases as the cladding index decreases according to:
For Teflon®AF 2400 with nclad~1.31, ξ=0.11; for Teflon®AF 1600 with nclad~1.29, ξ=0.16; in the limiting case of a cladding with nclad=1(air), ξ=0.46. Hence, cladding materials with refractive indices lower than that of Teflon®AF could provide a substantial increase in the efficiency of microfluidic sensors based on fluorescence detection.
Nanoporous dielectrics are an alternative to Teflon®AF that adhere well to common substrates without additional surface treatment, that provide an interface with the aqueous solution that can be made either hydrophobic or hydrophilic and can be chemically functionalized, and that can have considerably lower refractive indices than Teflon®AF. Refractive indices as low as nclad~1.15 have been achieved with nanoporous dielectrics, corresponding to ξ=0.34; hence, factors of 2–3 enhancement in fluorescence collection efficiency should be achievable using nanoporous dielectrics instead of Teflon®AF.
2. Experimental details
2.1 Preparation of the nanoporous films
The nanoporous dielectrics used in the experiments described here were made by the “sacrificial porogen” approach, in which an organic macromolecular (porogen) phase is selectively removed from a phase-separated organic/inorganic polymer hybrid [15]. The nanoscopic pores created by this process are much smaller (diameters~10–15 nm) than the wavelength of visible light; hence, the refractive index of the resulting film is approximately the volume-weighted average of the pore index (n~1) and the matrix index (n~1.37). The volume fraction occupied by pores in the solid film can be controlled by selecting the weight fraction of porogen incorporated into the original mixture. Refractive indices in the range n~1.37-1.15 can be achieved, depending on the degree of porogen loading. The nanoporous films used here were produced by dissolving the inorganic matrix material (poly(methylsilsesquioxane) or PMSSQ) and the porogen (PS-PEG, a star-shaped polymer consisting of a polystyrene core surrounded by a poly(ethylene oxide) corona) in a mutual solvent (1-methoxy-2-propanol acetate) and spin-coating the solution onto fused silica substrates. The samples were heated briefly to 80°C for solvent removal before curing under Argon atmosphere at 450°C for two hours after an initial ramp rate of 5°C/min. A white-light interferometer (Filmetrics, F20 thin film measurement system) was used to determine the thickness (t) and refractive index (n) of the films, which are summarized in Table 1.
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.)
2.2 Waveguide assembly
In order to investigate the fluorescence collection and transport properties of waveguides having an aqueous core and a nanoporous cladding, we used the arrangement shown in Fig. 1. Two wafers bearing nanoporous films on their surfaces were held facing each other, separated by a 50-µm-thick U-shaped polyester spacer (Artus Corp. MM2-P-002). The space between the nanoporous films was filled with an aqueous solution prepared by mixing three parts by volume of HPLC water (J. T. Baker 4218) with one part of a commercially-available aqueous suspension of 0.02-µm-diameter polystyrene spheres impregnated with a fluorescent dye having spectral characteristics similar to fluorescein (Microprobes FluoSpheres® F-8787).
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.
While aqueous-core waveguides used for microfluidic applications would be fabricated by coating the walls of an etched channel (as was done with Teflon®AF in [13] and [14]), this “parallel plate” arrangement has a number of advantages when the experimental objective is a preliminary assessment of waveguiding behavior. This configuration does not require the additional process steps required to etch channels; the behavior of the waveguide is not influenced by the resulting wall roughness; alignment and sealing of the top and bottom halves of the channel is not required; wafers bearing films of different composition can be quickly compared. A disadvantage is that if the films are hydrophobic, it may be difficult to uniformly fill the interior space and to control the distribution of fluid. In most of the films examined here, surface tension was sufficient to hold the aqueous solution in place, although maintaining a continuous fluid film over the entire length of the structure became challenging when the lowest-index (highest porosity) film was used. We also attempted to study waveguides made with Teflon® AF films using this same geometry, but found them to be so hydrophobic (even after a UV/ozone treatment [16] that somewhat reduced their hydrophobicity) that we could not retain the fluid within the waveguide long enough to make a reliable measurement.
2.3 Intrinsic loss of waveguide modes
The refractive index profile across the transverse dimension of the waveguide is shown in Fig. 2. If the low-index nanoporous cladding were very thick, the evanescent fields of the modes guided in the aqueous core would decay to a negligible level at the cladding-substrate interface and the behavior of the waveguide would approach that of a classical three-layer slab waveguide. Absent material losses, these modes would have purely real propagation constants and hence no intrinsic attenuation. However, the relatively thin (~0.8 µm) nanoporous films used in these experiments produce waveguide modes with evanescent tails that do interact with the substrate (Fig. 2). This interaction results in attenuation of the guided modes, because the evanescent tail can excite radiating waves in the high-index fused silica substrate (or, put differently, photons can tunnel through the thin cladding and escape into the substrate). The result is that the modes have complex propagation constants and, hence, an intrinsic attenuation that depends on how rapidly the evanescent tail decays across the low-index nanoporous layer. We used the Reflection Pole Method (RPM) [17] to find the complex propagation constant of each mode, from which both the effective index and the attenuation coefficient can be extracted, as shown in Fig. 3(a). The attenuation is lowest for low-order modes (effective indices approaching that of water), which are more tightly bound to the core; the attenuation becomes progressively higher for higher-order modes (effective indices approaching that of the cladding), which are more loosely confined to the core.
Fluorescence emitted from a small volume of excited molecules within the aqueous core is uniformly distributed in angle. If we think of each mode as corresponding to a ray bouncing back and forth across the waveguide core at an angle cos-1(neff/ncore) with respect to the waveguide axis, we would thus expect that each mode should be equally excited. Therefore, the efficiency with which fluorescent photons are captured should increase with the number of available modes. A single mode waveguide would capture only a small fraction of the emitted fluorescence; a highly multimode waveguide would capture a much larger fraction. In order for the captured fluorescence to be efficiently transported, the waveguide modes must also have a reasonably low loss. Hence, the number of low-loss modes available in such a waveguide is important for fluorescence applications.
In Fig. 3(b), the data of Fig. 3(a) have been reformatted to emphasize the number of modes propagating with an attenuation coefficient less than some chosen value. For example, when a 0.8-µm thick cladding with refractive index n=1.27 is used, approximately 40 modes propagate with a power attenuation of 1 dB/cm or less, while if a cladding of the same thickness but with refractive index n=1.15 is used, approximately 100 modes propagate with losses below this value. Hence, although the modes of waveguides having the structure shown in Fig. 2 are inherently lossy, a large number of low-loss modes exist in waveguides having the dimensions of interest for most microfluidic applications. Also shown in Fig. 3 (b) are similar plots for cladding layers twice as thick (1.6 µm) as those used in these experiments, demonstrating that the waveguide losses can be greatly reduced with the application of just one additional layer of cladding on each side. Fabrication of multilayer structures with up to six layers of nanoporous dielectric has been achieved [15].
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. (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.
2.4 Scanning fluorescence measurement
A 488-nm air-cooled argon ion laser was used to excite the fluorescent aqueous solution that forms the waveguide core. The circular Gaussian beam emitted by the laser (1/e2 diameter ~1 mm) was expanded by a 5× telescope, then focused by a cylindrical lens to form an elliptical Gaussian spot approximately 22 µm by 5 mm (1/e2 diameters) in the aqueous core (Fig. 4). The cylindrical focusing lens and a turning mirror were mounted on a stepper-motor-actuated translation stage to allow scanning of the focused spot along the waveguide under computer control. A laser line filter (Omega Filters XL06, not shown in the figure) was used to improve the spectral purity of the pump beam.
The fluorescence reaching the edge of the waveguide was collected by a 0.6-NA lens having a broadband anti-reflection coating (Newport KPA031-C) and placed one focal length away from the end of the waveguide. The collected fluorescence passed through a bandpass filter to reject any residual pump light (Omega Filters 545AF75) and was focused by an identical 0.6-NA lens onto a 50-µm slit placed immediately in front of a large-area silicon detector. This arrangement produces a 1:1 image of the end of the waveguide at the slit, but provides collimated light at the location of the bandpass filter. The spatial filtering by the slit of the image of the waveguide endface discriminates against fluorescence that does not emerge from the end of the waveguide. The pump beam was mechanically chopped at 1 kHz, and lock-in detection was used to measure the amplitude of the 1-kHz component of the detected signal.
2.5 Results and interpretation
As the focused spot from the argon laser is scanned along the waveguide, the detected fluorescence should vary as shown in Fig. 5. Several qualitative features of this variation can be identified.
As the pump spot scans across the edge of the waveguide, the fluorescence comes from those dye-doped polystyrene spheres that are very near the water-air interface. Fluorescence photons propagating at angles with respect to the waveguide axis smaller than the water-air critical angle (θc=48.7°) escape from the water layer and are refracted into the air; thus a fraction of the fluorescence power emitted in the forward direction escapes the waveguide. Since this emission comes from particles located immediately adjacent to the water-air interface, the fraction escaping is independent of the refractive index of the nanoporous cladding.
As the focused spot from the argon laser is scanned away from the water-air interface, the signal drops because of vignetting by the finite width of the opening at the end of the waveguide. The signal would continue to drop according to the dotted curve in Fig. 5, were it not for the fact that photons impinging upon the interface between the aqueous core and the nanoporous cladding at angles smaller than the critical value experience total internal reflection and reach the water-air interface at the end of the waveguide. The fraction of emitted photons captured through total internal reflection is given by Eq. 1. As nclad→1, ξ→0.46, so that at best, only about one-half of the light emitted in each direction can be captured and transported by a waveguide. Table I gives the fraction that should be captured by each of the nanoporous films used in this experiment and by the two varieties of Teflon AF used in earlier work.
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.
As the focused spot from the argon laser moves farther along the waveguide, the detected fluorescence signal decreases because of absorption, scattering and leakage in the waveguide. Hence, three distinct features of the curve in Fig. 5 may be identified:
1) a peak occurring when the focused argon laser spot crosses the edge of the waveguide, the magnitude of which does not depend on the refractive index of the cladding.
2) a transition to the regime where total internal reflection at the core-cladding interface dominates the detected signal. The magnitude of the signal at this transition does depend on the refractive index of the cladding.
3) a region where the detected signal decreases exponentially with propagation distance owing to intrinsic attenuation, absorption, scattering, and geometrical factors in the unguided direction.
In practice, the magnitude of the peak varies somewhat from sample to sample because of the alignment of the two wafers, the quality of the edges, the proximity of the aqueous layer to the edge, and the alignment of optical system. Since, according to the discussion above, the peak value should not depend on the cladding index, we have normalized the traces for the four different samples (three with nanoporous films and one with bare silica wafers for comparison) to the same peak height in Fig. 6(a), so that the relative values of the transition to guiding behavior, which should depend on cladding index, can be compared. It is clear from Fig. 6(a) that the capture and transport of fluorescence excited in the aqueous layer is improved when the fused silica substrate is coated with a low-index nanoporous dielectric film and that the magnitude of the fluorescence signal increases as the refractive index of the nanoporous film decreases.
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
Each curve corresponding to a waveguide with nanoporous cladding exhibits a shoulder that corresponds to feature #2 described above: the capture of the emitted fluorescence through total internal reflection. The height of this shoulder relative to the peak for each of the nanoporous films is shown in Fig. 6(b).
In Fig. 7, a single scale factor has been applied to all the shoulder heights determined from Fig. 6(b), in order to obtain the best least-squares fit to Eq. (1). Figure 7 also shows the efficiency expected if Teflon® AF films are used as the cladding.
The detected fluorescence signal decreases exponentially as a function of propagation distance as shown in Fig. 6(b). Fits to the exponential tail of the response produced the attenuation values shown in the figure, which were essentially the same for all three films. Because of the arrangement used for this study, this attenuation value includes not only basic materials and waveguide properties but geometrical factors which may not apply in the case of a two-dimensional channel waveguide.
3. Conclusion
We have demonstrated the use of nanoporous dielectric films as cladding layers for planar waveguides with aqueous cores and have shown that these materials offer advantages over Teflon®AF in these applications. Even though using such thin (~1µm) layers of nanoporous materials produces modes that leak energy into the high-index fused silica substrate, the number of low-loss modes is large enough that the presence of the nanoporous cladding improves the capture and transport of fluorescence generated in the aqueous core. The leakage can be mitigated by application of multiple layers of nanoporous films, a process that has already been demonstrated.
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