Biophotonic Sensors

Today, integrated optics is widely used for telecommunications, wireless systems, sensors etc. Waveguides act as "optical wires" transporting light analogous to charge-transporting wires in integrated circuits. Interestingly, no integrated waveguides for liquids and gases exist. For instance, microfluidic devices often combine functional elements with optical detection, but light is never guided within the liquid channels to connect different elements. If such integrated devices were available, exciting opportunities would be created in areas such as biology, molecular biology, chemistry, toxicology, and (atomic) physics. Non-solid materials can be studied with the same advantages provided by conventional integrated optics: miniaturization, parallelism (= many identical devices on chip), single-mode propagation, and higher-level integration (e.g. waveguides + other functional elements). Natural applications are rapid sensing of gases and liquids using a small, robust integrated device, expansion of the capabilities of microfluidic systems, and exploitation of the narrow linewidth of atomic optical transitions, e.g. for frequency standards.

In conventional waveguides, light is guided in a medium with higher refractive index than its surroundings (e.g. silica fiber/air). When the refractive index situation is reversed (e.g. in microcapillaries) light cannot be guided in the central low-index region (core) and will leak out as shown in Fig. 1

Fig. 1: Propagation through a 5 micron wide low-index core clad by high-index (n=1.5) material. After only a few microns propagation distance intensity leaks out into the claddings.

Multilayer claddings can provide the solution to this problem. If the thickness of the high-index cladding layer is chosen correctly, light at certain wavelengths can be guided through a low-index gas or liquid core. Multiple layers improve the guiding capabilities and reduce the propagation loss. Such structures can be realized as anti-resonant reflecting optical waveguides (ARROWs) [1] or - if the cladding layers are periodic - Bragg waveguides or photonic crystals [2,3].

Application of such multi-layer cladding layers to integrated optical devices with gaseous or liquid cores is being investigated. Using the ARROW principle, we have built the first integrated ARROW waveguides with liquid and hollow cores in collaboration with the University of California Santa Cruz. Efficient light propagation in these non-solid media over chip-scale distances has been demonstrated along with the smallest optical mode volume for an optical mode guided in air and liquid [4]. (see Figure 2)

Figure 2: Integrated optical waveguide with hollow core.
Left: SEM image of finished device showing 3.5 by 12mm hollow core.
Center: Optical mode propagating through structure. This is the smallest optical mode tobserved in a non-solid core to date.
Right: Fluorescence signal from core when filled with Alexa 647 dye molecules and excited with a He-Ne laser.

Several applications for this technology are being pursued presently. One of these applications is to build an integrated, planar optical platform for fluorescence measurements of single DNA molecules in solution [5]. These devices allow for massively parallel optical measurements on biological samples with small volumes driven through a semiconductor chip (Figure 3). The unique waveguide design and choice of materials allows for taking advantage of well-established techniques in integrated photonics and fiber optics. A section of such a device is shown in Figure 3. In addition, the technology is compatible with the integration of nanoscopic openings, so-called nanopores, that allow for controlled introduction of individual molecules into the waveguide core.

Figure 3: Schematic of an integrated ARROW-based device for optical measurements on fluids containing biomolecules.

References

[1] M.A. Duguay, Y. Kokubun, T. Koch, and L. Pfeiffer, "Antiresonant reflecting optical waveguides in SiO2-Si multilayer structures", Appl. Phys. Lett., 49, 13, (1986).

[2] P. Yeh, A. Yariv, and C-S. Hong, J. Opt. Soc. Am., 67, 423, (1977).

[3] Y. Fink, J.N. Winn, S. Fan, C. Chen, J.Michel, J.D.Joannopoulos, and E.L.Thomas, Science, 282, 1679, (1998).

[4] D. Yin, J.P. Barber, A.R. Hawkins, and H. Schmidt, "Integrated ARROW waveguides with hollow cores", Optics Express, 12, 2710, (2004). D. Yin, J.P. Barber, D.W. Deamer, A.R. Hawkins, and H. Schmidt, "Integrated optical waveguides with liquid cores", Applied Physics Letters, 85, 3477 (2004).

[5] H. Schmidt, D. Yin, D. Deamer, J.P. Barber, and A.R. Hawkins, "Integrated ARROW waveguides for gas/liquid sensing", Proceedings of the SPIE, Denver, CO, August 2-6, vol. 5515, 67 (2004).