Optofluidic Waveguiding for Biomedical Sensing

We review an optofluidic waveguiding lab-on-a-chip used to sense bioparticles. The sensor uses a liquid filled Anti-Resonant Reflecting Optical Waveguide (ARROW) that is interfaced with standard ridge waveguides. The ridge waveguides are coupled to off-chip lasers and detectors via optical fiber. A perpendicular intersection between the ARROW and a ridge waveguide is especially useful for detecting fluorescently tagged particles. Light coupled into the ridge waveguide can fluorescently excite these particles within a very small volume. Fluorescent signal can then be guided within the ARROW and subsequently off chip to a detector. We also discuss how such devices are fabricated. Both the ARROW and ridge waveguides are made using alternating thin films of tantalum oxide and silicon dioxide on silicon substrates. These thin films are deposited by either sputtering or plasma enhanced chemical vapor deposition (PECVD). The waveguides are patterned using a combination of standard photolithographic processes, reactive ion etching, and sacrificial etching. Low-loss optical guiding is very dependent on both the waveguide structure and the materials used. The latest processes for maximizing detection sensitivity are reviewed. We also present results using the optofluidic waveguiding sensor for detecting a variety of different types of particles such as fluorescently labeled nanobeads, viruses, ribosomes, and RNA.