Light guiding through a fluid is a natural phenomenon that takes place every time a ray of light enters a free flowing stream of water. Tyndell recognized this as one of the first examples of total internal reflection (Hecht 1998
). For decades after these observations, optical waveguiding was relegated primarily to solid materials, especially silica based glasses. Years of research in this area of course culminated in optical fiber with incredible purity and light transmission properties. Until recently, research in fluid waveguiding has been somewhat sporadic, coming either from attempts to compete with glass optical fiber or to build light based fluid sensors. The past decade, however, has seen a surge in interest in the topic (Psaltis et al. 2006
; Monat et al. 2007
). This coincides somewhat with the introduction of photonic crystal structures in many different variations. These structures were originally designed to allow for guiding through air, but their operating principles can also be applied to other low refractive index materials like water. Optofluidic devices have also benefited from the technology development that has taken place in the telecommunication and microelectronic industries. Techniques used to create all-glass optical fiber are now used to create hollow waveguiding fibers that can be filled with liquids. Semiconductor fabrication has been adopted to create mechanical structures on chips, including waveguides and photonic circuits and fluid based labs-on-a-chip.
Optofluidic waveguides have found applications in liquid analysis in which optical beams are used to probe lengths of a fluid column. Guided modes within the waveguides can confine light into small areas, leading to high intensities over long distances. Fields such as analytical chemistry, toxicology, and environmental monitoring can benefit from liquid waveguide based systems. Biosensing is another promising application. The small dimensions and fluid volumes in an optofluidic waveguide are suitable for very high sensitivity detection including single molecule detection (Craighead 2006
). Recently a number of active optical devices have also emerged that rely on these waveguides including fluidic dye lasers (Balslev and Kristensen 2005
This paper reviews fabrication techniques and structures for optofluidic waveguides including those that are fiber based and those that can be integrated on a chip. Structures will be included that have demonstrated light guiding through a liquid medium or through an air medium (with a suitable light confining mechanism for fluid guiding). While no paper can hope to be entirely inclusive, the intention is to provide the reader with an understanding of what has already been made and insight into what future optofluidic based waveguides are possible, given our current fabrication methodologies. The waveguides cited include a variety of light confinement mechanisms. In the context of microfluidics and the integration of optofluidic waveguides with other optofluidic devices, we will include an emphasis on the integrated waveguides our research groups have recently developed. These planar Anti Resonant Reflecting Optical Waveguides (ARROWs) provide a good example of marrying multiple microfluidic and optical functions on a compact platform.
This paper is organized as follows. Section 2 provides a very brief review of the two most common waveguiding mechanisms. Sections 3 and 4 then provide a review of fabrication methods and structures, first for fiber based waveguides and then on-chip implementations. These sections are organized according to general fabrication approach, with specific structures using various waveguiding mechanisms. Section 5 concentrates on planar ARROW waveguides and on-chip test platforms constructed using thin-film deposition and sacrificial etching.