Liquid-core waveguide promises novel sensors

Researchers at the University of California (UC), Santa Cruz, have demonstrated a technology that enables light propagation through small volumes of liquids on a chip using integrated optical waveguides with liquid cores. The new technology has a range of potential applications, including chemical and biological sensors with single-molecule sensitivityGuiding light waves through liquids and gases is a challenge because of their relatively low refractive indexes. Whereas in an optical fiber, light travels through a core with a high index of refraction surrounded by cladding with a lower index of refraction. The difference in refractive indexes results in “total internal reflection” of light waves, allowing transmission of optical signals over long distancesTo build a waveguide with a liquid or gas core, Holger Schmidt, an assistant professor of electrical engineering at UC Santa Cruz, relied on the principle of antiresonant reflecting optical waveguides (ARROW). ARROW waveguides with solid cores have been used for semiconductor lasers and other applications. The technique uses multiple layers of materials of precise thicknesses as cladding to reflect light back into the core. Schmidt’s group has achieved low-loss propagation of light over the length of a semiconductor chip in hollow-core ARROW waveguides containing air or liquids.“Liquids and gases are the natural environment for molecules in biology and chemistry. If you can guide light through water and air, all of the fields that rely on nonsolid materials can take advantage of integrated optics technology,” Schmidt says.Schmidt’s team has made two-dimensional arrays of waveguides that connect with each other at 90° angles, a feature made possible by silicon microfabrication techniques, which are carried out at Brigham Young University, Provo, Utah.The researchers have been able to detect molecular fluorescence from a liquid sample in the core of the waveguide, using light from a helium-neon laser to stimulate a fluorescent dye. The experiment detected fluorescence from 800 molecules of dye in a sample volume of 200 picoliters (a picoliter is one trillionth of a liter). Further refinements should enable detection of single molecules, Schmidt says. He also sees potential applications for gas-core waveguides in the areas of atomic physics and quantum optics.