Optical sensing: Researchers see the light

InTech, Dec 2004

CHEMICAL AND BIOLOGICAL SENsors with single-molecule sensitivity may be the result of a new technology that enables light propagation through small volumes of liquids on a chip.

"It is an enabling technology that opens up a wide range of fields to the use of optics on integrated semiconductors to do experiments or build devices," said Holger Schmidt, an assistant professor of electrical engineering at the University of California at Santa Cruz.

Schmidt and graduate student Dongliang Yin designed liquid-core waveguides so they could make them using the standard silicon fabrication technology used on an industrial scale to make computer chips. The fabrication process yields a hollow-core waveguide that works whether the core has liquid or gas.

Guiding light waves through liquids and gases is a challenge because of their relatively low refractive indexes. 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 distances.

To build a waveguide with a liquid or gas core, Schmidt relied on the principle of antiresonant reflecting optical waveguides (ARROWS). ARROWS with solid cores see use in 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 distances in hollow-core ARROWs 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 said.

Schmidt is working toward chemical sensing of single molecules using liquid-core waveguides. He also sees potential applications for gas-core waveguides in the areas of atomic physics and quantum optics.

The researchers chose silicon nitride and silicon dioxide as cladding materials for the hollow-core waveguides because of their compatibility with microfabrication techniques and the potential for integration with silicon-based electronics. The cladding layers go over a sacrificial layer that later etches away to create the hollow core, which has a rectangular shape. With a thickness of 3.5 microns and a width of 9 microns, it is the smallest hollow light guide ever made.

"We can make many waveguides in parallel on a chip, so you can imagine probing 20-30 channels at one time, with each channel containing a different sample," Schmidt said. "And because it is all silicon technology, we can integrate it with electrical contacts and even put a silicon photodetector right on the chip."

Schmidt's team has also made two-dimensional arrays of waveguides that connect with each other at 90-degree angles, another useful feature made possible by silicon microfabrication techniques.

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 said.

Fiberoptic connections can channel light into the waveguides, which could also be coupled with microfluidics systems-or "labs on a chip"-to control the flow of samples into and out of the waveguide cores.