Light Pipes Detect Vapors

A portable laboratory the size of a microchip uses light to detect gases.

Brigham Young University (BYU) and University of California-Santa Cruz (UC-Santa Cruz) researchers have achieved the advance by focusing light through special tubes tiny enough for use on the scarce silicon real estate.

Light normally disperses in all directions from its source. But these "light pipes" are designed to focus intense beams of light on a small number of atoms inside.

This tiny chip uses light to detect gases. The faint lines are "light pipes," which focus beams on the gas atoms inside. (Photo courtesy Brigham Young University)

The light pipes have a leg up on more common light "waveguides" like fiber optics, which are solid. Their hollow waveguides can contain gases or fluids; therefore, they can use light to identify substances by the wavelengths of light they reflect, a process called atomic spectroscopy. All on a chip the size of a fingernail.

"We've taken what is typically done on a lab bench and tried to shrink all of that to a little chip," said BYU electrical engineering professor Aaron Hawkins. "You could use the optical phenomenon produced in a portable setting for a variety of applications."

Those applications could include sensors that alert users to dangerous gases or gauges to measure air quality. Other applications of this process include highly secure quantum communications and advances in atomic clocks, which are useful for global positioning satellite systems and cell phone networks.

Hawkins and UC-Santa Cruz professor Holger Schmidt have been working together on the application of these hollow waveguides to optical problems for about four years. Their work is funded by the Defense Advanced Research Projects Agency and the National Science Foundation. The findings were recently reported in Nature Photonics.
"This is the first paper that shows how we can combine optical waveguiding and integrated optics with atomic spectroscopy -- nobody has done that before," Schmidt said.

Electrical engineering professor Aaron Hawkins and his graduate student Donald Conkey teamed up on a paper in Nature Photonics. Graduate student John Hulbert is assisting with similar work. (Photo courtesy Brigham Young University)

Hawkins was assisted by his graduate student Donald Conkey, a co-author of the paper, in the delicate maneuverings required to create such a small device. They built the chip in BYU's dust-free "clean room," then faced the challenge posed by the nature of the gas that was to be tested inside. Rubidium vapor degrades when exposed to oxygen, so it need to be inserted into the small and fragile chips inside a sealed container called a glove box.

"It was easy to conceive, but doing it was very difficult," Hawkins said. "Sticking rubidium, which is extremely reactive to water and oxygen, into tiny on-chip reservoirs and then getting it to fill up the open tubes was the hard part."

After the chip was built and rubidium was added, the device was sent to the team at UC-Santa Cruz for testing.

Looking forward, the research team is also collaborating on projects involving liquid in small waveguides, funded by the National Institutes of Health. This work is aimed at looking for viruses in biological fluids, which could lead to medical testing of blood and saliva. Results from these liquid-filled waveguides were recently published in Applied Physics Letters.

Hawkins said the teams are fulfilling a common engineering goal: to take something useful and make it smaller.

"This approach brings what others are doing in the lab closer to mass production," he said. "We are taking cutting-edge science and transitioning it into something we could conceivably see in a product."

For more information, visit: http://byu.edu