Tiny Holes Capture Light, Could Boost Sensor Capability

Scientists at Ohio State University have found a way to boost the light absorption of a metal mesh up to 1000 times, possibly paving the way for powerful chemical sensors and laboratory instruments.

Key to the technology is a new coating technique that enables the mesh to capture and transmit more light through its microscopic holes than would normally be possible.

James V. Coe, associate professor of chemistry at Ohio State, and his colleagues also found that if they coated the mesh with molecules of fat, they could use heat to control the amount of light passing through.

Coe and doctoral student Kenneth R. Rodriguez discussed the recent results of their work Wednesday, September 10, in two poster presentations at the American Chemical Society national meeting in New York.

“With the right coating process, we found we could precisely control the diameter of the holes, and the amount of light transmitted. In effect, the mesh acts like an optical switch,” Coe said, referring to devices that control light signals in optoelectronics. “With the addition of heat controls, you could call it a thermo-optical switch.”

The university has filed two patent applications for the technology, and is looking for commercial partners to develop it further.

Rodriguez explained the light-boosting phenomenon, which was first discovered in 1998 at the NEC Research Institute in Princeton, New Jersey. There, researchers noticed that nanometer-sized arrays of silver atoms could transmit an extraordinary amount of light in the form of energy packets called surface plasmons. The word “plasmon” is a cross between “plasma” and “photon.”

“The light is excited, and it dances along the surface of the metal and comes out the other side, so you get more light,” Rodriguez said.

The Ohio State researchers wanted to see if they could create plasmons in other metals, using infrared light instead of ultraviolet, as had been done at NEC. For their experiments, they purchased commercially available nickel mesh.

To the naked eye, the mesh looks like flexible metal tape. The perforations are less than 13 millionths of a meter apart — a distance many times smaller than the width of a human hair.

When Coe and his team tried to coat the mesh with copper, traditional methods didn’t work. The copper atoms clumped together, leaving part of the mesh uncovered.

The researchers devised a new method that applied an even layer of copper atoms. They soon discovered that they could tune the process to fill in the edges of the perforations and shrink the holes to whatever size they wanted. When the size of the holes becomes comparable to the wavelength of light hitting the mesh, plasmons are created.

Since the holes normally cover only 25 percent of the surface, the mesh should only transmit 25 percent of any incoming light. But in tests, the Ohio State-coated mesh transmitted 75 percent of the light, which suggested that light incident upon the metal was being transmitted.

When the researchers added a layer of fat — specifically, molecules of trans-fats obtained from soybeans — the surface absorbed even more light — up to 1,000 times more than in any other plasmon experiment ever reported.

The fat molecules can be used to control the amount of light passing through because of their shape, Coe said. “At room temperature, the molecules form long chains that stand straight up on the surface of the mesh. As the temperature rises, the chains melt, changing the polarization of light throughout the hole,” he said.

According to undergraduate student Amanda Stafford, the effect can be repeated over and over, as the fatty parts of the molecules are heated and cooled. “At 100 degrees C, the water in the fat boils away,” she said, “but as long as you keep the molecules hydrated, it works.”

Properly coated, the mesh could be used to study the interaction of cholesterol and cells, or the effect of heat on DNA. It could also be combined with the laboratory technique known as combinatorial chemistry, with which researchers create many thousands of different chemicals simultaneously and screen them for useful properties.

“If you put a different chemical in each hole, you could shine light through and get a detailed view of what was going on. That could speed up lab tests,” Coe said.

Other members of the research team include doctoral students Shaun M. Williams and Shannon Kennedy. Visiting undergraduate student Keith Zomchek from Bradley University in Peoria, IL, is also working on the project, along with Trisha Rogers, an Ohio State undergraduate student.

The American Chemical Society funded this project.

Contact: James V. Coe, (614) 292-9489; Coe.1@osu.edu
Written by Pam Frost Gorder, (614) 292-9475; Gorder.1@osu.edu