An Easier Way to Make a Nano Optical Device

To make something useful, one typically doesn’t think of breaking things and pulling them apart. But researchers at Princeton University have found that the approach could work in making a key nano component of optical devices. In a Nature Nanotechnology paper, they describe a method of sandwiching a thin, brittle polymer film between two silicon wafers and then pulling the wafers apart to make nanoscale gratings, or periodically arranged lines that are used to manipulate light in optics applications. “This is a low-cost way to make very high-quality gratings,” says Stephen Chou, an electrical-engineering professor at Princeton.

Gratings, which act like prisms and split light into its different wavelengths, are used in biosensors that detect pathogens, proteins, and analytes such as glucose; they work by measuring the light that the molecules absorb. Optical gratings are also used to store data on CDs and DVDs, and in optical communications devices. Nanoscale gratings hold promise for higher-density DVDs and CDs, faster communications, and more-accurate sensing.

Chou says that the new technique could also bring down the cost of liquid crystal displays (LCDs) because the gratings could replace polymer polarizer films currently used in the displays. Polarizer films only allow light aligned in a certain direction to pass through.

Existing methods to make gratings with lines that are less than a micrometer apart are complicated and expensive. Researchers are constantly looking for simple ways to make submicrometer structures, says Karl Berggren, an electrical-engineering professor at MIT. The new polymer-fracturing method “is a very elegant discovery, and one I’m sure will immediately be put to use by researchers,” he says.

Chou and his colleagues sandwich a 30-to500-nanometer-thick layer of a polymer such as polystyrene between two silicon wafers, heating and pressing the sandwich together. Then they insert a razor blade at one edge and pry the wafers apart. This splits the polymer film and leaves a polymer layer on each wafer. The surfaces of both layers have evenly spaced ridges, resulting in two gratings.

The researchers found that the distance between the ridges on the gratings, also known as the period of the gratings, is always four times the thickness of the sandwiched polymer film; depending on the thickness, the distance ranges from 120 nanometers to 200 micrometers. This property holds true for the different types of brittle polymers that the researchers used.

The results are surprising because generally, when a brittle material is broken, the cracks don’t follow a regular pattern. Fracturing a material is typically thought of as a “violent and nonlinear process,” says John Rogers, a materials-science and engineering professor at the University of Illinois at Urbana-Champaign. But the Princeton researchers have converted it into a reliable technique for fabricating nanoscale patterns. “It’s a delightful piece of work,” Rogers says.

According to Berggren, the new method will have a substantial near-term impact on research. But the technique would have to be more controlled and reliable in order for it to be practically useful in sensors and other applications.

Chou says that the technique creates high-quality gratings every time, as long as the polymer film is firmly attached to the two wafers. What his group is working on now, he says, is a better-controlled and more reproducible way to separate the wafers, as opposed to using a razor blade.

The researchers also want to make larger grating areas, Chou says. Right now, they can make gratings over an area of two square centimeters. This could be useful for polarizing films in cell-phone displays, but the grating would have to be several square feet if it is to be used in computer monitors and TVs.