Building New Materials With Light

Since the 1970s, physicists have used laser beams to trap and study small objects, from cells down to individual atoms. More recently, researchers have developed multilaser systems that can manipulate many particles at once. But these systems are large and complex, so they have limited applications.

Now, scientists have developed a simple system that uses a single laser beam to direct hundreds of particles at one time to assemble into two-dimensional structures (Nano Lett., DOI: 10.1021/nl400918x). This compact optical trap could eventually help researchers make materials for new types of sensors, optical devices, and chemical filters.

In optical traps, a concentrated beam of light puts a force on particles, causing them to move toward the most intense part of the beam. Current systems that can manipulate many particles at once do so by generating complex light fields using space-hogging setups with many lenses.

Michelle Povinelli, an electrical engineer at the University of Southern California, wanted to develop a simple system for generating these complex light fields. She envisioned one that was small enough to fit on a chip. That way, researchers could readily integrate the traps into devices like photonic circuits or chemical sensors, making them more amenable to applications.

Povinelli’s optical trap uses a patterned slab of silicon called a photonic crystal. She and her collaborators etched into the crystal a regular array of 300-nm-diameter holes, spaced 860 nm from one another. They immersed the slab in a suspension of 520-nm-diameter polystyrene particles and illuminated it from below with a laser. The particles floating above the crystal then moved into the holes, forming a square crystal lattice measuring 13 μm on each side.

The holes in the silicon interact with laser light to create an intense light field above the slab. This field holds the particles in the lattice. With the current device, the particles take about an hour to assemble because they must drift very close to the slab. “Once they get over a hole—boom—they’re pulled in, but first the particles have to wander over by chance,” Povinelli says.

She plans to speed up the assembly process by pumping a more concentrated stream of the particles over the trap. The optical trap should also work with other types of materials, such as semiconductors and metals, Povinelli says.

Paul V. Braun, a materials scientist at the University of Illinois, Urbana-Champaign, says that compared with other optical traps, the USC device offers simplicity and easy integration with other devices. But the existing systems can hold particles in different configurations, while the photonic crystal requires them to stay in a fixed pattern dictated by the pattern of holes. So the devices are easy to make but aren’t as flexible as current optical traps, he says.

Povinelli says she is now working out how to design photonic crystals that can switch the particles between different configurations when she changes the wavelength of laser light.