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Web Date: April 29, 2013

Building New Materials With Light

Nanotechnology: A simple optical system assembles two-dimensional structures of nanoparticles using a laser beam and could lead to new materials for sensors and photonic devices
Department: Science & Technology
News Channels: Nano SCENE, Materials SCENE
Keywords: self-assembly, optical trapping, optical materials, nanomaterials, photonic crystals
A video shows 520-nm-diameter polystyrene particles assembling into a square shape over the pattern of holes on a silicon photonic crystal. The black scale bar represents 5 µm.
Credit: Nano Lett.

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.

Light Force
A patterned slab of silicon (gray) concentrates laser light (red) to pull particles (blue) into two-dimensional shapes.
Credit: Nano Lett.
Illustration of nanoparticles being drawn to a photonic crystal illuminated by a laser
Light Force
A patterned slab of silicon (gray) concentrates laser light (red) to pull particles (blue) into two-dimensional shapes.
Credit: Nano Lett.

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.

Chemical & Engineering News
ISSN 0009-2347
Copyright © American Chemical Society
Aurovrata (Thu May 02 09:30:43 EDT 2013)
Could this be the solution to build carbon-nanotube cables from space down to the equator for a Space Elevator?
Travis J. Wells (Thu May 02 09:45:48 EDT 2013)
This is amazing! So if I understand correctly this could literally be a precursor to Star Trek replicator technology? My question is, once these particles are brought together does Weak Nuclear Force bind them together into one solid object at that point?
Bobloblaw (Sat May 11 17:34:49 EDT 2013)
No. These particles are around 300 nm in diameter. Moreover, they are dielectric. These photonics crystals do not trap individual atoms - their pitch is on the length scale of light. The weak force is an atomic force. The electromagnetic force is what is employed here to trap the particles.
Zeeshan Ahmad Abbasi (Thu May 02 12:50:17 EDT 2013)
Evolution of Photoelectric Effect,If electrons can be moved from metal surface with the energy of Electromagnetic radiation,why not the nano particles with intense Laser beam.
Michael Smith (Sat May 11 17:41:03 EDT 2013)
Not the same effect at all. The photoelectric effect involves electromagnetic radiation incident upon a metallic surface. If the incident energy (determine by the frequency of light) is above the work function of the metal, the light can eject an electron upon absorption. If I understand this experiment correctly, the laser is used to excite modes within the photonics crystal that possess a high quality factor. The evanescence fields created by these modes are trap the particles.
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