Light surrounds us; it reflects; it emits. But did you know that it is also a powerful force that can push objects? Light is constantly exerting force that can move everything from a microscopic plastic ball to a human being. Shine a laser beam at a person and the light will give them a tiny push, even though they won’t be able to feel it.
Since the 1970’s, engineers and physicists have been putting this concept into practice, using laser beams to hold, trap or push microscopic objects, such as atoms. These optical tweezers, as they are called, are used to study and measure DNA, cells and bacteria. While this technique has primarily been demonstrated using a single or few particles, researchers at USC are taking this concept a step further.
USC Viterbi professor Michelle Povinelli and a team of her students have shown that a single laser beam can be used to guide hundreds of synthetic nanoparticles into an array of reconfigurable, two-dimensional patterns. Povinelli’s new findings have been published in the April edition of Nano Letters, a publication of the American Chemical Society (ACS).
“We have discovered that by using light, we can force particles to arrange themselves in particular patterns that would not otherwise occur in nature,” Povinelli said.
Povinelli uses a technique called LATS (light-assisted templated self-assembly) to form these multi-particle patterns. The collection of particles can be thought of as a new material, put together from the nanoscale up. This composite material can have very different properties than the individual particles it is made of – just as opal, made of glass particles, has strikingly different characteristics than a single slab of glass.
“LATS can be used to make a wide variety of nanostructured materials, with precise control over composition and internal structure,” Povinelli said.
Her research may have future applications for optical communications and fiber optics. For instance, it can be used tune photonic devices, filters or sensors in real-time. In the bioscience world, it could be used to study and arrange different biological interactions between organisms.
To “trap” these particles, Povinelli takes a silicon-based slab with an array of cylindrical holes etched into it, or "template". These holes have been strategically placed to create the desired shape from the particles. Synthetic spherical plastic balls are placed into a water-based solvent and allowed to float freely.
The laser is positioned below the template and light is beamed upwards. The light is trapped in the template, boosting the local electromagnetic field intensity just above its surface. The balls naturally gravitate toward the area where the light is strongest, causing them to assemble on the surface. In the team’s current experiment, one sphere settles into each hole, forming a square array of particles.
“You can trick light into behaving the way you want it to behave,” said Eric Jaquay, a Ph.D. candidate in electrical engineering at USC Viterbi.
This preliminary research has broad implications for future structuring and reconfiguration. Povinelli’s research suggests that adjusting the wavelength of a laser light can effectively change the formation of the particles.
For instance, shining a laser on the particles may initially configure them into a square lattice. If the wavelength of the laser is tuned, the spots on the surface of the template where the field intensity is highest will change. The particles will then reconfigure into another pattern, such as a rectangular lattice. Turn the light off completely, and the balls will disperse and flow freely as before. This type of control can give unprecedented flexibility in changing the properties of materials, Povinelli explains.
Though still in its preliminary stages, Jaquay suggests this technique can be used on a plethora of materials, as well as expanded for use on larger objects. While the experiments have been limited to inorganic objects, Jaquay envisions using LATS to configure bio-particles, including cells, proteins and DNA into new structures.
"In our current experiments, we are expanding the types of particles that can be assembled and the range of patterns they form," Povinelli said. "This basic scientific advance should allow a range of applications we can now only guess at."