Developing Motors of Light to Power Small Electronic Devices

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University of Chicago physicists have successfully measured the angular momentum carried by tiny rings of light called optical vortices, an important step in harnessing their energy to power microelectromechanical (MEMs) devices. University of Chicago Physics Professor David Grier and a former Ph.D. student, Jennifer Curtis, now a researcher at the University of Heidelberg in Germany, present their data in the April 4 issue of Physical Review Letters.

Industry observers say that MEMs devices could lead to the production of everything from nanorobots to laboratories-on-a-chip by bringing together silicon-based microelectronics with micromachining technology.

"One of the sticking points for these systems is the need for a motor," Grier said. "There are many beautiful designs for MEMs motors, including electrostatic drives and magnetic drives. They all share the problem that they go a little bit slowly and they wear out very, very fast."

But optical vortices need no microfabricated micromotor. In fact, they have no moving parts. "You can just project an optical vortex onto the device and away it will go," Grier said.

Optical vortices are rings of light that rapidly spin microscopic spheres suspended in fluid around their circumference. Scientists have created optical vortices using a variety of methods since 1995, but Grier and Curtis used dynamic holographic optical tweezers to make theirs. Grier co-invented the HOT technology with another of his former Ph.D. students. The technology uses forces exerted by strongly focused, computer-generated holograms to create large arrays of optical traps. Each trap can suspend a microscopic object motionlessly in three dimensions, as if stuck in a miniature Star Trek tractor beam.

The research was supported by grants from the Arryx Inc. (the commercial licensee of HOT technology), the National Science Foundation and the W.M. Keck Foundation.

HOT technology, among other methods, can transform the light forming ordinary optical traps into so-called helical modes. This changes the traps into optical vortices that can impart angular momentum to trapped objects, making them spin. All photons of light already carry an intrinsic angular momentum endowed by nature, but the added helical twist superimposes an additional orbital angular momentum onto the photons. This resembles the extra angular momentum acquired by an electron as it traces its orbit around the nucleus of an atom.

"That's what makes these optical vortices and that's what makes them useful," Grier said.

HOT technology creates optical vortices by twisting ordinary waves of light into a corkscrew pattern. Each ray passing through the helix has an out-of-phase counterpart with which it destructively interferes when the beam converges with a lens. This destructive interference creates a dark spot at the focal point, surrounded by a ring of light a few micrometers across. Any object that absorbs or scatters this light experiences a torque due to the beam's orbital angular momentum. The strong intensity gradients in a well-focused optical vortex also tend to draw small objects toward the focal ring where they can become trapped while they spin.

Other research groups have created no more than eight parallel twists or "ramps" in their corkscrews of light. Using HOT technology, Grier's group can create an incredible 200 twists in the corkscrew. This allows the scientists to observe new phenomena when the resulting optical vortices are projected onto small objects suspended in fluid.

It also allows them to precisely control and tune the characteristics of the beam. For example, this enables the team to direct the fluid flow with an array of rapidly spinning particles trapped in one or more rings of light.

"Other optical traps grab things. This trap allows you to grab something and exert a twist as well. You can exert both a force and a torque," Grier said.

An array of four vortices that can be configured into a 12-way pump fits into a space measuring five to 10 microns across, which is just a fraction of the width of a human hair.

Grier and Curtis realized while making and studying the optical vortex pumps that they had actually produced the laboratory equivalent of graphic artist M.C. Escher's impossible staircase. Escher created the impossible staircase, a work titled "Ascending and Descending," in 1960. The work shows a group of monks walking up and down a continuous staircase that would bring them back to their starting point after making one complete circuit.

The team noted that as a particle makes its way around the circumference of the ring of light created by the optical vortex, it traverses a corrugated landscape that resembles a tilted washboard. Every time the particle falls into a depression on its way down the washboard, it loses a bit of energy. It stays lodged in the well until a kick of heat energy from the surrounding fluid propels it into the next depression.

At every step along the circumference of the vortex, the particle takes a step down in energy. But at the end of its journey, it finds itself back where it started. "It's forever taking steps downward and coming back to where it started, just as in Escher's impossible staircase," Grier said.

The impossible staircase works through a trick of perspective. The Chicago experiment works because of the particle's interactions with the heat bath. With every step it takes, the particle gives up a bit of its energy but then takes it back from the surrounding fluid.

"In fact, after every step down you end up with the same energy you started with. It looks like you're considering only the forces exerted by the light and not the forces being exerted by the surrounding environment," Grier said.