Light spins resin rotors

By Kimberly Patch, Technology Research News

It is obvious that wind can make windmills rotate, but not so obvious that the energy contained in photons of light can do something similar on the micron scale.

Researchers from the Hungarian Academy of Sciences have shown it is possible to harness the pressure exerted by photons of near-infrared light and use it to spin tiny resin rotors. The researchers also use light to construct the rotors, which can be as small as half a micron.

They got the idea to power tiny objects with light after noticing that microbeads they were trying to manipulate with laser tweezers rotated while trapped in the tweezer's light.

The researchers found literature describing the phenomenon, and adapted the process for making light driven rotors, said Péter Galajda, a Ph.D. student at the Hungarian Academy of Sciences. "The rotation speed of the rotors is easily controllable by the laser intensity," he said.

To fashion the rotors, the researchers focused green laser light through a microscope onto a sample of viscous resin. By moving the photosensitive resin around in the light the researchers were able to create three-dimensional objects. Once an object was finished, they used acetone to dissolve the remaining liquid resin.

The process allowed the researchers to make parts ranging in size from half a micron to several microns -- sizes similar to biological cells like the two-micron-long E. coli bacterium and the five-micron-diameter red blood cell.

Normally, ultraviolet light is used to solidify the photosensitive resin. But the researchers made their parts via two-photon polymerization, using the green light in place of ultraviolet. The resin doesn't normally absorb photons of green light, which carry half the energy of ultraviolet photons, but if the beam is intense, the resin will absorb a pair of green photons at the same time, reacting to them as if they were a single ultraviolet photon.

The two-photon process allowed the researchers to fashion smaller parts, because it solidifies resin at a higher resolution than is possible using ultraviolet light. Using two photons essentially squares the intensity of the beam, which improves the spatial resolution, said Galajda.

The process of making the microscopic objects is cheap and flexible, said Galajda. "The costs are fractions of those of clean labs using silicon techniques, [and] arbitrary shapes can be easily generated," he said.

Designing the shape of the rotors so they would rotate in light was tricky due to an unfortunate detail in the physics involved. "Since the size of these objects is close to the order of the wavelength range for visible light... it's hard to treat the problem analytically," said Galajda.

Because the object size and wavelength size are so close, it is difficult to tell the differences among the diffraction, scattering and reflection of the laser light by the rotors. This makes it impossible to calculate the position of a complex shape like a rotor when it is trapped in the laser light, Galajda said. The trouble is, this calculation is crucial to finding the exact the physics of the rotation, he said.

Lacking this information, the researchers simply made the assumption that it was the reflection of light that made objects rotate, and modeled the rotors after windmills. "We tried to find optimal shapes for the rotors intuitively," said Galajda.

Because the shapes they designed with this in mind rotated successfully in the light, the assumptions were probably correct, he said. "It seems that the rotation is similar to a light-driven windmill. Here the role of the wind is played by light," he said.

It's a useful process, said Stephen Quake, an associate professor of applied physics at The California Institute of Technology. "I think it's a very clever way to make the micromachine device and actuate it," he said. Although other researchers have moved objects using light, this process is potentially more useful, said Quake. "This is a much more general result, because they can actually engineer the things they are moving."

The researchers plan to continue the work by producing more complicated devices. "We demonstrated that it's possible [to] construct a working machine with a specified task. We plan in the near future to make working micromachines," such as microscopic pumps, Galajda said.

Eventually, the rotors could be used in lab-on-a-chip technology, said Galajda. "I think that light-induced rotors can find promising applications as part of complex micromachines. These devices could be used to perform... chemical, biochemical or physical analysis on extremely small amounts of samples in a user-friendly way."

The rotor could also be used to measure and manipulate molecules, he said. "The rotor itself [could] be used for... measuring viscosity of microscopic samples or even twisting attached macromolecules and measuring properties of those molecules," he said.

The researchers are continuing to work on understanding the physics of the light rotation process to explain fully the effects they have seen with the rotors, said Galajda. Although it is obvious that an object's position relative to the laser beam is important, it is not clear exactly how this works, he said. "We are in the very beginning of studying this problem," he said.

Using the rotors in practical applications also depends on developments in the laser industry, said Galajda. "Cheap high-energy semiconductor lasers with good beam quality would boost the applications," he said. The process could be used in practical applications within the next few years, he added.

Galajda's research colleague was Pál Ormos of the Hungarian Academy of Sciences. They published the research in the January 8, 2001 issue of Applied Physics Letters. The research was funded by the Hungarian Research Fund.

Timeline:   3-5 years
Funding:   Government
TRN Categories:  MicroElectroMechanical Systems (MEMS)
Story Type:   News
Related Elements:  Technical paper, "Complex Micromachines Produced and Driven by Light," Applied Physics Letters, January 8, 2000.


January 17, 2001

Page One

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