Focus: A Nanoscale Tractor Beam

Phys. Rev. Focus 21, 21
A new theory on the interactions of nanoparticles with laser light predicts some surprising effects when more than one particle is involved, such as a particle being drawn toward the source of the beam, against the flow of photons.

A laser beam can push a nanoscale particle away with the pressure of its photons, but the particle may also be drawn toward the light when other particles are nearby–like the “tractor beams” of science fiction–according to a theory in the June Physical Review B. The theory also predicts that in the presence of light, two particles can attract or repel one another, and that a third particle can amplify the force between the first two by 100 times. The work suggests ways of manipulating particles that may be used to build nano-devices or nano-composite materials.

For decades, researchers have used lasers to manipulate microscopic objects in water, such as bacteria or glass beads, for many kinds of experiments, and they’ve also trapped clouds of cold atoms with lasers in vacuum. But few have managed to move particles at an intermediate scale of tens of nanometers–the size range essential for future nanotechnology.

Such particles are so small that it’s hard to hit them with enough light to have much effect. But researchers have had some success when the laser is tuned to a frequency that matches the energy difference between two electron states in the particle. This “resonance” technique allows the particle to absorb a lot more photons, and be pushed by the beam the same way the spray from a garden hose can push a beach ball. The amount of absorption depends on the details of the particle, so eventually, particles could be separated by size or quantum state, or be placed in specific groups to engineer a device or novel material.

Takuya Iida and Hajime Ishihara of Osaka Prefecture University in Japan predicted the effectiveness of a resonant beam five years ago using computer simulations ([1] Focus story), and now they have worked out a detailed mathematical theory, starting from first principles of electromagnetism and quantum mechanics. Their work attempts to bridge the gap between purely quantum-mechanical theories of laser-atom interactions and purely classical theories of micron-sized particles.

The team analyzed several configurations of two or three 20-nanometer-diameter semiconductor spheres being hit with laser light. The theorists included effects of the quantum states of electrons in each sphere and of the electromagnetic forces of the particles on one another. For example, with three particles arranged one above the next, and laser light shining from the side, the team discovered that the middle particle could be drawn toward the laser–the equivalent of the beach ball moving against the flow from the garden hose. “At first, we were very surprised,” says Iida, but the team proved that the law of momentum conservation was not violated when the momentum of all three particles and the radiation were included. The effect occurs when the electrons in the middle particle–moving up and down in response to the laser’s oscillating electric field–get out of step with those in the upper and lower particles, thanks to electromagnetic interactions among all three. In another scenario, under conditions where two nanoparticles would be attracted to each other, interactions with a third particle between them can boost the attraction by 100 times.

The researchers hope their theory will lead to new ways of sorting and building with nanoparticles. Matthew Pelton of Argonne National Lab in Illinois finds some of the results intriguing but suspects that they will be “very difficult to verify in the lab” with current technology. Munir Nayfeh of the University of Illinois at Urbana-Champaign agrees that experiments will be difficult, given the current lack of control of characteristics of nanoparticles. Iida counters that the nanoparticle sorting suggested by his work could improve that control. Nayfeh adds that if it can be accomplished, the Osaka team’s concept “would revolutionize the capability of mass production of nanostructures on demand.”

–Pam Frost Gorder

Pam Frost Gorder is a freelance science writer in Columbus, OH.

References

1. T. Iida and H. Ishihara, “Theoretical Study of the Optical Manipulation of Semiconductor Nanoparticles under an Excitonic Resonance Condition,” Phys. Rev. Lett. 90, 057403 (2003); see also Focus story from 2003

Nanophysics

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