Light-Based ‘Tractor Beam’ Generated for Nanoscale Assembly

Researchers from the University of Washington (UW) have developed a method that could make reproducible manufacturing at the nanoscale possible.

The team adapted an existing technique pioneered by Arthur Ashkin, optical tweezers, to operate in a water-free liquid environment of carbon-rich organic solvents. Traditional optical tweezing methods have been used almost exclusively in water or vacuum-based environments.

Focused laser light generates an optical "tractor beam," which can manipulate and orient semiconductor nanorods (red) with metal tips (blue) in an organic solvent solution. The energy from the laser superheats the metallic tip of the trapped nanorod, allowing the aligned nanorods to be welded together end-to-end in a solution-based "nanosoldering" process. Courtesy of Vincent Holmberg/Matthew Crane/Elena Pandres/Peter Pauzauskie.

The optical tweezers work like a light-based “tractor beam.” The technology isn’t able to lift cows or capture spaceships like its sci-fi namesake; instead, it shows its strength at the nanoscale where highly focused laser light is able to trap materials nearly one billion times shorter than a meter.

The photons that make up the laser beam exert force on objects in the immediate vicinity of the optical trap. The laser’s properties can be adjusted so that the force can either trap or release an object.

To demonstrate the technique, researchers used the optical tweezers to build a novel nanowire heterostructure, which is a nanowire consisting of distinct sections composed of different materials. The structure was built from shorter “nanorods” of crystalline germanium, each just a few hundred nanometers long and tens of nanometers in diameter — or about 5000× thinner than a human hair. Each was capped with a metallic bismuth nanocrystal.

Then the researchers used the “tractor beam” and soldered the rods end to end, thanks to the molten bismuth cap at the end. The researchers could then repeat the process until they had assembled a patterned nanowire heterostructure with repeating semiconductor metal junctions that was 5× to 10× longer than the individual building blocks.

“This is a new approach to nanoscale manufacturing,” said Peter Pauzauskie, a UW associate professor of materials science and engineering, faculty member at the Molecular Engineering and Sciences Institute and the Institute for Nano-Engineered Systems, and a senior scientist at the Pacific Northwest National Laboratory. “There are no chamber surfaces involved in the manufacturing process, which minimizes the formation of strain or other defects. All of the components are suspended in solution, and we can control the size and shape of the nanostructure as it is assembled piece by piece.”

Nanowires that contain junctions between materials — such as the germanium-bismuth junctions synthesized by the UW team — may eventually be a route to creating topological qubits for applications in quantum computing.