Laser tweezers manipulate objects just 50 nanometers wide

A new set of laser tweezers offers scientists unprecedented control over objects just tens of billionths of a meter in size. The device could allow biologists to probe individual viruses and proteins without risk of frying them.

“It’s a very clever method,” says Phil Jones, an optics physicist at University College London. “You can trap much smaller objects with much less laser power.”

Since the 1980s, scientists have studied molecules, bacteria and other minuscule objects under the microscope by trapping them with laser light. Lenses focus the light toward the sample, and subtle forces exerted by the light nudge the object toward the center of the beam.

But this technique has limitations: It has trouble trapping objects much smaller than the laser light’s wavelength of several hundred nanometers. If scientists want to probe smaller biological curiosities such as proteins, they have to either turn up the laser power (which can overheat samples) or tether the samples to larger objects (which could cause specimens to behave differently than they would on their own).

Romain Quidant, a nano-optics physicist at the Institute of Photonic Sciences in Barcelona, is one of many researchers working on ways to overcome optical tweezers’ size limitation. In 2009 he and his group proposed a tweezing technique based on plasmonics, the study of light interacting with components of matter that are smaller than its wavelength (SN: 11/7/09, p. 26). The researchers found that electrons swimming around in metals such as gold can act collectively as a nano-sized lens to focus light into tiny spaces.

Now Quidant and his team have put that idea into practice with a contraption made up of an optical fiber attached to a motor. They tapered the tip of the fiber and attached a thin gold film with a hole between 130 and 180 nanometers wide shaped like a bow tie. Then they shined a laser through the fiber and its custom-made tip into a tank of water littered with plastic beads 50 nanometers in diameter. “The beads cannot be trapped with conventional tweezers,” Quidant says.

In a study published March 2 in Nature Nanotechnology, Quidant’s team reports that the super-focused laser light grabbed hold of individual beads for minutes at a time. When a bead tried to sneak away, the light pushed the bead back in place. Once the researchers had trapped a bead, they used the motor to move the fiber, and thus the bead, in all directions: left to right, forward and backward, even up and down.

While these laser tweezers could be useful for all kinds of nanotechnology, Jones says they carry the most promise for biology. The laser requires just a few milliwatts of power — about as much as a laser pointer does — minimizing the risk of samples absorbing the laser’s energy and overheating. Jones envisions biologists grabbing viruses and sticking them onto a cell to watch them attack. “You could put these particles exactly where you want them,” he says.

Editor’s Note: This story was updated on March 11, 2014, to correct the size of the bowtie-shaped hole and the power the laser required.