How Things Work: Optical tweezers

Brian Lee Mar 30, 2009

If anyone were to list similarities among outer space science fiction movies, the results would be predictable: giant spaceships, warp speed, intelligent aliens; there might even be audible explosions in the vacuum of space. While none of these actually exist yet (that we know of), technology is rapidly catching up to our childhood interstellar dreams. Optical tweezers are one such form of technology. Optical tweezers are loosely based on the fictional tractor beams, popularized by Star Wars and Star Trek.
Tractor beams are a kind of energy beam that focus on an object in space, forcing it to move toward the ship without actually coming into contact with it. Or, they might hold cargo in place, or even rip distant objects apart.

Of course, this is complete science fiction. Large-scale tractor beams do not exist yet outside of our imagination. However, an optical tweezer can be thought of as a microscopic tractor beam; it is a focused laser used to attract or repel microscopic objects. Holding particles, called optical trapping, facilitates the study and manipulation of atoms and other small or large molecules.

An article on the Bell Labs website explains that the first atomic trap, using a focused laser beam, was conducted at Bell laboratories in 1984. The paper written by Arthur Ashkin of Bell Labs was published in the Proceedings of the National Academy of Sciences of the United States of America (PNAS) and explained the working of the optical traps.

When a laser beam is focused on an object through a microscope lens, the laser beam creates an electric field. Forces are produced by the object’s interaction with the electric field and the light itself. The laser beam produces a strong electrical field gradient, with the region of the strongest electric field in the center of the beam. This is kind of like a garden hose with a thin nozzle — the farther one is from the center of the water spray, the less water there is.
The object tends to move to the center of the laser beam, where the electric field is strongest. In addition, forces are produced when the laser light is scattered as it hits the object, and when the laser light is refracted through the object.

The reason the particle stays in the center of the beam is because of the sum of the forces acting upon the object itself. In the center, rays of light refract or scatter through the object the same way on both sides, resulting in the cancellation of forces moving the object sideways. When the object drifts to one side, it returns to the center, because the greater electric field causes a greater force to be exerted on the object in the direction opposite to its drift. This is analogous to a spring — when displaced from its equilibrium position, an opposing force will cause it to accelerate back to the center.

Optical tweezers are truly a revolutionary connection of physics and biology. Currently, optical tweezers have trapped many objects for study, including viruses, bacteria, living cells, and strands of DNA. There are many practical applications in various scientific fields. Cells can be sorted or organized, as optical tweezers can place them exactly where a researcher wants. In addition, optical tweezers have been used for measurement in studies involving forces.

Researchers can attach a single cellular motor molecule, a kind of molecule involved in cellular movement, to microscopic beads. In this way, they are able to conduct experiments on a single molecule. This is a scientific breakthrough, as cells usually have many motor molecules to help it move in its environment.

The production of optical tweezers is still extremely expensive, and usually involves the modification of a commercial optical microscope. Many lasers have to be equipped extremely precisely, and researchers have to keep in mind that biological samples must not be killed during experiments. There have been many successful implementations of optical tweezers, and better methods of trapping particles are constantly being studied and proposed.

The potential of optical tweezers for full-scale tractor beams is extremely slim; however, for medical research, it is promising. Researchers can alter larger cellular structures like cell membranes, test breaking strengths of DNA strands, and track bacteria movement.
Of course, the entertainment field is quick to capitalize; there is a video online of Tetris being played with glass microspheres, which have a diameter of only 1 micrometer. The video can be found at this site: http://www.nat.vu.nl/~joost/tetris.