How do optical tweezers work

This is how optical tweezers work

In science fiction films, viewers are always spellbound when people step into a beam of light, only to be raised and then to disappear. This is already a reality on the microscopic level.

On the occasion of the award of the Nobel Prize in Physics to Arthur Ashkin, we are republishing this text from our archive.

In numerous science fiction films, viewers are repeatedly spellbound when people step into a beam of light only to be raised and then disappear. What has to be done with trick techniques in the cinema for the time being is already reality on the microscopic level. Microscopic particles and structures can actually be manipulated and even moved with light.

As early as the late 1980s, entire genes were introduced into plant cells using lasers and then complete plants were regenerated. And only recently, a research group from the University of Jena succeeded in opening cell membranes with a pulsed laser beam and inserting pieces of DNA. The DNA carried the genetic information for a protein that could fluoresce green when irradiated. The cells survived the procedure, continued to grow undisturbed and produced the green fluorescent protein.

Thanks to the invention of the laser, it is thanks to the invention of the laser that today you can work with light like very fine tweezers. In contrast to the light from an ordinary lamp, the light from a laser is focused. The light energy per unit area hardly weakens with distance. With the appropriate energy, laser light can therefore exert a high pressure. If, for example, the beam of a 1-watt laser is focused on a spot with a diameter of one micrometer, the light exerts a force that is around 700,000 times greater than the earth's gravity. This makes “optical tweezers” a versatile tool for the micro world.

Similar to mechanical tweezers with their two gripping arms, optical tweezers also consist of two functional components. With a relatively low-energy laser, a microscopic examination object is first held and then processed with a relatively high-energy laser. One of the undesirable side effects of optical tweezers is the absorption of light by cellular structures. This can lead to the development of heat and thus damage to the cell. The “gripper arm” of optical tweezers therefore mostly works with a neodymium-yttrium-aluminum-garnet laser, which is only slightly absorbed by biological materials and hardly develops any heat. It holds particles in a kind of trap so that they can be displaced by moving the laser beam.

In this way, for example, chloroplasts can be moved in a plant cell. When the laser is switched off, the organelles return to their original position. Your position is now again determined by the cell architecture. You can also use the laser beam to move entire cells in the tissue - for example, to fuse them with other cells.

Other lasers are used for the “working arm” of the optical tweezers: These are intended to cut or drill, i.e. use their energy to deliberately destroy them. The size of the working point and the functional specificity of optical tweezers are determined by the wavelength of the laser light used. With precise focusing, the laser only unfolds its energy at the target point. Outside of the focussing point, the effect of the laser light remains small.

One of the most important areas of application of the optical tweezers in the future will be genetic engineering and, in particular, gene processing or also gene repair. The ability to penetrate inside cells and act on cellular structures without killing the cell makes surgical treatment of genetic defects an option. Different genes can be imported or exported into the cell with the optical tweezers. If the location of a disease-causing gene is known, it can be cut out and replaced with an intact gene.