The phenomenon of microscopic particle trapping in a laser beam was first described in 1970 by Arthur Ashkin of Bell Telephone Laboratories (USA), who was then studying the effects of light pressure on microscale objects. Later, Ashkin and his fellows demonstrated the capacity of an optical trap based on an infra-red laser to catch, hold and move different biological objects, such as virus particles, individual bacterial and yeast cells and organelles in living cells of algae. Caught in the optical trap, the cells kept on dividing, which proved that an infra-red laser does not damage biological objects. Eventually, one of Ashkin’s fellows Steven Chu received the Nobel prize in physics in 1997 for the study of entrapment and cooling of atoms with an optical trap.

In laser tweezers, optically transparent microparticles larger than the wavelength of incident light (e.g., polystyrol or latex balls around 1 micron in diameter or living cells) reflect and refract a laser beam at the same time, which results, subject to the second law of motion, in the forces repulsing these particles from the light source and, at the same time, returning them to their initial position. When a particle is placed in the focus of a laser beam, these forces become balanced, and the particle becomes trapped. Its shift from this position results in another force that returns the particle back. Dielectric particles smaller than the wavelength of the laser can also be trapped in a highly focused laser beam. Their behaviour can be explained by the electromagnetic theory. Dielectric particles are polarised in the non-homogeneous electric field of a laser beam and move towards the beam’s axis, where the field has the highest intensity. Ashkin’s discovery entailed the development of a branch of study concerned with the optical manipulation of microscale objects, as well as the development of new types of optical traps. Today, optical traps and tweezers use one or more lasers and acousto-optical converters that help to create fixed and moving traps and work with more than one object at the same time. These instruments make it possible to measure the force produced by a single molecule of a molecular motor, such as myosin and kinesin, as well as elementary steps made by such molecules during motion. The capability to manipulate submicron objects, including individual atoms, and measure picoNewton forces and nanometre spatial shifts makes optical tweezers one of the key instruments of nanotechnology.