Half of the esteemed Nobel Prize for Physics went to Arthur Ashkin, from Bell Laboratories in the US, who invented the 'optical tweezer'. The method works to trap atoms, particles and viruses between two beams of light, using the phenomenon radiation pressure. The method works by pushing objects, using radiation pressure, towards the most intense part of a light beam, which is in the middle, holding them there for examination or removal.
This process means that scientists can capture and study living bacteria, without causing them any harm. Optical tweezers are a crucial tool, allowing biologists to study ecosystems without damaging or changing them.
Optical Tweezers use light to manipulate microscopic objects as small as a single atom. The radiation pressure from a focused laser beam is able to trap small particles. In the biological sciences, these instruments have been used to apply forces in the pN-range and to measure displacements in the nm range of objects ranging in size from 10 nm to over 100 mm.
The most basic form of an optical trap is diagramed above. A laser beam is focused by a high-quality microscope objective to a spot in the specimen plane. This spot creates an "optical trap" which is able to hold a small particle at its center. The forces felt by this particle consist of the light scattering and gradient forces due to the interaction of the particle with the light. Most frequently, optical tweezers are built by modifying a standard optical microscope. These instruments have evolved from simple tools to manipulate micron-sized objects to sophisticated devices under computer control that can measure displacements and forces with high precision and accuracy.
Optical tweezers have been used to trap dielectric spheres, viruses, bacteria, living cells, organelles, small metal particles, and even strands of DNA. Applications include confinement and organization (e.g. for cell sorting), tracking of movement (e.g. of bacteria), application and measurement of small forces, and altering of larger structures (such as cell membranes). Two of the main uses for optical traps have been the study of molecular motors and the physical properties of DNA. In both areas, a biological specimen is biochemically attached to a micron-sized glass or polystyrene bead that is then trapped. By attaching a single molecular motor (such as kinesin, myosin, RNA polymerase etc.) to such a bead, we have been able to probe motor properties such as: Does the motor take individual steps? What is the step size? How much force can the motor produce? Similarly, by attaching the beads to the ends of single pieces of DNA, experiments have measured the elasticity of the DNA, as well as the forces under which the DNA breaks or undergoes a phase transition.
