Originally called ‘single-beam gradient force trap’, optical tweezers are scientific instruments that use a highly focused laser beam to provide an attractive, or repulsive force, depending on the refractive index mismatch to physically hold and move microscopic dielectric objects similar to tweezers. Optical tweezers have been particularly successful in studying a variety of biological systems in recent years.
The detection of optical scattering and gradient forces on micron sized particles was first reported in 1970 at Bell Labs. Years later, the first observation was reported in what is now commonly referred to as an optical tweezer: a tightly focused beam of light capable of holding microscopic particles stable in three dimensions.
Optical tweezers have proven useful in many areas of biology. For instance, in 2003, the techniques of optical tweezers were applied to the field of cell sorting; by creating a large optical intensity pattern over the sample area, cells can be sorted by their intrinsic optical characteristics. They can also be used to probe the cytoskeleton, measure the visco-elastic properties of biopolymers, and study cell motility.
They are capable of manipulating micrometer-sized particles, living cells, or subcellular organisms by exerting piconewton forces via a highly focused laser beam. The beam is focused by sending it through a microscope objective. The narrowest point of the focused beam, known as the beam waist, contains a very strong light gradient. Dielectric particles are attracted along the gradient to the region of brightest light, the center of the beam. Using an infrared laser, an invisible optical trap is created. The laser light also tends to apply a force on particles in the beam along the direction of beam propagation. This is due to conservation of momentum: photons that are absorbed or scattered by the tiny dielectric particle impart momentum to the dielectric particle. This is known as the scattering force and results in the particle being displaced slightly downstream from the exact position of the beam waist.
Optical trapping is highly sensitive and capable of detecting movement of dielectric particles in the sub-nanometer scale. It is possible to study individual molecules by attaching them to beads and manipulating the bead in the laser trap. This method is widely used to study the physical properties of DNA and in the study of molecular motors. Both movement and small forces exerted within a system can be measured.
The most basic optical tweezer setup will likely include the following components: a laser (usually Nd:YAG), a beam expander, some optics used to steer the beam location in the sample plane, a microscope objective and condenser to create the trap in the sample plane, a position detector (e.g. quadrant photodiode) to measure beam displacements and a microscope illumination source coupled to a CCD camera.
Universe Optics is on the cutting edge of precision lenses to be used in optical tweezer systems. With the entire setup being constructed of several optical systems, you can count on accuracy and reliability in all of our designs. We design and manufacture each lens to minute specifications, guaranteeing clear, concise images necessary for diagnostics, research and development within the field of medicine and other forms of biological studies.
The technical advances in optical tweezers, promoted over the past two decades have brought the technique to a state in which they have become a laboratory tool instead of one for research and development. Reliable, accurate and stable measurements are now performed routinely by increasing numbers in various disciplines of life sciences worldwide.