Optical tweezers

An optical tweezers or dipole trap is a photonic device to manipulate, ie for holding and moving the smallest objects. The function is based on the fact that light microscopic objects (such as microspheres, some biological cells or cell organelles or even atoms) exerting force and which are thus pulled toward the focus of a strongly focused light beam. A typical embodiment, a reflected laser beam into an optical microscope, which is thus focused in the object plane. The manipulative parts must be transparent at the wavelength used. If the laser is set to one time, that the object is in focus, each positional deviation result, it is drawn through the pulse transmission in the refractive back into focus.

Except by focusing optics are also used holographic focusing of laser light.

Through use of a second laser having a wavelength that is absorbed by the object (usually ultraviolet), in addition it has a cutting instrument (Micro laser scalpel ) is available.

History and discovery

The first scientific study of forces on particles on the order of micrometers, caused by scattering of light and gradient forces, was published in 1970 by Arthur Ashkin, then a physicist at Bell Laboratories (USA). Some years later, Ashkin and colleagues reported the first observation of the ability to capture with the help of a highly focused beam of light microscopic particles in the three spatial dimensions. This discovery was the basis for the development of the optical trap. One of the coauthors of this publication was Steven Chu, the further-developed the technique for laser cooling and storage of atoms. Chu received together with the theoretical physicist Claude Cohen- Tannoudji and William D. Phillips for the development of methods to cool and trap atoms with laser light the Nobel Prize in Physics in 1997. During an interview Steven Chu described how Ashkin was the first to Optical forceps had described as a method for capturing atoms. It Ashkin was possible to catch large particles ( 10-10,000 nm diameter). Chu improved this technique to small particles of up to 0.1 nm in diameter. The first work in the living biological objects ( cells) were successfully manipulated with optical tweezers, is from Ashkin and Dziedzic.

Operation

The optical forces, exercise the Optical Tweezers on a silicone or latex beads in the micrometer or nanometer size range from one piconewtons and more than a nano- Newton. These forces are sufficient to keep freely diffusing particles in water calm or to influence biological molecules physiologically relevant. Most optical tweezers to manipulate particles in solution may be used ( for example in water or air).

A small dielectric sphere, which is significantly smaller than the incident wavelength interacts with the electromagnetic field of an incident light beam by an electric dipole is induced. The resulting interaction between the induced dipole and inducing field leads to a force along the electric field gradient ( gradient force / dipole force ) towards the town of maximum light intensity.

This force is superimposed on a second effect, interpreted as the intuitive classical radiation pressure. Reflection and refraction of the light beam on the surface of the sphere lead to a momentum transfer according to the rules of conservation of momentum. Effectively the result is a force and thus movement of the bead in the propagation direction of the light beam.

If the beam is strong enough focused, the gradient force dominates the force due to the radiation pressure. Manipulation of the location of a bead in a plane which is perpendicular to the laser beam is possible. The bead "follows" the beam. In detail, from electrodynamics semiclassically the light force can be separated into spontaneous force and dipole force, the former being above " radiation pressure " generated, temptation, the " Gradienten-/Dipolkraft ".

With special beam shaping optics and the "self - healing " properties of Bessel beams can be used for optical tweezers.