What are optical tweezers?!

Light carries Momentum

Light can behave like particles (called photons) and hence has momentum. This means if its direction is changed by an object then that object will feel a force exerted on it. We can use this to move tiny transparent particles, this is called optical tweezing.

Light particle interaction

A small particle interacts with a light field, shown in the image to the left. The particle is drawn to the point of the highest intensity of the beam (gradient Force). Once it reaches the maximum intensity point of the beam it is getting pushed along its propagation direction (scattering Force). This causes the bead to be pushed away, however if the beam is strongly focused the intensity gradient overcomes the push of the beam.

Basic Tweezer setup

Unlike normal microscopes we do not view with the naked eye as the laser light could cause serious injury. Instead a camera is used. A basic optical tweezer setup is shown to the right.

The image above showes a microsphere being trapped within the strong intensity gradient of a laser beam - this gradient in XYZ - plane is achieved by using a high NA microscope objective.

Microscopic size

Though the force exerted by a focussed laser beam is only a few million-millionths of the force of a 1kg weight due to gravity, the mass of a particle
scales with the cube of its dimensions so that the mass of a micro-scopic sphere is only a billion-trillionth that of car.

Non contact manipulation

Optical tweezers are often called miniature tractor beams.This is because they give the ability to manipulate microscopic objects without physically touching them.

A tweezer setup is shown to the right - similar tweezer rigs are used within the experimental ATOM 3D project work.

The animation beyond shows such a system in ACTION! A bead (3 micron ) resides in a fiber optical trap and a second bead is captured with an optical tweezer and loaded into the fiber trap. Both beads form an optically bound array.The behaviour of such arrays is studied within ATOM 3D.

Within ATOM 3D experiments like the one above are utilized to investigate the physical properties of tweezers and also to develope new techniques in micromanipulation with applications in the field of life science.

For individual experiments carried out, please see the research section.