Physics

The Concept of Optical Tweezers Explained



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Since the 1970s, the idea of tweezers changed drastically for some people. Instead of using a pair of tweezers with a magnifying glass and a light attached to them, scientists have been using an entirely different machine called Optical tweezers. The goal of optical tweezers is not to move and control tiny bits of bead, wood or string, but to manipulate biological material at the molecular level. Currently, optical tweezers are being used in the biological sciences to bring physics and biology together, but the University of Dresden has an optical tweezers for working with nanoparticles.

Imagine being able to build a nano machine from molecules or cells or being able to unwind a chromosome. Optical tweezers allow this to happen, not with metal prongs that come together to grasp an object, but with incredibly small force fields that are created by laser light. The process basically uses the light scattering and gradient forces that come when particles interact with the light. Stanford University describes optical tweezers this way:

"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."

Originally, optical tweezers were created by modifying a regular microscope. Now, specially built machines allow incredibly accurate, specific and delicate controls. Scientists are able to measure and use displacement and force with incredible precision and accuracy.

Trapping is one popular reason for using optical tweezers. When scientists want to trap microscopic entities like dielectric spheres, viruses, bacteria, living cells, organelles, small metal particles, and even strands of DNA, the new optical tweezers allow incredible control.

Other uses include:

Confinement

Organization

Tracking movement: for bacteria

Measuring and applying smaller forces

Altering larger structures like cell membranes

Making and observing the behavior of molecular motors

Optical tweezers work this way:

When light is bent, there is a transfer of momentum.

Light momentum is proportional to its energy and the direction that it comes from.

Change the direction of light and the momentum of light will change.

If an object changes the direction of light, the object will also change momentum in an equal an opposite way.

This is how the light seems to act as if a force is being applied to the object.

How optical tweezers are built:

Starting with a high quality optical microscope, high quality lasers and other assets are added. Basically, a laser or multiple lasers, a way to focus the beam or beams, condensers, traps, and video cameras are common modifications for an optical tweezer.

A computer is needed to control the operation of the optical tweezers, and skills in microscopy, optics, and laser techniques are required to use such a device.

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