Optical tweezers use light to immobilize microscopic particles as small as a single atom in 3D space. The basic principle behind optical tweezers is the momentum transfer between light and the object being hold. Much analogous to the water pushing on a dam that blocks the stream, light pushes onto objects (and also attracts them) that make the light bend.

This so-called optical force can be designed to point to a certain point in space, where the particle will be held. In fact, the optical trapping technique has so far won two Nobel Prizes, one in 1997 for holding and cooling down single atoms, a second in 2018 for offering biologists with a tool to study single biomolecules such as DNA and proteins.

Researchers led by Prof. Yuanjie Pang at Huazhong University of Science and Technology (HUST), China, are interested in fiber optical tweezers, where the light and the particles are manipulated at the tip of an optical fiber. This technique eliminates the requirement of conventional, bulky, optical accessories such as microscope objectives, lenses and mirrors.

Their idea is to start with a perfectly annular symmetric light mode that can only transmitted in the optical fiber and will not leak into the surrounding space through the fiber tip, and have a particle to break the mode symmetry and thereby scatter light into the space.

This way, by changing the symmetry and the momentum of the light, the particle receives a reactive force that holds it at the fiber tip. The researchers predict potential applications such as performing an in-vivo single bioparticle manipulating experiment by using the fiber optical tweezer as an endoscope in the interior of a living animal.

Research Report: "Optical trapping using transverse electromagnetic (TEM)-like mode in a coaxial nanowaveguide"

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Huazhong University of Science and Technology
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