When light loses its symmetry, it has the ability to contain particles.

 

Mode symmetry-broken mechanism for enhancing optical trapping behavior. Credit: Yuanhao Lou, Xiongjie Ning, Bei Wu, Yuanjie Pang

Light is used by optical tweezers to imprison minuscule particles as small as a single atom in three-dimensional space. The momentum transfer between light and the thing being held is the core principle behind optical tweezers. Light, like water pushing against a dam that blocks a stream, pushes against and attracts objects that bend the light. This so-called optical force can be directed to a specific location in space where a particle will be held. 

In fact, the optical trapping technology has received two Nobel Prizes so far, one in 1997 for retaining and cooling single atoms and the other in 2018 for providing scientists with a tool to examine single biomolecules like DNA and proteins.

 

The usage of fibre optical tweezers, where light and particles are manipulated at the tip of an optical fibre, is of interest to researchers lead by Prof. Yuanjie Pang at Huazhong University of Science and Technology (HUST) in China. This technology does away with the need for traditional, cumbersome optical gear like microscopes, lenses, and mirrors. 

They propose starting with a fully circular symmetric light mode that can only be transmitted in the optical fibre and will not leak into the surrounding space through the fibre tip, and then using a particle to break the mode symmetry and disperse light into 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.

 

Potential applications, according to the researchers, include executing an in-vivo single bioparticle manipulation experiment employing fibre optical tweezers as an endoscope within a living animal. The paper "Optical trapping in a coaxial nanowaveguide utilising a transverse electromagnetic (TEM)-like mode" was featured on the cover of Frontiers of Optoelectronics.

Reference:

Yuanhao Lou et al, Optical trapping using transverse electromagnetic (TEM)-like mode in a coaxial nanowaveguide, Frontiers of Optoelectronics (2021). DOI:10.1007/s12200-021-1134-3