Shaping Light to Change Particle Behavior

Laser light is coupled into a microfibre from the left and causes attractive and repulsive binding forces on small particles (Source: OIST)

Laser light is coupled into a microfibre from the left and causes attractive and repulsive binding forces on small particles (Source: OIST)

Light can take many different forms. Even in our day-to-day life, sunlight is vastly different from fluorescent light. In physics, when studying inter­actions between light and tiny particles, the shape of the light can make a big difference. Scientists from Okinawa Institute of Science and Technology Graduate University OIST and colla­borators at the University of Innsbruck in Austria found that the inter­actions between particles trapped in light distributed along an optical microfiber, as well as the speed of particle movement were different based on the light’s charac­teristics.

Distri­buting light across an optical micro­fiber is used as a way to mani­pulate tiny particles for a variety of possible appli­cations not only in the world of physics, but also biology. There are two main ways to work with light and optical micro­fibers: in the fundamental mode and the higher order mode. The funda­mental mode is the basic light shape where the energy is strongest in the middle of the beam of light and fades at the edges. If the light is any other shape, it can be classified as a higher order mode, which can be created by shining the light through a certain type of crystal.

The OIST team had pre­viously found that the use of higher order modes trapped and moved single particles more rapidly than the funda­mental mode. This time, they looked more closely at the dif­ferences between particle inter­actions and speed changes when dealing with more than one particle, in the funda­mental or the higher order mode. When there are multiple particles trapped in the light sur­rounding an optical micro­fiber, they align in a specific order, which is called the optical binding effect. To explore these particle inter­actions, the researchers trapped up to five particles using optical tweezers. They then moved the particles towards the optical micro­fiber and released them into the light field around the microfiber. The team measured the speed at which the particles were traveling along the microfiber.

“We did measure­ments for both funda­mental and higher order modes,” Aili Maimaiti, OIST Special Research Student said. “We found that higher order modes had a different effect on the particles. In higher order modes, the col­lective particle speed slows down when more particles are added, while the opposite is true for the funda­mental mode.” They also calcu­lated the distance between multiple particles as they moved. They did this calculation each time they added a particle up to the maximum of five particles. The team found that the particles farther from the light source have a smaller space in between them – or inter­particle distance –  but as you move closer to the light source, the space is larger. When they looked at the dif­ferences between funda­mental and higher order modes, they found that the inter­particle distance was smaller in higher order modes.

“This is proof that the binding effect is different under the higher order mode,” Maimaiti said. The researchers developed a theo­retical model that supported the experi­mental findings. The model explained that the particles act as mirrors that reflect and transmit the light in which they are trapped and this causes their inter­action. They high­lighted the importance of under­standing these inte­ractions between particles trapped in light. Physical pheno­mena, such as particle behavior in higher order modes not only allows for better control of the particle positions, but addi­tionally could be useful in studying quantum effects with chains of atoms in 1D crystal-like structures. (Source: OIST)

Reference: A. Maimaiti et al.: Nonlinear force dependence on optically bound micro-particle arrays in the evanescent fields of fundamental and higher order microfibre modes, Sci. Rep. 6, 30131; DOI: 10.1038/srep30131

Links: Light-Matter Interactions Unit, Okinawa Institute of Science and Technology, Okinawa, Japan • Inst. for Theoretical Physics, University of Innsbruck, Innsbruck, Austria

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