Evanescent waves bleeding out of optical fibers can be used to trap and move microparticles like a conveyor belt, a phenomenon that could be useful in drug delivery, cell research and even quantum computing. While the effect has been demonstrated before using light of the fundamental mode, researchers at the Okinawa Institute of Science and Technology Graduate University have taken it a step further by using higher-order modes. They were able to move 3-μm polystyrene beads up to eight times faster through water with higher-order modes than with the fundamental mode. “While it was theoretically proposed that higher-order modes would produce stronger forces, this is the first time, to our knowledge, that three-dimensional particle manipulation has been experimentally demonstrated,” said Dr. Viet Giang Truong. The fibers used in the experiments start with a diameter of 80 μm and taper down to 2 μm at the waist. As the light travels through the fiber, it cannot fit inside the extremely thin waist, so it spreads out, creating an evanescent field around the fiber. This light field can trap particles close to the fiber surface and move them in the direction of light propagation. The shape of light in the fundamental mode, top, versus a higher-order mode, bottom, is shown in a simulation, left, and in the experiment, right. Images courtesy of the Okinawa Institute of Science and Technology Graduate University. Higher-order modes create larger evanescent fields in such tapered fibers. But part of the increase in the speed of the microparticles is also likely due to microfluidic dynamics, said doctoral student Aili Maimaiti. As the particle picks up speed, it lifts slightly away from the fiber, reducing drag and allowing it to move even faster. “This experiment proves the capability of higher-order modes in microfibers to trap and propel particles,” Truong said. “The next step is to control multiple particles in three dimensions around the microfiber. We are also keen to demonstrate similar behavior of atoms around the nanofibers.” Researchers used the microfiber in conjunction with optical tweezers, a tool widely used in research to trap and move individual particles. The higher-order-mode microfiber gives the tweezers greater control over particles to be manipulated, the researchers said. In the future, microfibers could also increase optical tweezers’ sensitivity by communicating more precise information about the trapped particle. Because of the change in energy distribution, the evanescent field from the higher-order mode (red line) extends farther outside the microfiber (gray area) than from the fundamental mode (black line). Optical trapping and manipulation of particles using optical micro- or nanofibers has the potential to help deliver drugs to specific locations, such as inside a target cell. The effect could also be used to measure the interaction forces between cell components, or to study the proteins involved in the DNA and RNA transcription and translation processes. “The beauty of ultrathin optical fibers is that they are a very noninvasive tool allowing us to probe many different physical systems while only affecting specific parameters that we choose,” said professor Dr. Sile Nic Chormaic. “While this work focuses on trapping micron-sized particles using higher-order light modes in optical microfibers, we can use similar techniques at the atomic level for creating some of the building blocks in quantum networks.” The research was published in Scientific Reports (doi: 10.1038/srep09077). For more information, visit www.oist.jp.